1. Declarations and Initializations

1.1 How should I decide which integer type to use?

1.2 Why aren't the sizes of the standard types precisely defined?

1.3 Since C doesn't define sizes exactly, I've been using typedefs like int16 and int32 . I can then define these typedefs to be int , short , long , etc. depending on what machine I'm using. That should solve everything, right?

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

1.5 What's wrong with this declaration?

char* p1, p2;

p2

1.6 I'm trying to declare a pointer and allocate some space for it, but it's not working. What's wrong with this code?

char *p; *p = malloc(10);

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

1.8 How can I implement opaque (abstract) data types in C?

1.9 How can I make a sort of ``semi-global'' variable, that is, one that's private to a few functions spread across a few source files?

1.10 Do all declarations for the same static function or variable have to include the storage class static ?

1.11 What does extern mean in a function declaration?

1.12 What's the auto keyword good for?

1.13 What's the difference between using a typedef or a #define for a user-defined type?

1.14 I can't seem to define a linked list successfully. I tried

typedef struct { char *item; NODEPTR next; } *NODEPTR;

1.15 How can I define a pair of mutually referential structures? I tried

typedef struct { int afield; BPTR bpointer; } *APTR; typedef struct { int bfield; APTR apointer; } *BPTR;

BPTR

1.16 What's the difference between these two declarations?

struct x1 { ... }; typedef struct { ... } x2;

1.17 What does

typedef int (*funcptr)();

1.18 I've got the declarations

typedef char *charp; const charp p;

p

const

1.19 I don't understand why I can't use const values in initializers and array dimensions, as in

const int n = 5; int a[n];

1.20 What's the difference between const char *p , char const *p , and char * const p ?

1.20b

What does it mean for a function parameter to be const ? What do the two const 's in

int f(const * const p)

1.21 How do I construct declarations of complicated types such as ``array of N pointers to functions returning pointers to functions returning pointers to char '', or figure out what similarly complicated declarations mean?

1.22 How can I declare a function that can return a pointer to a function of the same type? I'm building a state machine with one function for each state, each of which returns a pointer to the function for the next state. But I can't find a way to declare the functions--I seem to need a function returning a pointer to a function returning a pointer to a function returning a pointer to a function..., ad infinitum.

1.23 Can I declare a local array (or parameter array) of a size matching a passed-in array, or set by another parameter?

1.24 I have an extern array which is defined in one file, and used in another:

file1.c: file2.c: int array[] = {1, 2, 3}; extern int array[];

sizeof

array

file2.c

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

1.25b What's the right declaration for main ?

Is void main() correct?

1.26 My compiler is complaining about mismatched function prototypes which look fine to me.

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

1.28 My compiler isn't letting me declare a big array like

double array[256][256];

1.29 How can I determine which identifiers are safe for me to use and which are reserved?

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

If global variables start out as ``zero'', is that good enough for null pointers and floating-point zeroes?

1.31 This code, straight out of a book, isn't compiling:

int f() { char a[] = "Hello, world!"; }

1.31b What's wrong with this initialization?

char *p = malloc(10);

1.32 What is the difference between these initializations?

char a[] = "string literal"; char *p = "string literal";

p[i]

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

1.34 I finally figured out the syntax for declaring pointers to functions, but now how do I initialize one?

1.35 Can I initialize unions?

2. Structures, Unions, and Enumerations

2.1 What's the difference between these two declarations?

struct x1 { ... }; typedef struct { ... } x2;

2.2 Why doesn't

struct x { ... }; x thestruct;

2.3 Can a structure contain a pointer to itself?

2.4 How can I implement opaque (abstract) data types in C?

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

2.5 Why does the declaration

extern int f(struct x *p);

2.6 I came across some code that declared a structure like this:

struct name { int namelen; char namestr[1]; };

namestr

namelen

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

2.8 Is there a way to compare structures automatically?

2.9 How are structure passing and returning implemented?

2.10 How can I pass constant values to functions which accept structure arguments? How can I create nameless, immediate, constant structure values?

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

2.12 Why is my compiler leaving holes in structures, wasting space and preventing ``binary'' I/O to external data files? Can I turn this off, or otherwise control the alignment of structure fields?

