C interview Questions - Part 1 Interview question

Section 1. Declarations and Initializations

1.1: How do you decide which integer type to use?

A: If you might need large values (tens of thousands), use long.
Otherwise, if space is very important, use short.  Otherwise,
use int.

1.4: What should the 64-bit type on a machine that can support it?

A: C9X specifies long long.

1.7: What's the best way to declare and define global variables?

A: The best arrangement is to place each definition in some
relevant .c file, with an external declaration in a header file.

1.11: What does extern mean in a function declaration?

A: Nothing, really; the keyword extern is optional here.

1.12: What's the auto keyword good for?

A: Nothing.

1.14: I can't seem to define a linked list node which contains a
pointer to itself.

A: Structures in C can certainly contain pointers to themselves;
the discussion and example in section 6.5 of K&R make this
clear.  Problems arise if an attempt is made to define (and use)
a typedef in the midst of such a declaration; avoid this.

1.21: How do I declare an array of N pointers to functions returning
pointers to functions returning pointers to characters?

A: char *(*(*a[N])())();
Using a chain of typedefs, or the cdecl program, makes these
declarations easier.

1.22: How can I declare a function that returns a pointer to a
function of its own type?

A: You can't quite do it directly.  Use a cast, or wrap a struct
around the pointer and return that.

1.25: My compiler is complaining about an invalid redeclaration of a
function, but I only define it once.

A: Calling an undeclared function declares it implicitly as
returning int.

1.25b: What's the right declaration for main()?

A: See questions 11.12a to 11.15.

1.30: What am I allowed to assume about the initial values
of variables which are not explicitly initialized?

A: Uninitialized variables with "static" duration start out as 0,
as if the programmer had initialized them.  Variables with
"automatic" duration, and dynamically-allocated memory, start
out containing garbage (with the exception of calloc).

1.31: Why can't I initialize a local array with a string?

A: Perhaps you have a pre-ANSI compiler.

1.31b: What's wrong with "char *p = malloc(10);" ?

A: Function calls are not allowed in initializers for global or
static variables.

1.32: What is the difference between char a[] = "string"; and
char *p = "string"; ?

A: The first declares an initialized and modifiable array; the
second declares a pointer initialized to a not-necessarily-
modifiable constant string.

1.34: How do I initialize a pointer to a function?

A: Use something like "extern int func(); int (*fp)() = func;" .


Section 2. Structures, Unions, and Enumerations

2.1: What's the difference between struct x1 { ... }; and
typedef struct { ... } x2; ?

A: The first structure is named by a tag, the second by a typedef
name.

2.2: Why doesn't "struct x { ... }; x thestruct;" work?

A: C is not C++.

2.3: Can a structure contain a pointer to itself?

A: See question 1.14.

2.4: What's the best way of implementing opaque (abstract) data types
in C?

A: One good way is to use structure pointers which point to
structure types which are not publicly defined.

2.6: I came across some code that declared a structure with the last
member an array of one element, and then did some tricky
allocation to make it act like the array had several elements.
Is this legal or portable?

A: An official interpretation has deemed that it is not strictly
conforming with the C Standard.

2.7: I heard that structures could be assigned to variables and
passed to and from functions, but K&R1 says not.

A: These operations are supported by all modern compilers.

2.8: Is there a way to compare structures automatically?

A: No.

2.10: Can I pass constant values to functions which accept structure
arguments?

A: Not yet.  As of this writing, C has no way of generating
anonymous structure values.

2.11: How can I read/write structures from/to data files?

A: It is relatively straightforward to use fread and fwrite.

2.12: How can I turn off structure padding?

A: There is no standard method.

2.13: Why does sizeof report a larger size than I expect for a
structure type?

A: The alignment of arrays of structures must be preserved.

2.14: How can I determine the byte offset of a field within a
structure?

A: ANSI C defines the offsetof() macro, which should be used if
available.

2.15: How can I access structure fields by name at run time?

A: Build a table of names and offsets, using the offsetof() macro.

2.18: I have a program which works correctly, but dumps core after it
finishes.  Why?

A: Check to see if a structure type declaration just before main()
is missing its trailing semicolon, causing main() to be declared
as returning a structure.  See also questions 10.9 and 16.4.

2.20: Can I initialize unions?

A: The current C Standard allows an initializer for the first-named
member.

2.22: What is the difference between an enumeration and a set of
preprocessor #defines?

A: At the present time, there is little difference.  The C Standard
states that enumerations are compatible with integral types.

2.24: Is there an easy way to print enumeration values symbolically?

A: No.


Section 3. Expressions

3.1: Why doesn't the code "a[i] = i++;" work?

A: The variable i is both referenced and modified in the same
expression.

3.2: Under my compiler, the code "int i = 7;
printf("%d\n", i++ * i++);" prints 49.  Regardless of the order
of evaluation, shouldn't it print 56?

A: The operations implied by the postincrement and postdecrement
operators ++ and -- are performed at some time after the
operand's former values are yielded and before the end of the
expression, but not necessarily immediately after, or before
other parts of the expression are evaluated.

3.3: What should the code "int i = 3; i = i++;" do?

A: The expression is undefined.

3.3b: Here's a slick expression: "a ^= b ^= a ^= b".  It swaps a and b
without using a temporary.

A: Not portably; its behavior is undefined.

3.4: Don't precedence and parentheses dictate order of evaluation?

A: Operator precedence and explicit parentheses impose only a
partial ordering on the evaluation of an expression, which does
not generally include the order of side effects.

3.5: But what about the && and || operators?

A: There is a special exception for those operators: left-to-right
evaluation is guaranteed.

3.8: What's a "sequence point"?

A: A point (at the end of a full expression, or at the ||, &&, ?:,
or comma operators, or just before a function call) at which all
side effects are guaranteed to be complete.

3.9: So given a[i] = i++; we don't know which cell of a[] gets
written to, but i does get incremented by one, right?

A: *No*.  Once an expression or program becomes undefined, *all*
aspects of it become undefined.

3.12: If I'm not using the value of the expression, should I use i++
or ++i to increment a variable?

A: Since the two forms differ only in the value yielded, they are
entirely equivalent when only their side effect is needed.

3.14: Why doesn't the code "int a = 1000, b = 1000;
long int c = a * b;" work?

A: You must manually cast one of the operands to (long).

3.16: Can I use ?: on the left-hand side of an assignment expression?

A: No.


Section 4. Pointers

4.2: What's wrong with "char *p; *p = malloc(10);"?

A: The pointer you declared is p, not *p.

4.3: Does *p++ increment p, or what it points to?

A: *p++ increments p.  To increment the value pointed to by p, use
(*p)++ .

4.5: I want to use a char * pointer to step over some ints.  Why
doesn't "((int *)p)++;" work?

A: In C, a cast operator is a conversion operator, and by
definition it yields an rvalue, which cannot be assigned to, or
incremented with ++.

4.8: I have a function which accepts, and is supposed to initialize,
a pointer, but the pointer in the caller remains unchanged.

A: The called function probably altered only the passed copy of the
pointer.

4.9: Can I use a void ** pointer as a parameter so that a function
can accept a generic pointer by reference?

A: Not portably.

4.10: I have a function which accepts a pointer to an int.  How can I
pass a constant like 5 to it?

A: You will have to declare a temporary variable.

4.11: Does C even have "pass by reference"?

A: Not really, though it can be simulated.

4.12: I've seen different methods used for calling functions via
pointers.

A: The extra parentheses and explicit * are now officially
optional, although some older implementations require them.


Section 5. Null Pointers

5.1: What is this infamous null pointer, anyway?

A: For each pointer type, there is a special value -- the "null
pointer" -- which is distinguishable from all other pointer
values and which is not the address of any object or function.

5.2: How do I get a null pointer in my programs?

A: A constant 0 in a pointer context is converted into a null
pointer at compile time.  A "pointer context" is an
initialization, assignment, or comparison with one side a
variable or expression of pointer type, and (in ANSI standard C)
a function argument which has a prototype in scope declaring a
certain parameter as being of pointer type.  In other contexts
(function arguments without prototypes, or in the variable part
of variadic function calls) a constant 0 with an appropriate
explicit cast is required.

5.3: Is the abbreviated pointer comparison "if(p)" to test for non-
null pointers valid?

A: Yes.  The construction "if(p)" works, regardless of the internal
representation of null pointers, because the compiler
essentially rewrites it as "if(p != 0)" and goes on to convert 0
into the correct null pointer.

5.4: What is NULL and how is it #defined?

A: NULL is simply a preprocessor macro, #defined as 0 (or
((void *)0)), which is used (as a stylistic convention, in
preference to unadorned 0's) to generate null pointers.

5.5: How should NULL be defined on a machine which uses a nonzero bit
pattern as the internal representation of a null pointer?

A: The same as on any other machine: as 0.  (The compiler makes the
translation, upon seeing a 0, not the preprocessor; see also
question 5.4.)

5.6: If NULL were defined as "((char *)0)," wouldn't that make
function calls which pass an uncast NULL work?

A: Not in general.  The complication is that there are machines
which use different internal representations for pointers to
different types of data.  A cast is still required to tell the
compiler which kind of null pointer is required, since it may be
different from (char *)0.

5.9: If NULL and 0 are equivalent as null pointer constants, which
should I use?

A: Either; the distinction is entirely stylistic.

5.10: But wouldn't it be better to use NULL, in case the value of NULL
changes?

A: No.  NULL is a constant zero, so a constant zero is equally
sufficient.

5.12: I use the preprocessor macro "#define Nullptr(type) (type *)0"
to help me build null pointers of the correct type.

A: This trick, though valid, does not buy much.

5.13: This is strange.  NULL is guaranteed to be 0, but the null
pointer is not?

A: A "null pointer" is a language concept whose particular internal
value does not matter.  A null pointer is requested in source
code with the character "0".  "NULL" is a preprocessor macro,
which is always #defined as 0 (or ((void *)0)).

5.14: Why is there so much confusion surrounding null pointers?

