7/1/08

C Interview Questions and Answers I

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?

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

A: The extra parentheses and explicit * are now>

Transfer interrupted!

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: When will the next Obfuscated C Code Contest be held?

How can I get a copy of previous winning entries?

A: See the full list, or http://www.ioccc.org/index.html .

20.37: 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.

20.38: 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.

20.39: How do you pronounce "char"?

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

20.39b: 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.


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