2.13 Why does sizeof report a larger size than I expect for a structure type, as if there were padding at the end?

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

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

2.16 Does C have an equivalent to Pascal's with statement?

2.17 If an array name acts like a pointer to the base of an array, why isn't the same thing true of a structure?

2.18 This program works correctly, but it dumps core after it finishes. Why?

struct list { char *item; struct list *next; } /* Here is the main program. */ main(argc, argv) { ... }

2.19 What's the difference between a structure and a union, anyway?

2.20 Can I initialize unions?

2.21 Is there an automatic way to keep track of which field of a union is in use?

2.22 What's the difference between an enumeration and a set of preprocessor #define s?

2.23 Are enumerations really portable?

Aren't they Pascalish?



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

2.25 I came across some structure declarations with colons and numbers next to certain fields, like this:

struct record { char *name; int refcount : 4; unsigned dirty : 1; };

2.26 Why do people use explicit masks and bit-twiddling code so much, instead of declaring bit-fields?

3. Expressions

3.1 Why doesn't this code:

a[i] = i++;

3.2 Under my compiler, the code

int i = 7; printf("%d

", i++ * i++);

3.3 I've experimented with the code

int i = 3; i = i++;

i

3.3b Here's a slick expression:

a ^= b ^= a ^= b

a

b

3.4 Can I use explicit parentheses to force the order of evaluation I want, and control these side effects? Even if I don't, doesn't precedence dictate it?

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

I see code like `` while((c = getchar()) != EOF && c != '

') '' ...

3.6 Is it safe to assume that the right-hand side of the && and || operators won't be evaluated if the left-hand side determines the outcome?

3.7 Why did

printf("%d %d", f1(), f2());

f2

3.8 How can I understand complex expressions like the ones in this section, and avoid writing undefined ones? What's a ``sequence point''?

3.9 So if I write

a[i] = i++;

a[]

i

3.10a People keep saying that the behavior of i = i++ is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected.

3.10b People told me that if I evaluated an undefined expression, or accessed an uninitialized variable, I'd get a random, garbage value. But I tried it, and got zero. What's up with that?

3.11 How can I avoid these undefined evaluation order difficulties if I don't feel like learning the complicated rules?

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

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

3.13 I need to check whether one number lies between two others. Why doesn't

if(a < b < c)

3.14 Why doesn't the code

int a = 1000, b = 1000; long int c = a * b;

3.14b How can I ensure that integer arithmetic doesn't overflow?

3.15 Why does the code

double degC, degF; degC = 5 / 9 * (degF - 32);

3.16 I have a complicated expression which I have to assign to one of two variables, depending on a condition. Can I use code like this?

((condition) ? a : b) = complicated_expression;

3.17 I have some code containing expressions like

a ? b = c : d

3.18 What does the warning ``semantics of ` > ' change in ANSI C'' mean?

3.19 What's the difference between the ``unsigned preserving'' and ``value preserving'' rules?

4. Pointers

4.1 What are pointers really good for, anyway?

4.2 I'm trying to declare a pointer and allocate some space for it, but it's not working. What's wrong with this code?

char *p; *p = malloc(10);

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

4.4 I'm trying to use pointers to manipulate an array of int s. What's wrong with this code?

int array[5], i, *ip; for(i = 0; i < 5; i++) array[i] = i; ip = array; printf("%d

", *(ip + 3 * sizeof(int)));

4.5 I have a char * pointer that happens to point to some int s, and I want to step it over them. Why doesn't

((int *)p)++;

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

4.7 I've got some code that's trying to unpack external structures, but it's crashing with a message about an ``unaligned access.'' What does this mean?

4.8 I have a function which accepts, and is supposed to initialize, a pointer:

void f(int *ip) { static int dummy = 5; ip = &dummy; }

int *ip; f(ip);

4.9 Suppose I want to write a function that takes a generic pointer as an argument and I want to simulate passing it by reference. Can I give the formal parameter type void ** , and do something like this?

void f(void **); double *dp; f((void **)&dp);

4.10 I have a function

extern int f(int *);

int

f(&5);

4.11 Does C even have ``pass by reference''?

4.12 I've seen different syntax used for calling functions via pointers. What's the story?

4.13 What's the total generic pointer type? My compiler complained when I tried to stuff function pointers into a void * .

4.14 How are integers converted to and from pointers? Can I temporarily stuff an integer into a pointer, or vice versa?

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

4.16 What's wrong with this declaration?

char* p1, p2;

p2

4.17 What are ``near'' and ``far'' pointers?

5. Null Pointers

5.1 What is this infamous null pointer, anyway?

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

5.3 Is the abbreviated pointer comparison `` if(p) '' to test for non-null pointers valid? What if the internal representation for null pointers is nonzero?