A: The fact that null pointers are represented both in source code,
and internally to most machines, as zero invites unwarranted
assumptions.  The use of a preprocessor macro (NULL) may seem to
suggest that the value could change some day, or on some weird
machine.

5.15: I'm confused.  I just can't understand all this null pointer
stuff.

A: A simple rule is, "Always use `0' or `NULL' for null pointers,
and always cast them when they are used as arguments in function
calls."

5.16: Given all the confusion surrounding null pointers, wouldn't it
be easier simply to require them to be represented internally by
zeroes?

A: Such a requirement would accomplish little.

5.17: Seriously, have any actual machines really used nonzero null
pointers?

A: Machines manufactured by Prime, Honeywell-Bull, and CDC, as well
as Symbolics Lisp Machines, have done so.

5.20: What does a run-time "null pointer assignment" error mean?

A: It means that you've written, via a null pointer, to an invalid
location.  (See also question 16.8.)


Section 6. Arrays and Pointers

6.1: I had the definition char a[6] in one source file, and in
another I declared extern char *a.  Why didn't it work?

A: The declaration extern char *a simply does not match the actual
definition.  Use extern char a[].

6.2: But I heard that char a[] was identical to char *a.

A: Not at all.  Arrays are not pointers.  A reference like x[3]
generates different code depending on whether x is an array or a
pointer.

6.3: So what is meant by the "equivalence of pointers and arrays" in
C?

A: An lvalue of type array-of-T which appears in an expression
decays into a pointer to its first element; the type of the
resultant pointer is pointer-to-T.  So for an array a and
pointer p, you can say "p = a;" and then p[3] and a[3] will
access the same element.

6.4: Why are array and pointer declarations interchangeable as
function formal parameters?

A: It's supposed to be a convenience.

6.7: How can an array be an lvalue, if you can't assign to it?

A: An array is not a "modifiable lvalue."

6.8: What is the real difference between arrays and pointers?

A: Arrays automatically allocate space which is fixed in size and
location; pointers are dynamic.

6.9: Someone explained to me that arrays were really just constant
pointers.

A: An array name is "constant" in that it cannot be assigned to,
but an array is *not* a pointer.

6.11: I came across some "joke" code containing the "expression"
5["abcdef"] .  How can this be legal C?

A: Yes, array subscripting is commutative in C.  The array
subscripting operation a[e] is defined as being identical to
*((a)+(e)).

6.12: What's the difference between array and &array?

A: The type.

6.13: How do I declare a pointer to an array?

A: Usually, you don't want to.  Consider using a pointer to one of
the array's elements instead.

6.14: How can I set an array's size at run time?

A: It's straightforward to use malloc() and a pointer.

6.15: How can I declare local arrays of a size matching a passed-in
array?

A: Until recently, you couldn't; array dimensions had to be compile-
time constants.  C9X will fix this.

6.16: How can I dynamically allocate a multidimensional array?

A: The traditional solution is to allocate an array of pointers,
and then initialize each pointer to a dynamically-allocated
"row."  See the full list for code samples.

6.17: Can I simulate a non-0-based array with a pointer?

A: Not if the pointer points outside of the block of memory it is
intended to access.

6.18: My compiler complained when I passed a two-dimensional array to
a function expecting a pointer to a pointer.

A: The rule by which arrays decay into pointers is not applied
recursively.  An array of arrays (i.e. a two-dimensional array
in C) decays into a pointer to an array, not a pointer to a
pointer.

6.19: How do I write functions which accept two-dimensional arrays
when the width is not known at compile time?

A: It's not always particularly easy.

6.20: How can I use statically- and dynamically-allocated
multidimensional arrays interchangeably when passing them to
functions?

A: There is no single perfect method, but see the full list for
some ideas.

6.21: Why doesn't sizeof properly report the size of an array which is
a parameter to a function?

A: The sizeof operator reports the size of the pointer parameter
which the function actually receives.


Section 7. Memory Allocation

7.1: Why doesn't the code "char *answer; gets(answer);" work?

A: The pointer variable answer has not been set to point to any
valid storage.  The simplest way to correct this fragment is to
use a local array, instead of a pointer.

7.2: I can't get strcat() to work.  I tried "char *s3 =
strcat(s1, s2);" but I got strange results.

A: Again, the main problem here is that space for the concatenated
result is not properly allocated.

7.3: But the man page for strcat() says that it takes two char *'s as
arguments.  How am I supposed to know to allocate things?

A: In general, when using pointers you *always* have to consider
memory allocation, if only to make sure that the compiler is
doing it for you.

7.3b: I just tried the code "char *p; strcpy(p, "abc");" and it
worked.  Why didn't it crash?

A: You got "lucky".

7.3c: How much memory does a pointer variable allocate?

A: Only enough memory to hold the pointer itself, not any memory
for the pointer to point to.

7.5a: I have a function that is supposed to return a string, but when
it returns to its caller, the returned string is garbage.

A: Make sure that the pointed-to memory is properly (i.e. not
locally) allocated.

7.5b: So what's the right way to return a string?

A: Return a pointer to a statically-allocated buffer, a buffer
passed in by the caller, or memory obtained with malloc().

7.6: Why am I getting "warning: assignment of pointer from integer
lacks a cast" for calls to malloc()?

A: Have you #included ?

7.7: Why does some code carefully cast the values returned by malloc
to the pointer type being allocated?

A: Before ANSI/ISO C, these casts were required to silence certain
warnings.

7.8: Why does so much code leave out the multiplication by
sizeof(char) when allocating strings?

A: Because sizeof(char) is, by definition, exactly 1.

7.14: I've heard that some operating systems don't actually allocate
malloc'ed memory until the program tries to use it.  Is this
legal?

A: It's hard to say.

7.16: I'm allocating a large array for some numeric work, but malloc()
is acting strangely.

A: Make sure the number you're trying to pass to malloc() isn't
bigger than a size_t can hold.

7.17: I've got 8 meg of memory in my PC.  Why can I only seem to
malloc 640K or so?

A: Under the segmented architecture of PC compatibles, it can be
difficult to use more than 640K with any degree of transparency.
See also question 19.23.

7.19: My program is crashing, apparently somewhere down inside malloc.

A: Make sure you aren't using more memory than you malloc'ed,
especially for strings (which need strlen(str) + 1 bytes).

7.20: You can't use dynamically-allocated memory after you free it,
can you?

A: No.  Some early documentation implied otherwise, but the claim
is no longer valid.

7.21: Why isn't a pointer null after calling free()?

A: C's pass-by-value semantics mean that called functions can never
permanently change the values of their arguments.

7.22: When I call malloc() to allocate memory for a local pointer, do
I have to explicitly free() it?

A: Yes.

7.23: When I free a dynamically-allocated structure containing
pointers, do I also have to free each subsidiary pointer?

A: Yes.

7.24: Must I free allocated memory before the program exits?

A: You shouldn't have to.

7.25: Why doesn't my program's memory usage go down when I free
memory?

A: Most implementations of malloc/free do not return freed memory
to the operating system.

7.26: How does free() know how many bytes to free?

A: The malloc/free implementation remembers the size of each block
as it is allocated.

7.27: So can I query the malloc package to find out how big an
allocated block is?

A: Not portably.

7.30: Is it legal to pass a null pointer as the first argument to
realloc()?

A: ANSI C sanctions this usage, although several earlier
implementations do not support it.

7.31: What's the difference between calloc() and malloc()?

A: calloc() takes two arguments, and initializes the allocated
memory to all-bits-0.

7.32: What is alloca() and why is its use discouraged?

A: alloca() allocates memory which is automatically freed when the
function which called alloca() returns.  alloca() cannot be
written portably, is difficult to implement on machines without
a stack, and fails under certain conditions if implemented
simply.


Section 8. Characters and Strings

8.1: Why doesn't "strcat(string, '!');" work?

A: strcat() concatenates *strings*, not characters.

8.2: Why won't the test if(string == "value") correctly compare
string against the value?

A: It's comparing pointers.  To compare two strings, use strcmp().

8.3: Why can't I assign strings to character arrays?

A: Strings are arrays, and you can't assign arrays directly.  Use
strcpy() instead.

8.6: How can I get the numeric (character set) value corresponding to
a character?

A: In C, if you have the character, you have its value.

8.9: Why is sizeof('a') not 1?

A: Character constants in C are of type int.


Section 9. Boolean Expressions and Variables

9.1: What is the right type to use for Boolean values in C?

A: There's no one right answer; see the full list for some
discussion.

9.2: What if a built-in logical or relational operator "returns"
something other than 1?

A: When a Boolean value is generated by a built-in operator, it is
guaranteed to be 1 or 0.  (This is *not* true for some library
routines such as isalpha.)

9.3: Is if(p), where p is a pointer, valid?

A: Yes.  See question 5.3.


Section 10. C Preprocessor

10.2: I've got some cute preprocessor macros that let me write C code
that looks more like Pascal.  What do y'all think?

A: Bleah.

10.3: How can I write a generic macro to swap two values?

A: There is no good answer to this question.  The best all-around
solution is probably to forget about using a macro.

10.4: What's the best way to write a multi-statement macro?

A: #define Func() do {stmt1; stmt2; ... } while(0) /* (no trailing ;) */

10.6: What are .h files and what should I put in them?

A: Header files (also called ".h files") should generally contain
common declarations and macro, structure, and typedef
definitions, but not variable or function definitions.

10.7: Is it acceptable for one header file to #include another?

A: It's a question of style, and thus receives considerable debate.

10.8a: What's the difference between #include <> and #include "" ?

A: Roughly speaking, the <> syntax is for Standard headers and ""
is for project headers.

10.8b: What are the complete rules for header file searching?

A: The exact behavior is implementation-defined; see the full list
for some discussion.

10.9: I'm getting strange syntax errors on the very first declaration
in a file, but it looks fine.

A: Perhaps there's a missing semicolon at the end of the last
declaration in the last header file you're #including.

10.10b: I'm #including the header file for a function, but the linker
keeps saying it's undefined.

A: See question 13.25.

10.11: Where can I get a copy of a missing header file?

A: Contact your vendor, or see question 18.16 or the full list.

10.12: How can I construct preprocessor #if expressions which compare
strings?