5.4 What is NULL and how is it defined?

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

5.6 If NULL were defined as follows:

#define NULL ((char *)0)

NULL

5.7 My vendor provides header files that #define NULL as 0L . Why?

5.8 Is NULL valid for pointers to functions?

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

5.10 But wouldn't it be better to use NULL (rather than 0 ), in case the value of NULL changes, perhaps on a machine with nonzero internal null pointers?

5.11 I once used a compiler that wouldn't work unless NULL was used.

5.12 I use the preprocessor macro

#define Nullptr(type) (type *)0

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

5.14 Why is there so much confusion surrounding null pointers? Why do these questions come up so often?

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

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

5.17 Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?

5.18 Is a run-time integral value of 0, cast to a pointer, guaranteed to be a null pointer?

5.19 How can I access an interrupt vector located at the machine's location 0? If I set a pointer to 0 , the compiler might translate it to some nonzero internal null pointer value.

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

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?

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

6.3 So what is meant by the ``equivalence of pointers and arrays'' in C?

6.4 If they're so different, then why are array and pointer declarations interchangeable as function formal parameters?

6.4b So arrays are passed by reference, even though the rest of C uses pass by value?

6.5 Why can't I do something like this?

extern char *getpass(); char str[10]; str = getpass("Enter password: ");

6.6 If you can't assign to arrays, then how can

int f(char str[]) { if(str[0] == '\0') str = "none"; ... }

6.6b And what about this? Isn't this an array assignment?

char a[] = "Hello, world!

";

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

6.8 Practically speaking, what is the difference between arrays and pointers?

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

6.10 I'm still mystified. Is a pointer a kind of array, or is an array a kind of pointer?

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

6.12 Since array references decay into pointers, if arr is an array, what's the difference between arr and &arr ?

6.13 How do I declare a pointer to an array?

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

How can I avoid fixed-sized arrays?

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

6.16 How can I dynamically allocate a multidimensional array?

6.17 Here's a neat trick: if I write

int realarray[10]; int *array = &realarray[-1];

array

6.18 My compiler complained when I passed a two-dimensional array to a function expecting 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?

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

6.21 Why doesn't sizeof properly report the size of an array when the array is a parameter to a function? I have a test routine

f(char a[10]) { int i = sizeof(a); printf("%d

", i); }

6.22 How can code in a file where an array is declared as extern (i.e. it is defined, and its size determined, in some other file) determine the size of the array? sizeof doesn't seem to work.

6.23 I want to know how many elements are in an array, but sizeof yields the size in bytes.

6.24 Is there a way to have an array of bits?

7. Memory Allocation

7.1 Why doesn't this fragment work?

char *answer; printf("Type something:

"); gets(answer); printf("You typed \"%s\"

", answer);

7.2 I can't get strcat to work. I tried

char *s1 = "Hello, "; char *s2 = "world!"; char *s3 = strcat(s1, s2);

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?

7.3b I just tried the code

char *p; strcpy(p, "abc");

7.3c How much memory does a pointer variable allocate?

7.4 I'm reading lines from a file into an array, with this code:

char linebuf[80]; char *lines[100]; int i; for(i = 0; i < 100; i++) { char *p = fgets(linebuf, 80, fp); if(p == NULL) break; lines[i] = p; }

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.

7.5b So what's the right way to return a string or other aggregate?

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

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

7.7b What's wrong with casting malloc 's return value?

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

7.8 I see code like

char *p = malloc(strlen(s) + 1); strcpy(p, s);

malloc((strlen(s) + 1) * sizeof(char))

7.9 I wrote a little wrapper around malloc , but it doesn't work:

#include <stdio.h> #include <stdlib.h> mymalloc(void *retp, size_t size) { retp = malloc(size); if(retp == NULL) { fprintf(stderr, "out of memory

"); exit(EXIT_FAILURE); } }

7.10 I'm trying to declare a pointer and allocate some space for it, but it's not working. What's wrong with this code?

char *p; *p = malloc(10);

7.10a What's wrong with this initialization?

char *p = malloc(10);

7.10b How can I shut off the ``warning: possible pointer alignment problem'' message which lint gives me for each call to malloc ?

7.11 How can I dynamically allocate arrays?

7.12 How can I find out how much memory is available?

7.13 What should malloc(0) do? Return a null pointer or a pointer to 0 bytes?

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?

7.15 malloc is returning crazy pointer values, but I did read question 7.6 and I have included the line

extern void *malloc();

7.16 I'm allocating a large array for some numeric work, using the line

double *array = malloc(300 * 300 * sizeof(double));

malloc

malloc

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

7.18 My application depends heavily on dynamic allocation of nodes for data structures, and malloc / free overhead is becoming a bottleneck. What can I do?

7.19 My program is crashing, apparently somewhere down inside malloc , but I can't see anything wrong with it. Is there a bug in malloc ?

7.19b

I'm dynamically allocating an array, like this:

int *iarray = (int *)malloc(nints);

malloc

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

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

How unsafe is it to use (assign, compare) a pointer value after it's been freed?

7.22 When I call malloc to allocate memory for a pointer which is local to a function, do I have to explicitly free it?

7.23 I'm allocating structures which contain pointers to other dynamically-allocated objects. When I free a structure, do I also have to free each subsidiary pointer?

7.24 Must I free allocated memory before the program exits?

7.25 I have a program which malloc s and later free s a lot of memory, but I can see from the operating system that memory usage doesn't actually go back down.

7.26 How does free know how many bytes to free?

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

7.28 Why doesn't sizeof tell me the size of the block of memory pointed to by a pointer?

7.29 Having dynamically allocated an array (as in question 6.14), can I change its size?

7.30 Is it legal to pass a null pointer as the first argument to realloc ? Why would you want to?

7.31 What's the difference between calloc and malloc ? Which should I use? Is it safe to take advantage of calloc 's zero-filling? Does free work on memory allocated with calloc , or do you need a cfree ?

7.32 What is alloca and why is its use discouraged?

8. Characters and Strings

8.1 Why doesn't

strcat(string, '!');

8.2 I'm checking a string to see if it matches a particular value. Why isn't this code working?

char *string; ... if(string == "value") { /* string matches "value" */ ... }

8.3 If I can say

char a[] = "Hello, world!";

char a[14]; a = "Hello, world!";

8.4 I can't get strcat to work. I tried

char *s1 = "Hello, "; char *s2 = "world!"; char *s3 = strcat(s1, s2);

8.5 What is the difference between these initializations?

char a[] = "string literal"; char *p = "string literal";

p[i]

8.6 How can I get the numeric value (i.e. ASCII or other character set code) corresponding to a character, or vice versa?

8.7 Does C have anything like the ``substr'' (extract substring) routine present in other languages?

8.8 I'm reading strings typed by the user into an array, and then printing them out later. When the user types a sequence like

, why isn't it being handled properly?

8.9 I think something's wrong with my compiler: I just noticed that sizeof('a') is 2, not 1 (i.e. not sizeof(char) ).

8.10 I'm starting to think about multinational character sets, and I'm worried about the implications of making sizeof(char) be 2 so that 16-bit character sets can be represented.

9. Boolean Expressions and Variables

9.1 What is the right type to use for Boolean values in C? Is there a standard type? Should I use #define s or enums for the true and false values?

9.2 Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered ``true'' in C? What if a built-in logical or relational operator ``returns'' something other than 1?

9.3 Is if(p) , where p is a pointer, a valid and portable test?

9.4 Should I use symbolic names like TRUE and FALSE for Boolean constants, or plain 1 and 0?

9.5 A third-party header file I just started using is defining its own TRUE and FALSE values incompatibly with the code I've already developed. What can I do?

10. C Preprocessor

10.1 I'm trying to define a few simple little function-like macros such as

#define square(x) x * x

10.2 Here are some cute preprocessor macros:

#define begin { #define end }

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

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

10.5 What's the difference between using a typedef or a #define for a user-defined type?

10.5b What's the difference between

const MAXSIZE = 100;

and

#define MAXSIZE 100

10.6 I'm splitting up a program into multiple source files for the first time, and I'm wondering what to put in .c files and what to put in .h files. (What does `` .h '' mean, anyway?)

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

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

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

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

10.10 I'm using header files which accompany two different third-party libraries, and they are ``helpfully'' defining common macros such as TRUE , FALSE , Min() , and Max() , but the definitions clash with each other and with definitions I'd already established in my own header files. What can I do?