A: You can't do it directly; try #defining several manifest
constants and implementing conditionals on those.

10.13: Does the sizeof operator work in preprocessor #if directives?

A: No.

10.14: Can I use an #ifdef in a #define line, to define something two
different ways?

A: No.

10.15: Is there anything like an #ifdef for typedefs?

A: Unfortunately, no.

10.16: How can I use a preprocessor #if expression to detect
endianness?

A: You probably can't.

10.18: How can I preprocess some code to remove selected conditional
compilations, without preprocessing everything?

A: Look for a program called unifdef, rmifdef, or scpp.

10.19: How can I list all of the predefined identifiers?

A: If the compiler documentation is unhelpful, try extracting
printable strings from the compiler or preprocessor executable.

10.20: I have some old code that tries to construct identifiers with a
macro like "#define Paste(a, b) a/**/b", but it doesn't work any
more.

A: Try the ANSI token-pasting operator ##.

10.22: What does the message "warning: macro replacement within a
string literal" mean?

A: See question 11.18.

10.23-4: I'm having trouble using macro arguments inside string
literals, using the `#' operator.

A: See questions 11.17 and 11.18.

10.25: I've got this tricky preprocessing I want to do and I can't
figure out a way to do it.

A: Consider writing your own little special-purpose preprocessing
tool, instead.

10.26: How can I write a macro which takes a variable number of
arguments?

A: Here is one popular trick.  Note that the parentheses around
printf's argument list are in the macro call, not the
definition.

#define DEBUG(args) (printf("DEBUG: "), printf args)

if(n != 0) DEBUG(("n is %d\n", n));


Section 11. ANSI/ISO Standard C

11.1: What is the "ANSI C Standard?"

A: In 1983, the American National Standards Institute (ANSI)
commissioned a committee to standardize the C language.  Their
work was ratified as ANS X3.159-1989, and has since been adopted
as ISO/IEC 9899:1990, and later amended.

11.2: How can I get a copy of the Standard?

A: Copies are available from ANSI in New York, or from Global
Engineering Documents in Englewood, CO, or from any national
standards body, or from ISO in Geneva, or republished within one
or more books.  See the unabridged list for details.

11.2b: Where can I get information about updates to the Standard?

A: See the full list for pointers.

11.3: My ANSI compiler is complaining about prototype mismatches for
parameters declared float.

A: You have mixed the new-style prototype declaration
"extern int func(float);" with the old-style definition
"int func(x) float x;".  "Narrow" types are treated differently
according to which syntax is used.  This problem can be fixed by
avoiding narrow types, or by using either new-style (prototype)
or old-style syntax consistently.

11.4: Can you mix old-style and new-style function syntax?

A: Doing so is currently legal, for most argument types
(see question 11.3).

11.5: Why does the declaration "extern int f(struct x *p);" give me a
warning message?

A: A structure declared (or even mentioned) for the first time
within a prototype cannot be compatible with other structures
declared in the same source file.

11.8: Why can't I use const values in initializers and array
dimensions?

A: The value of a const-qualified object is *not* a constant
expression in the full sense of the term.

11.9: What's the difference between "const char *p" and
"char * const p"?

A: The former declares a pointer to a constant character; the
latter declares a constant pointer to a character.

11.10: Why can't I pass a char ** to a function which expects a
const char **?

A: The rule which permits slight mismatches in qualified pointer
assignments is not applied recursively.

11.12a: What's the correct declaration of main()?

A: int main(int argc, char *argv[]) .

11.12b: Can I declare main() as void, to shut off these annoying "main
returns no value" messages?

A: No.

11.13: But what about main's third argument, envp?

A: It's a non-standard (though common) extension.

11.14: I believe that declaring void main() can't fail, since I'm
calling exit() instead of returning.

A: It doesn't matter whether main() returns or not, the problem is
that its caller may not even be able to *call* it correctly.

11.15: The book I've been using always uses void main().

A: It's wrong.

11.16: Is exit(status) truly equivalent to returning the same status
from main()?

A: Yes and no.  (See the full list for details.)

11.17: How do I get the ANSI "stringizing" preprocessing operator `#'
to stringize the macro's value instead of its name?

A: You can use a two-step #definition to force a macro to be
expanded as well as stringized.

11.18: What does the message "warning: macro replacement within a
string literal" mean?

A: Some pre-ANSI compilers/preprocessors expanded macro parameters
even inside string literals and character constants.

11.19: I'm getting strange syntax errors inside lines I've #ifdeffed
out.

A: Under ANSI C, #ifdeffed-out text must still consist of "valid
preprocessing tokens."  This means that there must be no
newlines inside quotes, and no unterminated comments or quotes
(i.e. no single apostrophes).

11.20: What are #pragmas ?

A: The #pragma directive provides a single, well-defined "escape
hatch" which can be used for extensions.

11.21: What does "#pragma once" mean?

A: It is an extension implemented by some preprocessors to help
make header files idempotent.

11.22: Is char a[3] = "abc"; legal?

A: Yes, in ANSI C.

11.24: Why can't I perform arithmetic on a void * pointer?

A: The compiler doesn't know the size of the pointed-to objects.

11.25: What's the difference between memcpy() and memmove()?

A: memmove() offers guaranteed behavior if the source and
destination arguments overlap.

11.26: What should malloc(0) do?

A: The behavior is implementation-defined.

11.27: Why does the ANSI Standard not guarantee more than six case-
insensitive characters of external identifier significance?

A: The problem is older linkers which cannot be forced (by mere
words in a Standard) to upgrade.

11.29: My compiler is rejecting the simplest possible test programs,
with all kinds of syntax errors.

A: Perhaps it is a pre-ANSI compiler.

11.30: Why are some ANSI/ISO Standard library functions showing up as
undefined, even though I've got an ANSI compiler?

A: Perhaps you don't have ANSI-compatible headers and libraries.

11.31: Does anyone have a tool for converting old-style C programs to
ANSI C, or for automatically generating prototypes?

A: See the full list for details.

11.32: Why won't frobozz-cc, which claims to be ANSI compliant, accept
this code?

A: Are you sure that the code being rejected doesn't rely on some
non-Standard extension?

11.33: What's the difference between implementation-defined,
unspecified, and undefined behavior?

A: If you're writing portable code, ignore the distinctions.
Otherwise, see the full list.

11.34: I'm appalled that the ANSI Standard leaves so many issues
undefined.

A: In most of these cases, the Standard is simply codifying
existing practice.

11.35: I just tried some allegedly-undefined code on an ANSI-conforming
compiler, and got the results I expected.

A: A compiler may do anything it likes when faced with undefined
behavior, including doing what you expect.


Section 12. Stdio

12.1: What's wrong with the code "char c; while((c = getchar()) !=
EOF) ..."?

A: The variable to hold getchar's return value must be an int.

12.2: Why won't the code "while(!feof(infp)) {
fgets(buf, MAXLINE, infp); fputs(buf, outfp); }" work?

A: EOF is only indicated *after* an input routine fails.

12.4: My program's prompts and intermediate output don't always show
up on the screen.

A: It's best to use an explicit fflush(stdout) whenever output
should definitely be visible.

12.5: How can I read one character at a time, without waiting for the
RETURN key?

A: See question 19.1.

12.6: How can I print a '%' character with printf?

A: "%%".

12.9: How can printf() use %f for type double, if scanf() requires
%lf?

A: C's "default argument promotions" mean that values of type float
are promoted to double.

12.9b: What printf format should I use for a typedef when I don't know
the underlying type?

A: Use a cast to convert the value to a known type, then use the
printf format matching that type.

12.10: How can I implement a variable field width with printf?

A: Use printf("%*d", width, x).

12.11: How can I print numbers with commas separating the thousands?

A: There is no standard routine (but see ).

12.12: Why doesn't the call scanf("%d", i) work?

A: The arguments you pass to scanf() must always be pointers.

12.13: Why doesn't the code "double d; scanf("%f", &d);" work?

A: Unlike printf(), scanf() uses %lf for double, and %f for float.

12.15: How can I specify a variable width in a scanf() format string?

A: You can't.

12.17: When I read numbers from the keyboard with scanf "%d\n", it
seems to hang until I type one extra line of input.

A: Try using "%d" instead of "%d\n".

12.18: I'm reading a number with scanf %d and then a string with
gets(), but the compiler seems to be skipping the call to
gets()!

A: scanf() and gets() do not work well together.

12.19: I'm re-prompting the user if scanf() fails, but sometimes it
seems to go into an infinite loop.

A: scanf() tends to "jam" on bad input since it does not discard
it.

12.20: Why does everyone say not to use scanf()?  What should I use
instead?

A: scanf() has a number of problems.  Usually, it's easier to read
entire lines and then interpret them.

12.21: How can I tell how much destination buffer space I'll need for
an arbitrary sprintf call?  How can I avoid overflowing the
destination buffer with sprintf()?

A: Use the new snprintf() function, if you can.

12.23: Why does everyone say not to use gets()?

A: It cannot be prevented from overflowing the input buffer.

12.24: Why does errno contain ENOTTY after a call to printf()?

A: Don't worry about it.  It is only meaningful for a program to
inspect the contents of errno after an error has been reported.

12.25: What's the difference between fgetpos/fsetpos and ftell/fseek?

A: fgetpos() and fsetpos() use a special typedef which may allow
them to work with larger files than ftell() and fseek().

12.26: Will fflush(stdin) flush unread characters from the standard
input stream?

A: No.

12.30: I'm trying to update a file in place, by using fopen mode "r+",
but it's not working.

A: Be sure to call fseek between reading and writing.

12.33: How can I redirect stdin or stdout from within a program?

A: Use freopen().

12.34: Once I've used freopen(), how can I get the original stream
back?

A: There isn't a good way.  Try avoiding freopen.

12.36b: How can I arrange to have output go two places at once?

A: You could write your own printf variant which printed everything
twice.  See question 15.5.

12.38: How can I read a binary data file properly?

A: Be sure to specify "rb" mode when calling fopen().


Section 13. Library Functions

13.1: How can I convert numbers to strings?