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

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?

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

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

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

#define a b \ #ifdef whatever c d #else e f g #endif

10.15 Is there anything like an #ifdef for typedef s?

10.16 How can I use a preprocessor #if expression to tell whether a machine's byte order is big-endian or little-endian?

10.17 I'm getting strange syntax errors inside lines I've #ifdef fed out.

10.18 I inherited some code which contains far too many #ifdef 's for my taste. How can I preprocess the code to leave only one conditional compilation set, without running it through the preprocessor and expanding all of the #include 's and #define 's as well?

10.19 How can I list all of the predefined identifiers?

10.20 I have some old code that tries to construct identifiers with a macro like

#define Paste(a, b) a/**/b

10.21 I have an old macro

#define CTRL(c) ('c' & 037)

10.22 Why is the macro

#define TRACE(n) printf("TRACE: %d

", n)

TRACE(count);

printf("TRACE: %d\count", count);

10.23 How can I use a macro argument inside a string literal in the macro expansion?

10.24 I'm trying to use the ANSI ``stringizing'' preprocessing operator ` # ' to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value.

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

10.26 How can I write a macro which takes a variable number of arguments, or use the preprocessor to ``turn off'' a function call with a variable number of arguments?

10.27 How can I include expansions of the __FILE__ and __LINE__ macros in a general-purpose debugging macro?

11. ANSI/ISO Standard C

11.1 What is the ``ANSI C Standard?''

11.2 How can I get a copy of the Standard?

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

11.3 My ANSI compiler complains about a mismatch when it sees

extern int func(float); int func(x) float x; { ...

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

11.5 Why does the declaration

extern int f(struct x *p);

11.6 I had a frustrating problem which turned out to be caused by the line

printf("%d", n);

n

long int

11.7 I heard that you have to #include <stdio.h> before calling printf . Why?

11.8 I don't understand why I can't use const values in initializers and array dimensions, as in

const int n = 5; int a[n];

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

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

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

11.11 I've got the declarations

typedef char *charp; const charp p;

p

const

11.11b What's the difference between

const MAXSIZE = 100;

and

#define MAXSIZE 100

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

11.12b Can I declare main as void , to shut off these annoying ``main returns no value'' messages?

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

11.14a I believe that declaring void main() can't fail, since I'm calling exit instead of returning, and anyway my operating system ignores a program's exit/return status.

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

11.15 The book I've been using, C Programing for the Compleat Idiot, always uses void main() .

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

11.17 I'm trying to use the ANSI ``stringizing'' preprocessing operator ` # ' to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value.

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

11.19 I'm getting strange syntax errors inside lines I've #ifdef fed out.

11.20 What are #pragma s and what are they good for?

11.21 What does `` #pragma once '' mean? I found it in some header files.

11.22 Is char a[3] = "abc"; legal? What does it mean?

11.23 Since array references decay into pointers, if arr is an array, what's the difference between arr and &arr ?

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

11.25 What's the difference between memcpy and memmove ?

11.26 What should malloc(0) do? Return a null pointer or a pointer to 0 bytes?

11.27 Why does the ANSI Standard place limits on the length and case-significance of external identifiers?

11.28 What was noalias and what ever happened to it?

11.29a My compiler is rejecting the simplest possible test programs, with all kinds of syntax errors. It's complaining about the first line of

main(int argc, char **argv) { return 0; }

11.29b What does the message ``Automatic aggregate intialization is an ANSI feature'' mean? My compiler is complaining about valid ANSI code.

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

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

11.32 Why won't the Frobozz Magic C Compiler, which claims to be ANSI compliant, accept this code? I know that the code is ANSI, because gcc accepts it.

11.33 People seem to make a point of distinguishing between implementation-defined, unspecified, and undefined behavior. What do these mean?

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

11.34 I'm appalled that the ANSI Standard leaves so many issues undefined. Isn't a Standard's whole job to standardize these things?

11.35 People keep saying that the behavior of i = i++ is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected.

12. Stdio

12.1 What's wrong with this code?

char c; while((c = getchar()) != EOF) ...

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? I see that EOF is defined by <stdio.h> to be -1; am I supposed to enter -1?

12.2 Why does the simple line-copying loop while(!feof(infp)) { fgets(buf, MAXLINE, infp); fputs(buf, outfp); } copy the last line twice?