A: Just use sprintf().

13.2: Why does strncpy() not always write a '\0'?

A: For mildly-interesting historical reasons.

13.5: Why do some versions of toupper() act strangely if given an
upper-case letter?

A: Older versions of toupper() and tolower() did not always work as
expected in this regard.

13.6: How can I split up a string into whitespace-separated fields?

A: Try strtok().

13.7: I need some code to do regular expression and wildcard matching.

A: regexp libraries abound; see the full list for details.

13.8: I'm trying to sort an array of strings with qsort(), using
strcmp() as the comparison function, but it's not working.

A: You'll have to write a "helper" comparison function which takes
two generic pointer arguments, converts them to char **, and
dereferences them, yielding char *'s which can be usefully
compared.

13.9: Now I'm trying to sort an array of structures, but the compiler
is complaining that the function is of the wrong type for
qsort().

A: The comparison function must be declared as accepting "generic
pointers" (const void *) which it then converts to structure
pointers.

13.10: How can I sort a linked list?

A: Algorithms like insertion sort and merge sort work well, or you
can keep the list in order as you build it.

13.11: How can I sort more data than will fit in memory?

A: You want an "external sort"; see the full list for details.

13.12: How can I get the time of day in a C program?

A: Just use the time(), ctime(), localtime() and/or strftime()
functions.

13.13: How can I convert a struct tm or a string into a time_t?

A: The ANSI mktime() function converts a struct tm to a time_t.  No
standard routine exists to parse strings.

13.14: How can I perform calendar manipulations?

A: The ANSI/ISO Standard C mktime() and difftime() functions
provide some support for both problems.

13.14b: Does C have any Year 2000 problems?

A: No, although poorly-written C programs do.  Make sure you know
that tm_year holds the value of the year minus 1900.

13.15: I need a random number generator.

A: The Standard C library has one: rand().

13.16: How can I get random integers in a certain range?

A: One method is something like

(int)((double)rand() / ((double)RAND_MAX + 1) * N)

13.17: Each time I run my program, I get the same sequence of numbers
back from rand().

A: You can call srand() to seed the pseudo-random number generator
with a truly random initial value.

13.18: I need a random true/false value, so I'm just taking rand() % 2,
but it's alternating 0, 1, 0, 1, 0...

A: Try using the higher-order bits: see question 13.16.

13.20: How can I generate random numbers with a normal or Gaussian
distribution?

A: See the longer versions of this list for ideas.

13.24: I'm trying to port this old program.  Why do I get "undefined
external" errors for some library functions?

A: Some semistandard functions have been renamed or replaced over
the years; see the full list for details.

13.25: I get errors due to library functions being undefined even
though I #include the right header files.

A: You may have to explicitly ask for the correct libraries to be
searched.

13.26: I'm still getting errors due to library functions being
undefined, even though I'm requesting the right libraries.

A: Library search order is significant; usually, you must search
the libraries last.

13.28: What does it mean when the linker says that _end is undefined?

A: You generally get that message only when other symbols are
undefined, too.


Section 14. Floating Point

14.1: When I set a float variable to 3.1, why is printf printing it as
3.0999999?

A: Most computers use base 2 for floating-point numbers, and many
fractions (including 0.1 decimal) are not exactly representable
in base 2.

14.2: Why is sqrt(144.) giving me crazy numbers?

A: Make sure that you have #included , and correctly
declared other functions returning double.

14.3: I keep getting "undefined: sin" compilation errors.

A: Make sure you're actually linking with the math library.

14.4: My floating-point calculations are acting strangely and giving
me different answers on different machines.

A: First, see question 14.2 above.  If the problem isn't that
simple, see the full list for a brief explanation, or any good
programming book for a better one.

14.5: What's a good way to check for "close enough" floating-point
equality?

A: The best way is to use an accuracy threshold which is relative
to the magnitude of the numbers being compared.

14.6: How do I round numbers?

A: For positive numbers, try (int)(x + 0.5) .

14.7: Where is C's exponentiation operator?

A: Try using the pow() function.

14.8: The predefined constant M_PI seems to be missing from .

A: That constant is not standard.

14.9: How do I test for IEEE NaN and other special values?

A: There is not yet a portable way, but see the full list for
ideas.

14.11: What's a good way to implement complex numbers in C?

A: It is straightforward to define a simple structure and some
arithmetic functions to manipulate them.

14.12: I'm looking for some mathematical library code.

A: See Ajay Shah's index of free numerical software at
ftp://ftp.math.psu.edu/pub/FAQ/numcomp-free-c .

14.13: I'm having trouble with a Turbo C program which crashes and says
something like "floating point formats not linked."

A: You may have to insert a dummy call to a floating-point library
function to force loading of floating-point support.


Section 15. Variable-Length Argument Lists

15.1: I heard that you have to #include  before calling
printf().  Why?

A: So that a proper prototype for printf() will be in scope.

15.2: How can %f be used for both float and double arguments in
printf()?

A: In variable-length argument lists, types char and short int are
promoted to int, and float is promoted to double.

15.3: Why don't function prototypes guard against mismatches in
printf's arguments?

A: Function prototypes do not provide any information about the
number and types of variable arguments.

15.4: How can I write a function that takes a variable number of
arguments?

A: Use the  header.

15.5: How can I write a function that takes a format string and a
variable number of arguments, like printf(), and passes them to
printf() to do most of the work?

A: Use vprintf(), vfprintf(), or vsprintf().

15.6: How can I write a function analogous to scanf(), that calls
scanf() to do most of the work?

A: C9X will support vscanf().

15.7: I have a pre-ANSI compiler, without .  What can I do?

A: There's an older header, , which offers about the
same functionality.

15.8: How can I discover how many arguments a function was actually
called with?

A: Any function which takes a variable number of arguments must be
able to determine *from the arguments' values* how many of them
there are.

15.9: My compiler isn't letting me declare a function that accepts
*only* variable arguments.

A: Standard C requires at least one fixed argument.

15.10: Why isn't "va_arg(argp, float)" working?

A: Because the "default argument promotions" apply in variable-
length argument lists, you should always use
va_arg(argp, double).

15.11: I can't get va_arg() to pull in an argument of type pointer-to-
function.

A: Use a typedef.

15.12: How can I write a function which takes a variable number of
arguments and passes them to some other function ?

A: In general, you cannot.

15.13: How can I call a function with an argument list built up at run
time?

A: You can't.


Section 16. Strange Problems

16.1b: I'm getting baffling syntax errors which make no sense at all,
and it seems like large chunks of my program aren't being
compiled.

A: Check for unclosed comments or mismatched preprocessing
directives.

16.1c: Why isn't my procedure call working?

A: Function calls always require parenthesized argument lists.

16.3: This program crashes before it even runs!

A: Look for very large, local arrays.
(See also questions 11.12b, 16.4, 16.5, and 18.4.)

16.4: I have a program that seems to run correctly, but then crashes
as it's exiting.

A: See the full list for ideas.

16.5: This program runs perfectly on one machine, but I get weird
results on another.

A: See the full list for a brief list of possibilities.

16.6: Why does the code "char *p = "hello, world!"; p[0] = 'H';"
crash?

A: String literals are not modifiable, except (in effect) when they
are used as array initializers.

16.8: What does "Segmentation violation" mean?

A: It generally means that your program tried to access memory it
shouldn't have, invariably as a result of stack corruption or
improper pointer use.


Section 17. Style

17.1: What's the best style for code layout in C?

A: There is no one "best style," but see the full list for a few
suggestions.

17.3: Is the code "if(!strcmp(s1, s2))" good style?

A: Not particularly.

17.4: Why do some people write if(0 == x) instead of if(x == 0)?

A: It's a trick to guard against the common error of writing
if(x = 0) .

17.5: I came across some code that puts a (void) cast before each call
to printf().  Why?

A: To suppress warnings about otherwise discarded return values.

17.8: What is "Hungarian Notation"?

A: It's a naming convention which encodes information about a
variable's type in its name.

17.9: Where can I get the "Indian Hill Style Guide" and other coding
standards?

A: See the unabridged list.

17.10: Some people say that goto's are evil and that I should never use
them.  Isn't that a bit extreme?

A: Yes.  Absolute rules are an imperfect approach to good
programming style.


Section 18. Tools and Resources

18.1: I'm looking for C development tools (cross-reference generators,
code beautifiers, etc.).

A: See the full list for a few names.

18.2: How can I track down these pesky malloc problems?

A: See the full list for a list of tools.

18.3: What's a free or cheap C compiler I can use?

A: See the full list for a brief catalog.

18.4: I just typed in this program, and it's acting strangely.  Can
you see anything wrong with it?

A: See if you can run lint first.

18.5: How can I shut off the "warning: possible pointer alignment
problem" message which lint gives me for each call to malloc()?

A: It may be easier simply to ignore the message, perhaps in an
automated way with grep -v.

18.7: Where can I get an ANSI-compatible lint?

A: See the unabridged list for two commercial products.

18.8: Don't ANSI function prototypes render lint obsolete?

A: No.  A good compiler may match most of lint's diagnostics; few
provide all.

18.9: Are there any C tutorials or other resources on the net?

A: There are several of them.

18.10: What's a good book for learning C?

A: There are far too many books on C to list here; the full list
contains a few pointers.

18.13: Where can I find the sources of the standard C libraries?

A: Several possibilites are listed in the full list.

18.13b: Is there an on-line C reference manual?

A: Two possibilities are
http://www.cs.man.ac.uk/standard_c/_index.html and
http://www.dinkumware.com/htm_cl/index.html .

18.13c: Where can I get a copy of the ANSI/ISO C Standard?

A: See question 11.2.

18.14: I need code to parse and evaluate expressions.

A: Several available packages are mentioned in the full list.

18.15: Where can I get a BNF or YACC grammar for C?

A: See the ANSI Standard, or the unabridged list.

18.15b: Does anyone have a C compiler test suite I can use?

A: See the full list for several sources.

18.15c: Where are some collections of useful code fragments and
examples?

A: See the full list for a few sources.

18.15d: I need code for performing multiple precision arithmetic.

A: See the full list for a few ideas.

18.16: Where and how can I get copies of all these freely distributable
programs?

A: See the regular postings in the comp.sources.unix and
comp.sources.misc newsgroups, or the full version of this list,
for information.


Section 19. System Dependencies

19.1: How can I read a single character from the keyboard without
waiting for the RETURN key?

A: Alas, there is no standard or portable way to do this sort of
thing in C.

19.2: How can I find out how many characters are available for
reading, or do a non-blocking read?

A: These, too, are entirely operating-system-specific.

19.3: How can I display a percentage-done indication that updates
itself in place, or show one of those "twirling baton" progress
indicators?

A: The character '\r' is a carriage return, and '\b' is a
backspace.

19.4: How can I clear the screen, or print text in color, or move the
cursor?

A: The only halfway-portable solution is the curses library.

19.5: How do I read the arrow keys?  What about function keys?

A: Such things depend on the keyboard, operating system, and
library you're using.

19.6: How do I read the mouse?

A: What system are you using?

19.7: How can I do serial ("comm") port I/O?

A: It's system-dependent.

19.8: How can I direct output to the printer?

A: See the full list for ideas.

19.9: How do I send escape sequences to control a terminal or other
device?

A: By sending them.  ESC is '\033' in ASCII.

19.10: How can I do graphics?

A: There is no portable way.

19.11: How can I check whether a file exists?

A: You can try the access() or stat() functions.  Otherwise, the
only guaranteed and portable way is to try opening the file.

19.12: How can I find out the size of a file, prior to reading it in?

A: You might be able to get an estimate using stat() or fseek/ftell
(but see the full list for caveats).

19.12b: How can I find the modification date of a file?

A: Try stat().

19.13: How can a file be shortened in-place without completely clearing
or rewriting it?

A: There are various ways to do this, but there is no portable
solution.

19.14: How can I insert or delete a line in the middle of a file?

A: Short of rewriting the file, you probably can't.

19.15: How can I recover the file name given an open file descriptor?

A: This problem is, in general, insoluble.  It is best to remember
the names of files yourself as you open them

19.16: How can I delete a file?

A: The Standard C Library function is remove().

19.16b: How do I copy files?

A: Open the source and destination files and copy a character or
block at a time, or see question 19.27.

19.17: What's wrong with the call fopen("c:\newdir\file.dat", "r")?

A: You probably need to double those backslashes.

19.18: How can I increase the allowable number of simultaneously open
files?

A: Check your system documentation.

19.20: How can I read a directory in a C program?

A: See if you can use the opendir() and readdir() functions.

19.22: How can I find out how much memory is available?

A: Your operating system may provide a routine which returns this
information.

19.23: How can I allocate arrays or structures bigger than 64K?

A: Some operating systems won't let you.

19.24: What does the error message "DGROUP exceeds 64K" mean?

A: It means that you have too much static data.

19.25: How can I access memory located at a certain address?

A: Set a pointer to the absolute address.

19.27: How can I invoke another program from within a C program?

A: Use system().

19.30: How can I invoke another program and trap its output?

A: Unix and some other systems provide a popen() function.

19.31: How can my program discover the complete pathname to the
executable from which it was invoked?

A: argv[0] may contain all or part of the pathname.  You may be
able to duplicate the command language interpreter's search path
logic to locate the executable.

19.32: How can I automatically locate a program's configuration files
in the same directory as the executable?

A: It's hard; see also question 19.31 above.

19.33: How can a process change an environment variable in its caller?

A: If it's possible to do so at all, it's system dependent.

19.36: How can I read in an object file and jump to locations in it?

A: You want a dynamic linker or loader.

19.37: How can I implement a delay, or time a user's response, with sub-
second resolution?

A: Unfortunately, there is no portable way.

19.38: How can I trap or ignore keyboard interrupts like control-C?

A: Use signal().

19.39: How can I handle floating-point exceptions gracefully?

A: Take a look at matherr() and signal(SIGFPE).

19.40: How do I...  Use sockets?  Do networking?  Write client/server
applications?

A: These questions have more to do with the networking facilities
you have available than they do with C.

19.40b: How do I...  Use BIOS calls?  Write ISR's?  Create TSR's?

A: These are very particular to a particular system.

19.40c: I'm trying to compile a program in which "union REGS" and
int86() are undefined.

A: Those have to do with MS-DOS interrupt programming.

19.41: But I can't use all these nonstandard, system-dependent
functions, because my program has to be ANSI compatible!

A: That's an impossible requirement.  Any real program requires at
least a few services which ANSI doesn't define.


Section 20. Miscellaneous

20.1: How can I return multiple values from a function?

A: Either pass pointers to several locations which the function can
fill in, or have the function return a structure containing the
desired values.

20.3: How do I access command-line arguments?

A: Via main()'s argv parameter.

20.5: How can I write data files which can be read on other machines
with different data formats?

A: The most portable solution is to use text files.

20.6: How can I call a function, given its name as a string?

A: The most straightforward thing to do is to maintain a
correspondence table of names and function pointers.

20.8: How can I implement sets or arrays of bits?

A: Use arrays of char or int, with a few macros to access the
desired bit at the proper index.

20.9: How can I determine whether a machine's byte order is big-endian
or little-endian?

A: The usual tricks involve pointers or unions.

20.10: How can I convert integers to binary or hexadecimal?

A: Internally, integers are already in binary.  During I/O, you may
be able to select a base.

20.11: Can I use base-2 constants (something like 0b101010)?
Is there a printf() format for binary?

A: No, on both counts.

20.12: What is the most efficient way to count the number of bits which
are set in an integer?

A: Many "bit-fiddling" problems like this one can be sped up and
streamlined using lookup tables.

20.13: What's the best way of making my program efficient?

A: By picking good algorithms and implementing them carefully.

20.14: Are pointers really faster than arrays?  How much do function
calls slow things down?

A: Precise answers to these and many similar questions depend on
the processor and compiler in use.

20.15b: People claim that optimizing compilers are good, but mine can't
even replace i/=2 with a shift.

A: Was i signed or unsigned?

20.15c: How can I swap two values without using a temporary?

A: The "clever" trick is a ^= b; b ^= a; a ^= b; see also question
3.3b.

20.17: Is there a way to switch on strings?

A: Not directly.

20.18: Is there a way to have non-constant case labels (i.e. ranges or
arbitrary expressions)?

A: No.

20.19: Are the outer parentheses in return statements really optional?

A: Yes.

20.20: Why don't C comments nest?  Are they legal inside quoted
strings?

A: C comments don't nest because PL/I's comments don't either.  The
character sequences /* and */ are not special within double-
quoted strings.

20.20b: What does a+++++b mean ?

A: Nothing.  It's interpreted as "a ++ ++ + b", and cannot be
parsed.

20.24: Why doesn't C have nested functions?

A: They were deliberately left out of C as a simplification.

20.24b: What is assert()?

A: It is a macro which documents an assumption being made by the
programmer; it terminates the program if the assumption is
violated.

20.25: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions
from C?

A: The answer is entirely dependent on the machine and the specific
calling sequences of the various compilers in use.

20.26: Does anyone know of a program for converting Pascal or FORTRAN
to C?

A: Several freely distributable programs are available, namely
ptoc, p2c, and f2c.  See the full list for details.

20.27: Can I use a C++ compiler to compile C code?

A: Not necessarily; C++ is not a strict superset of C.

20.28: I need to compare two strings for close, but not necessarily
exact, equality.

A: See the full list for ideas.

20.29: What is hashing?

A: A mapping of strings (or other data structures) to integers, for
easier searching.

20.31: How can I find the day of the week given the date?

A: Use mktime(), Zeller's congruence, or some code in the full
list.

20.32: Will 2000 be a leap year?

A: Yes.

20.34: How do you write a program which produces its own source code as
output?

A: Here's one:

char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}";
main(){printf(s,34,s,34);}

20.35: What is "Duff's Device"?

A: It's a devastatingly deviously unrolled byte-copying loop.  See
the full list for details.

20.36: What was the entry keyword mentioned in K&R1?

A: It was reserved to allow functions with multiple, differently-
named entry points, but it has been withdrawn.

20.37: Where does the name "C" come from, anyway?

A: C was derived from B, which was inspired by BCPL, which was a
simplification of CPL.

20.38: How do you pronounce "char"?

A: Like the English words "char," "care," or "car" (your choice).

20.39: What do "lvalue" and "rvalue" mean?

A: An "lvalue" denotes an object that has a location; an "rvalue"
is any expression that has a value.

C Interview Questions - Part 2 Interview question

2.4b: Is there a good way of simulating OOP-style inheritance, or
other OOP features, in C?

A: It's straightforward to implement simple "methods" by placing
function pointers in structures. You can make various clumsy,
brute-force attempts at inheritance using the preprocessor or by
having structures contain "base types" as initial subsets, but
it won't be perfect. There's obviously no operator overloading,
and overriding (i.e. of "methods" in "derived classes") would
have to be done by hand.

Obviously, if you need "real" OOP, you'll want to use a language
that supports it, such as C++.

==========

3.12a: What's the difference between ++i and i++?

A: If your C book doesn't explain, get a better one. Briefly:
++i adds one to the stored value of i and "returns" the new,
incremented value to the surrounding expression; i++ adds one
to i but returns the prior, unincremented value.

==========

4.15: How do I convert an int to a char *? I tried a cast, but it's
not working.

A: It depends on what you're trying to do. If you tried a cast
but it's not working, you're probably trying to convert an
integer to a string, in which case see question 13.1. If you're
trying to convert an integer to a character, see question 8.6.
If you're trying to set a pointer to point to a particular
memory address, see question 19.25.

==========

7.7c: In a call to malloc(), what does an error like "Cannot convert
`void *' to `int *'" mean?

A: It means you're using a C++ compiler instead of a C compiler.
See question 7.7.

==========

7.11: How can I dynamically allocate arrays?

A: See questions 6.14 and 6.16.

==========

11.8b: If you can't modify string literals, why aren't they defined as
being arrays of const characters?