12.3 I'm using fgets to read lines from a file into an array of pointers. Why do all the lines end up containing copies of the last line?

12.4 My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program.

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

12.6 How can I print a '%' character in a printf format string? I tried \% , but it didn't work.

12.7 Why doesn't

long int n = 123456; printf("%d

", n);

12.8 I thought that ANSI function prototypes were supposed to guard against argument type mismatches.

12.9 Someone told me it was wrong to use %lf with printf . How can printf use %f for type double , if scanf requires %lf ?

12.9b What printf format should I use for a typedef like size_t when I don't know whether it's long or some other type?

12.10 How can I implement a variable field width with printf ? That is, instead of something like %8d , I want the width to be specified at run time.

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

What about currency formatted numbers?

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

12.12b Why does the call

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

&

scanf

12.13 Why doesn't this code:

double d; scanf("%f", &d);

12.14 Why doesn't the code

short int s; scanf("%d", &s);

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

12.16 How can I read data from data files with particular formats?

How can I read ten floats without having to use a jawbreaker scanf format

like "%f %f %f %f %f %f %f %f %f %f" ?

How can I read an arbitrary number of fields from a line into an array?

12.17 When I read numbers from the keyboard with scanf and a "%d

" format, like this:

int n; scanf("%d

", &n); printf("you typed %d

", n);

12.18a I'm reading a number with scanf and %d , and then a string with gets() :

int n; char str[80]; printf("enter a number: "); scanf("%d", &n); printf("enter a string: "); gets(str); printf("you typed %d and \"%s\"

", n, str);

gets()

12.18b I'm using scanf %c to read a Y/N response, but later input gets skipped.

12.19 I figured I could use scanf more safely if I checked its return value to make sure that the user typed the numeric values I expect:

int n; while(1) { printf("enter a number: "); if(scanf("%d", &n) == 1) break; printf("try again: "); } printf("you typed %d

", n);

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

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 ?

12.22 What's the deal on sprintf 's return value? Is it an int or a char * ?

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

12.24 I thought I'd check errno after a long string of printf calls, to see if any of them had failed:

errno = 0; printf("This

"); printf("is

"); printf("a

"); printf("test.

"); if(errno != 0) fprintf(stderr, "printf failed: %s

", strerror(errno));

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

What are fgetpos and fsetpos good for?

12.26a How can I flush pending input so that a user's typeahead isn't read at the next prompt? Will fflush(stdin) work?

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

12.27 I wrote this routine which is supposed to open a file:

myfopen(char *filename, FILE *fp) { fp = fopen(filename, "r"); }

FILE *infp; myfopen("filename.dat", infp);

infp

12.28 I can't even get a simple fopen call to work! What's wrong with this call?

FILE *fp = fopen(filename, 'r');

12.28b How can I open files with names like `` file1 '', `` file2 '', `` file3 '', etc., where the numeric part is controlled by a variable? Basically I want `` file%d '', like printf .

12.29 fopen is failing for certain pathnames.

12.30 I'm trying to update a file in place, by using fopen mode "r+" , reading a certain string, and writing back a modified string, but it's not working.

12.31 How can I insert or delete a line (or record) in the middle of a file?

12.32 How can I recover the file name given an open stream?

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

12.34 Once I've used freopen , how can I get the original stdout (or stdin ) back?

12.35 How can I tell if standard input or output is redirected (i.e. whether `` < '' or `` > '' was used on the invocation command line)?

12.36 I'm trying to write a program like `` more .'' How can I get back to the interactive keyboard if stdin is redirected?

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

12.37 I want to read and write numbers between files and memory in a byte-at-a-time way, not as formatted characters the way fprintf and fscanf do. How can I do this?

12.38 How can I read a binary data file properly? I'm occasionally seeing 0x0a and 0x0d values getting garbled, and I seem to hit EOF prematurely if the data contains the value 0x1a .

12.39 I'm writing a ``filter'' for binary files, but stdin and stdout are preopened as text streams. How can I change their mode to binary?

12.40 What's the difference between text and binary I/O?

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

12.42 How can I write code to conform to these old, binary data file formats?

12.43 I'm reading strings typed by the user into an array, and then printing them out later. When the user types a sequence like

, why isn't it being handled properly?

13. Library Functions

13.1 How can I convert numbers to strings (the opposite of atoi )? Is there an itoa function?

13.2 Why does strncpy not always place a '\0' terminator in the destination string?

13.3 Does C have anything like the ``substr'' (extract substring) routine present in other languages?

13.4 How do I convert a string to all upper or lower case?

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

Why does some code call islower before toupper ?