A: One reason is that so very much code contains lines like

char *p = "Hello, world!";

which are not necessarily incorrect. These lines would suffer
the diagnostic messages, but it's really any later attempt to
modify what p points to which would be problems.

See also question 1.32.

==========

11.14b: So what could go wrong? Are there really any systems where
void main() doesn't work?

A: It has been reported that programs using void main() and
compiled using BC++ 4.5 can crash. Some compilers (including
DEC C V4.1 and gcc with certain warnings enabled) will complain
about void main().

==========

11.33b: What does it really mean for a program to be "legal" or "valid"
or "conforming"?

A: Simply stated, the Standard talks about three kinds of
conformance: conforming programs, strictly conforming programs,
and conforming implementations.

A "conforming program" is one that is accepted by a conforming
implementation.

A "strictly conforming program" is one that uses the language
exactly as specified in the Standard, and that does not depend
on any implementation-defined, unspecified, or undefined
behavior.

A "conforming implementation" is one that does everything the
Standard says it's supposed to.

References: ISO Sec. ; Rationale Sec. 1.7.

==========

12.1b: I have a simple little program that reads characters until EOF,
but how do I actually *enter* that "EOF" value from the
keyboard?

A: It turns out that the value of EOF as seen within your C program
has essentially nothing to do with the keystroke combination you
might use to signal end-of-file from the keyboard. Depending on
your operating system, you indicate end-of-file from the
keyboard using various keystroke combinations, usually either
control-D or control-Z.

==========

12.12b: Why *does* the call

char s[30];
scanf("%s", s);

work (without the &)?

A: You always need a *pointer*; you don't necessarily need an
explicit &. When you pass an array to scanf(), you do not need
the &, because arrays are always passed to functions as
pointers, whether you use & or not. See questions 6.3 and 6.4.

==========

12.26b: If fflush() won't work, what can I use to flush input?

A: It depends on what you're trying to do. If you're trying to get
rid of an unread newline or other unexpected input after calling
scanf() (see questions 12.18a-12.19), you really need to rewrite
or replace the call to scanf() (see question 12.20).
Alternatively, you can consume the rest of a partially-read line
with a simple code fragment like

while((c = getchar()) != '\n' && c != EOF)
/* discard */ ;

(You may also be able to use the curses flushinp() function.)

==========

12.27: fopen() is failing for certain pathnames.

A: See questions 19.17 and 19.17b.

==========

13.29: My compiler is complaining that printf is undefined!
How can this be?

A: Allegedly, there are C compilers for Microsoft Windows which do
not support printf(). It may be possible to convince such a
compiler that what you are writing is a "console application"
meaning that it will open a "console window" in which printf()
is supported.

==========

17.4b: I've seen function declarations that look like this:

extern int func __((int, int));

What are those extra parentheses and underscores for?

A: They're part of a trick which allows the prototype part of the
function declaration to be turned off for a pre-ANSI compiler.
Somewhere else is a conditional definition of the __ macro like
this:

#ifdef __STDC__
#define __(proto) proto
#else
#define __(proto) ()
#endif

The extra parentheses in the invocation

extern int func __((int, int));

are required so that the entire prototype list (perhaps
containing many commas) is treated as the single argument
expected by the macro.

==========

18.9b: Where can I find some good code examples to study and learn
from?

A: Here are a couple of links to explore:

ftp://garbo.uwasa.fi/pc/c-lang/00index.txt

http://www.eskimo.com/~scs/src/

(Beware, though, that there is all too much truly bletcherous
code out there, too. Don't "learn" from bad code that it's the
best anyone can do; you can do better.) See also questions
18.9, 18.13, 18.15c, and 18.16.

==========

19.9b: How can I access an I/O board directly?

A: In general, there are two ways to do this: use system-specific
functions such as "inport" and "outport" (if the device is
accessed via an "I/O port"), or use contrived pointer variables
to access "memory-mapped I/O" device locations. See question
19.25.

==========

19.10b: How can I display GIF and JPEG images?

A: It will depend on your display environment, which may already
provide these functions. Reference JPEG software is at
http://www.ijg.org/files/ .

==========

19.17b: fopen() isn't letting me open files like "$HOME/.profile" and
"~/.myrcfile".

A: Under Unix, at least, environment variables like $HOME, along
with the home-directory notation involving the ~ character, are
expanded by the shell, and there's no mechanism to perform these
expansions automatically when you call fopen().

==========

19.17c: How can I suppress the dreaded MS-DOS "Abort, Retry, Ignore?"
message?

A: Among other things, you need to intercept the DOS Critical Error
Interrupt, interrupt 24H. See the comp.os.msdos.programmer FAQ
list for more details.

==========

19.40d: What are "near" and "far" pointers?

A: These days, they're pretty much obsolete; they're definitely
system-specific. If you really need to know, see a DOS- or
Windows-specific programming reference.

==========

20.9b: How do I swap bytes?

A: V7 Unix had a swab() function, but it seems to have been
forgotten.

A problem with explicit byte-swapping code is that you have
to decide whether to call it or not; see question 20.9 above.
A better solution is to use functions (such as the BSD
networking ntohs() et al.) which convert between the known byte
order of the data and the (unknown) byte order of the machine,
and to arrange for these functions to be no-ops on those
machines which already match the desired byte order.

If you do have to write your own byte-swapping code, the two
obvious approaches are again to use pointers or unions, as in
question 20.9.

References: PCS Sec. 11 p. 179.

====================


Next, here are the significant changes.

==========

[Q1.1 How should I decide which integer type to use?]

If for some reason you need to declare something with an
*exact* size... be sure to encapsulate the choice behind
an appropriate typedef.
---
an appropriate typedef, such as those in C99's inttypes.h.

If you need to manipulate huge values, larger than the
guaranteed range of C's built-in types, see question 18.15d.

==========

[Q1.4 What should the 64-bit type be on a machine that can support it?]

A: The forthcoming revision to the C Standard (C9X) specifies type
long long as effectively being at least 64 bits, and this type
has been implemented by a number of compilers for some time.
(Others have implemented extensions such as __longlong.)
On the other hand, there's no theoretical reason why a compiler
couldn't implement type short int as 16, int as 32, and long int
as 64 bits, and some compilers do indeed choose this
arrangement.
---
A: The new C99 Standard specifies type long long as effectively
being at least 64 bits, and this type has been implemented by a
number of compilers for some time. (Others have implemented
extensions such as __longlong.) On the other hand, it's also
appropriate to implement type short int as 16, int as 32, and
long int as 64 bits, and some compilers do.

==========

[Q1.7 What's the best way to declare and define global variables...]

A: First, though there can be many "declarations" (and in many
translation units) of a single "global" (strictly speaking,
"external") variable or function, there must be exactly one
"definition". (The definition is the declaration that actually
allocates space, and provides an initialization value, if any.)
---
translation units) of a single global variable or function,
there must be exactly one "definition", where the definition is
the declaration that actually allocates space, and provides an
initialization value, if any.

==========

(Unix compilers and linkers typically use a "common model" which
allows multiple definitions, as long as at most one is
initialized; this behavior is mentioned as a "common extension"
by the ANSI Standard, no pun intended. A few very odd systems
may require an explicit initializer to distinguish a definition
from an external declaration.)
---
by the ANSI Standard, no pun intended.)

==========

[Q1.25 My compiler is complaining about an invalid redeclaration...]

A: Functions which are called without a declaration in scope
(perhaps because the first call precedes the function's
definition) are assumed to be declared as returning int (and
without any argument type information), leading to discrepancies
if the function is later declared or defined otherwise. Non-int
functions must be declared before they are called.
---
A: Functions which are called without a declaration in scope,
perhaps because the first call precedes the function's
definition, are assumed to be declared as returning int (and
without any argument type information), leading to discrepancies
if the function is later declared or defined otherwise. All
functions should be (and non-int functions must be) declared
before they are called.

==========

[Q1.30 What am I allowed to assume about the initial values...]

These rules do apply to arrays and structures (termed
"aggregates"); arrays and structures are considered "variables"
as far as initialization is concerned.

==========

[Q1.31 This code, straight out of a book, isn't compiling]

A: Perhaps you have a pre-ANSI compiler, which doesn't allow
initialization of "automatic aggregates" (i.e. non-static
local arrays, structures, and unions). (As a workaround, and
depending on how the variable a is used, you may be able to make
it global or static, or replace it with a pointer, or initialize
it by hand with strcpy() when f() is called.)
---
A: Perhaps you have an old, pre-ANSI compiler, which doesn't allow
initialization of "automatic aggregates" (i.e. non-static local
arrays, structures, or unions).

==========

[Q1.34 I finally figured out the syntax for... pointers to functions...]

An explicit declaration for the function is normally needed,
since implicit external function declaration does not happen in
this case (because the function name in the initialization is
not part of a function call).
---
A prior, explicit declaration for the function (perhaps in a
header file) is normally needed. The implicit external function
declaration that can occur when a function is called does not
help when a function name's only use is for its value.

==========

[Q2.4 How can I implement opaque (abstract) data types in C?]

A: One good way is for clients to use structure pointers (perhaps
additionally hidden behind typedefs) which point to structure
types which are not publicly defined. It's legal to declare
and use "anonymous" structure pointers (that is, pointers to
structures of incomplete type), as long as no attempt is made to
access the members -- which of course is exactly the point of an
opaque type.

==========

[Q2.6 I came across some code that declared a structure like this...]

these "chummy" structures must be used with care, since the
programmer knows more about their size than the compiler does.
(In particular, they can generally only be manipulated via
pointers.)

C9X will introduce the concept of a "flexible array member",
which will allow the size of an array to be omitted if it is
the last member in a structure, thus providing a well-defined
solution.
---
C99 introduces the concept of a "flexible array member", which
allows the size of an array to be omitted if it is the last
member in a structure, thus providing a well-defined solution.

==========

[Q2.10 How can I pass constant... structure arguments?]

A: As of this writing, C has no way of generating anonymous
structure values. You will have to use a temporary structure
variable or a little structure-building function.
---
A: Traditional C had no way of generating anonymous structure
values; you had to use a temporary structure variable or a
little structure-building function.

The C9X Standard will introduce "compound literals"; one form of
compound literal will allow structure constants. For example,
to pass a constant coordinate pair to a plotpoint() function
which expects a struct point, you will be able to call
---
C99 introduces "compound literals", one form of which provides
for structure constants. For example, to pass a constant
coordinate pair to a hypothetical plotpoint() function which
expects a struct point, you can call

Combined with "designated initializers" (another C9X feature),
it will also be possible to specify member values by name:
---
Combined with "designated initializers" (another C99 feature),
it is also possible to specify member values by name:

==========

2.12: My compiler is leaving holes in structures, which is wasting
space and preventing "binary" I/O to external data files. Can I
turn off the padding, or otherwise control the alignment of
---
space and preventing "binary" I/O to external data files. Why?
Can I turn this off, or otherwise control the alignment of

A: Your compiler may provide an extension to give you this control
(perhaps a #pragma; see question 11.20), but there is no
standard method.
---
A: Those "holes" provide "padding", which may be needed in order to
preserve the "alignment" of later fields of the structure. For
efficient access, most processors prefer (or require) that
multibyte objects (e.g. structure members of any type larger
than char) not sit at arbitrary memory addresses, but rather at
addresses which are multiples of 2 or 4 or the object size.

Your compiler may provide an extension to give you explicit
control over struct alignment (perhaps involving a #pragma; see
question 11.20), but there is no standard method.

C Interview Questions - Part 3 Interview question

[Q2.14 How can I determine the byte offset of a field within a structure?]

A: ANSI C defines the offsetof() macro, which should be used if
available; see stddef.h. If you don't have it, one possible
implementation is
---
A: ANSI C defines the offsetof() macro in stddef.h, which lets
you compute the offset of field f in struct s as
offsetof(struct s, f). If for some reason you have to code this
sort of thing yourself, one possibility is

==========

[2.15 How can I access structure fields by name at run time?]

A: Build a table of names and offsets, using the offsetof() macro.
The offset of field b in struct a is

offsetb = offsetof(struct a, b)

If structp is a pointer to an instance of this structure, and
field b is an int (with offset as computed above), b's value can
be set indirectly with
---
A: Keep track of the field offsets as computed using the offsetof()
macro (see question 2.14). If structp is a pointer to an
instance of the structure, and field f is an int having offset
offsetf, f's value can be set indirectly with

==========

[Q2.20 Can I initialize unions?]

A: The current C Standard allows an initializer for the first-named
member of a union. C9X will introduce "designated initializers"
which can be used to initialize any member.
---
A: In the original ANSI C, an initializer was allowed only for the
first-named member of a union. C99 introduces "designated
initializers" which can be used to initialize any member.

==========

[Q3.2 Under my compiler... Regardless of the order of evaluation...]

example, the compiler chose to multiply the previous value by
itself and to perform both increments afterwards.
---
itself and to perform both increments later.

[Q3.8 How can I understand these complex expressions?]

The second sentence can be difficult to understand. It says
that if an object is written to within a full expression, any
and all accesses to it within the same expression must be for
the purposes of computing the value to be written. This rule
effectively constrains legal expressions to those in which the
accesses demonstrably precede the modification.
---
and all accesses to it within the same expression must be
directly involved in the computation of the value to be written.
This rule effectively constrains legal expressions to those in
which the accesses demonstrably precede the modification. For
example, i = i + 1 is legal, but not a[i] = i++ (see question
3.1).

==========

[3.9 So... we don't know which..., but i does get incremented..., right?]

A: *No*. Once an expression or program becomes undefined, *all*
---
A: Not necessarily! Once an expression or program becomes
undefined, *all* aspects of it become undefined.

==========

[Q4.2 I'm trying to declare a pointer and allocate some space for it...]

A: The pointer you declared is p, not *p. To make a pointer point
somewhere, you just use the name of the pointer:
---
A: The pointer you declared is p, not *p. When you're manipulating
the pointer itself (for example when you're setting it to make
it point somewhere), you just use the name of the pointer:

==========

[4.3 Does *p++ increment p, or what it points to?]

A: Postfix ++ essentially has higher precedence than the prefix
---
A: The postfix ++ and -- operators essentially have higher
precedence than the prefix unary operators.

==========

[Q4.10 ...How can I pass a constant by reference?]

A: You can't do this directly. You will have to declare
---
A: In C99, you can use a "compound literal":

f((int[]){5});

Prior to C99, you couldn't do this directly; you had to declare
a temporary variable, and then pass its address to the function:

==========

[Q5.6 If NULL were defined as follows...]

At any rate, ANSI function prototypes ensure that most (though
not quite all; see question 5.2) pointer arguments are converted
correctly when passed as function arguments, so the question is
largely moot.

==========

5.20: What does a run-time "null pointer assignment" error mean?
How can I track it down?

A debugger may let you set a data watchpoint on location 0.
Alternatively, you could write a bit of code to stash away a
copy of 20 or so bytes from location 0, and periodically check
that the memory at location 0 hasn't changed. See also question
16.8.

==========

[Q6.15 How can I declare local arrays of a size matching a passed-in array?]

A: Until recently, you couldn't. Array dimensions in C
traditionally had to be compile-time constants. C9X will
introduce variable-length arrays (VLA's) which will solve this
---
A: Until recently, you couldn't; array dimensions in C
traditionally had to be compile-time constants. However, C99
introduces variable-length arrays (VLA's) which solve this
problem; local arrays may have sizes set by variables or other
expressions, perhaps involving function parameters.

==========

[Q6.16 How can I dynamically allocate a multidimensional array?]

You can also use sizeof(*array1) and sizeof(**array1)
instead of sizeof(int *) and sizeof(int).)

==========

[Q6.19 How do I write functions which accept two-dimensional arrays...]

C9X will allow variable-length arrays, and once compilers which
accept C9X's extensions become widespread, this will probably
---
C99 allows variable-length arrays, and once compilers which
accept C99's extensions become widespread, VLA's will probably
become the preferred solution.

==========

[Q7.27 So can I query the malloc package to find out how big...]

(Some compilers provide nonstandard extensions.)

==========

[Q7.32 What is alloca() and why is its use discouraged?]

Now that C99 supports variable-length arrays (VLA's),
they can be used to more cleanly accomplish most of the
tasks which alloca() used to be put to.

==========

[Q8.6 How can I get the numeric (character set) value corresponding to...]

To convert back and forth between the digit characters and the
corresponding integers in the range 0-9, add or subtract the
constant '0' (that is, the character value '0').

==========

[Q10.7 Is it acceptable for one header file to #include another?]

the prestigious Indian Hill Style Guide (see question 17.9)
disparages them; they can make it harder to find relevant
definitions; they can lead to multiple-definition errors if a file
is #included twice;
---
is #included twice; they can lead to increased compilation time;
and they make manual Makefile maintenance very difficult.

==========

10.11: I seem to be missing the system header file sgtty.h.
Can someone send me a copy?
---
10.11: I'm compiling a program, and I seem to be missing one of the
header files it requires. Can someone send me a copy?

A: There are several situations, depending on what sort of header
file it is that's "missing".

If the missing header file is a standard one, there's a problem
with your compiler. You'll need to contact your vendor, or
someone knowledgeable about your particular compiler, for help.

The situation is more complicated in the case of nonstandard
headers. Some are completely system- or compiler-specific.
Some are completely unnecessary, and should be replaced by their
Standard equivalents. (For example, instead of malloc.h, use
stdlib.h.) Other headers, such as those associated with
popular add-on libraries, may be reasonably portable.

Standard headers exist in part so that definitions appropriate
to your compiler, operating system, and processor can be
supplied. You cannot just pick up a copy of someone else's
header file and expect it to work, unless that person is using
exactly the same environment. Ask your compiler vendor why the
file was not provided (or to send a replacement copy).
---
exactly the same environment. You may actually have a
portability problem (see section 19), or a compiler problem.
Otherwise, see question 18.16.

==========

[Q10.26 How can I write a macro which takes a variable number of arguments?]

C9X will introduce formal support for function-like macros with
variable-length argument lists. The notation ... will appear at
---
C99 introduces formal support for function-like macros with
variable-length argument lists. The notation ... can appear at

definition will be replaced by the variable arguments during
---
definition is replaced by the variable arguments during

==========

[Q11.1 What is the "ANSI C Standard?"]

More recently, the Standard has been adopted as an international
standard, ISO/IEC 9899:1990, and this ISO Standard replaces the
earlier X3.159 even within the United States (where it is known
---
A year or so later, the Standard was adopted as an international
standard, ISO/IEC 9899:1990, and this ISO Standard replaced the
earlier X3.159 even within the United States (where it was known

As of this writing, a complete revision of the Standard is in
its final stages. The new Standard is nicknamed "C9X" on the
assumption that it will be finished by the end of 1999. (Many
of this article's answers have been updated to reflect new C9X
features.)
---
Most recently, a major revision of the Standard, "C99", has been
completed and adopted.

The original ANSI Standard included a "Rationale," explaining
many of its decisions, and discussing a number of subtle points,
including several of those covered here. (The Rationale was
"not part of ANSI Standard X3.159-1989, but... included for
information only," and is not included with the ISO Standard.
A new one is being prepared for C9X.)
---
Several versions of the Standard, including C99 and the original
ANSI Standard, have included a "Rationale," explaining many of
its decisions, and discussing a number of subtle points,
including several of those covered here.

==========

[Q11.2 How can I get a copy of the Standard?]

Note that ANSI derives revenues to support its operations
from the sale of printed standards, so electronic copies
are *not* available.
---
An electronic (PDF) copy is available on-line, for US$18,
from www.ansi.org.

The last time I checked, the cost was $130.00 from ANSI or
$400.50 from Global. Copies of the original X3.159 (including
the Rationale) may still be available at $205.00 from ANSI or
$162.50 from Global.

In the U.S., it may be possible to get a copy of the original
ANSI X3.159 (including the Rationale) as "FIPS PUB 160" from

National Technical Information Service (NTIS)
U.S. Department of Commerce
Springfield, VA 22161
703 487 4650

==========

[Q11.10 Why can't I pass a char ** to a function which expects...]

You must use explicit casts (e.g. (const char **) in this case)
when assigning (or passing) pointers which have qualifier
mismatches at other than the first level of indirection.
---
If you must assign or pass pointers which have qualifier
mismatches at other than the first level of indirection, you
must use explicit casts (e.g. (const char **) in this case),
although as always, the need for such a cast may indicate a
deeper problem which the cast doesn't really fix.

==========

11.27: Why does the ANSI Standard not guarantee more than six case-
insensitive characters of external identifier significance?
---
11.27: Why does the ANSI Standard place limits on the length and case-
significance of external identifiers?

The limitation is only that identifiers be *significant*
in the first six characters, not that they be restricted to
six characters in length. This limitation is marked in the
Standard as "obsolescent", and will be removed in C9X.
---
in some initial sequence of characters, not that they be
restricted to that many characters in total length.
(The limitation was to six characters in the original
ANSI Standard, but has been relaxed to 31 in C99.)

==========

[Q11.33 ...implementation-defined, unspecified, and undefined behavior.]

(A fourth defined class of not-quite-precisely-defined behavior,
without the same stigma attached to it, is "locale-specific".)

==========

[Q12.2 Why does the code... copy the last line twice?]

Usually, you should just check the return value of
the input routine (in this case, fgets() will return NULL on end-
of-file); often, you don't need to use feof() at all.
---
the input routine -- fgets(), for example, returns NULL on end-
of-file. In virtually all cases, there's no need to use feof()
at all.

==========

[Q12.21 How can I tell how much... buffer space... for a... sprintf call?]

The "obvious" solution to the overflow problem is a length-
limited version of sprintf(), namely snprintf(). It would be
used like this:
---
To avoid the overflow problem, you can use a length-limited
version of sprintf(), namely snprintf(). It is used like this:

It will be standardized in C9X.
---
It has finally been standardized in C99.

When the C9X snprintf() arrives, it will also be possible to use
it to predict the size required for an arbitrary sprintf() call.
C9X snprintf() will return the number of characters it would
have placed in the buffer, not just how many it did place.
Furthermore, it may be called with a buffer size of 0 and a
null pointer as the destination buffer. Therefore, the call
---
As an extra, added bonus, the C99 snprintf() provides a way
to predict the size required for an arbitrary sprintf() call.
C99's snprintf() returns the number of characters it would have
placed in the buffer, and it may be called with a buffer size
of 0. Therefore, the call

will compute the number of characters required for the fully-
---
predicts the number of characters required for the fully-

Yet another option is the (nonstandard) asprintf() function,
present in various C libraries including bsd's and GNU's, which
formats to (and returns a pointer to) a malloc'ed buffer, like
this:

char *buf;
asprintf(&buf, "%d = %s", 42, "forty-two");
/* now buf points to malloc'ed space containing formatted string */

==========

[Q12.23 Why does everyone say not to use gets()?]

A: Unlike fgets(), gets() cannot be told the size of the buffer
it's to read into, so it cannot be prevented from overflowing
that buffer. As a general rule, always use fgets().
---
that buffer. The Standard fgets() function is a vast
improvement over gets(), although it's not perfect, either.
(If long lines are a real possibility, their proper handling
must be carefully considered.)

==========

12.26: How can I flush pending input so that a user's typeahead isn't
---
12.26b: If fflush() won't work, what can I use to flush input?

There is no standard way to discard unread characters from a
stdio input stream, nor would such a way necessarily be
sufficient, since unread characters can also accumulate in
other, OS-level input buffers. You may be able to read and
discard characters until \n, or use the curses flushinp()
function, or use some system-specific technique. See also
questions 19.1 and 19.2.
---
other, OS-level input buffers. If you're trying to actively
discard typed-ahead input (perhaps in anticipation of issuing a
critical prompt), you'll have to use a system-specific
technique; see questions 19.1 and 19.2.

==========

[Q12.30 I'm trying to update a file in place...]

writing in the read/write "+" modes. Also, remember that you
can only overwrite characters with the same number of
replacement characters, and that overwriting in text mode may
truncate the file at that point. See also question 19.14.
---
truncate the file at that point, and that you may have to
preserve line lengths. See also question 19.14.

==========

[Q12.34 Once I've used freopen(), how can I get the original stdout... back?]

It is barely possible to save away information about a stream
before calling freopen(), such that the original stream can
later be restored, but the methods involve system-specific calls
such as dup(), or copying or inspecting the contents of a FILE
structure, which is exceedingly nonportable and unreliable.
---
It may be possible, in a nonportable way, to save away
information about a stream before calling freopen(), such that
the original stream can later be restored. One way is to use a
system-specific call such as dup() or dup2(), if available.
Another is to copy or inspect the contents of the FILE
structure, but this is exceedingly nonportable and unreliable.

==========

12.36b: How can I arrange to have output go two places at once,
e.g. to the screen and to a file?

Here is a simple example:

#include stdio.h
#include stdarg.h

void f2printf(FILE *fp1, FILE *fp2, char *fmt, ...)
{
va_list argp;
va_start(argp, fmt); vfprintf(fp1, fmt, argp); va_end(argp);
va_start(argp, fmt); vfprintf(fp2, fmt, argp); va_end(argp);
}

where f2printf() is just like fprintf() except that you give it
two file pointers and it prints to both of them.

==========

[Q13.2 Why does strncpy() not always place a '\0' terminator...]

You can get around the problem by using strncat() instead
of strncpy(): if the destination string starts out empty
(that is, if you do *dest = '\0' first), strncat()
does what you probably wanted strncpy() to do.

==========

[Q13.12 How can I get the current date or time of day in a C program?]

printf("It's %.24s.\n", ctime(&now));
---
printf("It's %s", ctime(&now));

If you need control over the format, use strftime().
If you need sub-second resolution, see question 19.37.

==========

[Q13.14 How can I add N days to a date?]

A: The ANSI/ISO Standard C mktime() and difftime() functions
provide some support for both problems.
---
provide some (limited) support for both problems.

These solutions are only guaranteed to work correctly
---
However, these solutions are guaranteed to work correctly only
for dates in the range which can be represented as time_t's.

(For conservatively-sized time_t, that range is often -- but not
always -- from 1970 to approximately 2037; note however that
there are time_t representations other than as specified by Unix
and Posix.)

Another approach to both problems,
which will work over a much wider range of dates,
is to use "Julian day numbers".

==========

[Q13.15 I need a random number generator.]

If you do find yourself needing to implement your own random
number generator, there is plenty of literature out there; see
the References. There are also any number of packages on the
net: look for r250, RANLIB, and FSULTRA (see question 18.16).
---
the References below or the sci.math.num-analysis FAQ list.
There are also any number of packages on the net: old standbys
are r250, RANLIB, and FSULTRA (see question 18.16), and there is
much recent work by Marsaglia, and Matumoto and Nishimura (the
"Mersenne Twister"), and some code collected by Don Knuth on his
web pages.

==========

[Q13.17 Each time I run my program, I get the same sequence of numbers...]

A: You can call srand() to seed the pseudo-random number generator
with a truly random initial value. Popular seed values are the
time of day, or the elapsed time before the user presses a key
(although keypress times are hard to determine portably;
---
with a truly random (or at least variable) initial value, such
as the time of day. Here is a simple example:

#include stdlib.h
#include time.h

srand((unsigned int)time((time_t *)NULL));

(Unfortunately, this code isn't perfect -- among other things,
the time_t returned by time() might be a floating-point type,
hence not portably convertible to unsigned int without the
possibility of overflow.

==========

[Q14.1 When I set a float... 3.1, why is printf printing it as 3.0999999?]

A: Most computers use base 2 for floating-point numbers as well as
for integers. In base 2, one divided by ten is an infinitely-
repeating fraction (0.0001100110011...), so fractions such as
3.1 (which look like they can be exactly represented in decimal)
---
for integers. Although 0.1 is a nice, polite-looking fraction
in base 10, its base-2 representation is an infinitely-repeating
fraction (0.0001100110011...), so exact decimal fractions such as 3.1
cannot be represented exactly in binary.

[Q14.3 ...I keep getting "undefined: sin" compilation errors.]

A: Make sure you're actually linking with the math library. For
instance, under Unix, you usually need to use the -lm option, at
---
instance, due to a longstanding bug in Unix and Linux systems,
you usually need to use an explicit -lm flag, at the *end* of
the command line, when compiling/linking.

==========

[Q14.5 What's a good way to check for "close enough" floating-point equality?]

use something like

#include math.h
if(fabs(a - b) = epsilon * fabs(a))

for some suitably-chosen degree of closeness epsilon (as long as
a is nonzero!).
---
where epsilon is a value chosen to set the degree of "closeness"
(and where you know that a will not be zero).

==========

[Q14.8 The predefined constant M_PI seems to be missing...]
machine's copy of math.h.

If you need pi, you'll have to define it yourself, or compute it
with 4*atan(1.0).
---
with 4*atan(1.0) or acos(-1.0).

==========

[Q14.9 How do I test for IEEE NaN and other special values?]

C9X will provide isnan(), fpclassify(), and several other
---
C99 provides isnan(), fpclassify(), and several other
classification routines.

==========

[Q14.11 What's a good way to implement complex numbers in C?]

C9X will support complex as a standard type.
---
C99 supports complex as a standard type.

==========

[Q15.6 How can I write a function analogous to scanf()...]

A: C9X will support vscanf(), vfscanf(), and vsscanf().
(Until then, you may be on your own.)
---
A: C99 (but *not* any earlier C Standard) supports vscanf(),
vfscanf(), and vsscanf().

==========

[Q16.1b I'm getting baffling syntax errors which make no sense at all...]

A: Check for unclosed comments or mismatched #if/#ifdef/#ifndef/
#else/#endif directives; remember to check header files, too.
---
A: Check for unclosed comments, mismatched #if/#ifdef/#ifndef/
#else/#endif directives, and perhaps unclosed quotes; remember
to check header files, too.