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

How can I duplicate the process by which main() is handed argc and argv ?

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

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

13.9 Now I'm trying to sort an array of structures with qsort . My comparison function takes pointers to structures, but the compiler complains that the function is of the wrong type for qsort . How can I cast the function pointer to shut off the warning?

13.10 How can I sort a linked list?

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

13.12 How can I get the current date or time of day in a C program?

13.13 I know that the library function localtime will convert a time_t into a broken-down struct tm , and that ctime will convert a time_t to a printable string. How can I perform the inverse operations of converting a struct tm or a string into a time_t ?

13.14 How can I add N days to a date? How can I find the difference between two dates?

13.14b Did C have any Year 2000 problems?

13.15 I need a random number generator.

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

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

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...

13.19 How can I return a sequence of random numbers which don't repeat at all?

13.19b How can I generate floating-point random numbers?

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

13.21 I'm porting this program, and it calls a routine drand48 , which my library doesn't have. What is it?

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

13.23 What's the difference between memcpy and memmove ?

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

13.25 I keep getting errors due to library functions being undefined, but I'm #including all the right header files.

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

13.27 Why is my simple program, which hardly does more than print ``Hello, world!'' in a window, compiling to such a huge executable (several hundred K)? Should I #include fewer header files?

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

13.29 My compiler is complaining that printf is undefined! How can this be? It's the world's most popular C function...

14. Floating Point

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

14.2 I'm trying to take some square roots, and I've simplified the code down to

main() { printf("%f

", sqrt(144.)); }

14.3 I'm trying to do some simple trig, and I am #including <math.h> , but the linker keeps complaining that functions like sin and cos are undefined.

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

14.4b I'm sure I've got the trig functions declared correctly, but they're still giving me wrong answers.

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

14.6 How do I round numbers?

14.7 Why doesn't C have an exponentiation operator?

14.8 The predefined constant M_PI seems to be missing from my machine's copy of <math.h> .

14.9 How do I set variables to, or test for IEEE NaN (``Not a Number'') and other special values?

14.10 How can I handle floating-point exceptions gracefully?

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

14.12 I'm looking for some code to do:

Fast Fourier Transforms (FFT's)

matrix arithmetic (multiplication, inversion, etc.)

complex arithmetic



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

15. Variable-Length Argument Lists

15.1 I heard that you have to #include <stdio.h> before calling printf . Why?

15.2 How can %f be used for both float and double arguments in printf ? Aren't they different types?

15.3 I had a frustrating problem which turned out to be caused by the line

printf("%d", n);

n

long int

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

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?

15.6 How can I write a function analogous to scanf , i.e. that accepts similar arguments, and calls scanf to do most of the work?

15.7 I have a pre-ANSI compiler, without <stdarg.h> . What can I do?

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

15.9 My compiler isn't letting me declare a function

int f(...) { }

15.10 I have a varargs function which accepts a float parameter. Why isn't

va_arg(argp, float)

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

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

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

16. Strange Problems

16.1 Why is this loop always executing once?

for(i = start; i < end; i++); { printf("%d

", i); }

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.

16.1c Why isn't my procedure call working? The compiler seems to skip right over it.

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

16.3 This program crashes before it even runs! (When single-stepping with a debugger, it dies before the first statement in main .)

16.4 I have a program that seems to run correctly, but it crashes as it's exiting, after the last statement in main() . What could be causing this?

16.5 This program runs perfectly on one machine, but I get weird results on another. Stranger still, adding or removing a debugging printout changes the symptoms...

16.6 Why does this code:

char *p = "hello, world!"; p[0] = 'H';

16.7

I've got some code that's trying to unpack external structures, but it's crashing with a message about an ``unaligned access.'' What does this mean? The code looks like this:

struct mystruct { char c; long int i32; int i16; } s; char buf[7], *p; fread(buf, 7, 1, fp); p = buf; s.c = *p++; s.i32 = *(long int *)p; p += 4; s.i16 = *(int *)p;

16.8 What do ``Segmentation violation'', ``Bus error'', and ``General protection fault'' mean? What's a ``core dump''?

17. Style

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

17.2 How should functions be apportioned among source files?

17.3 Here's a neat trick for checking whether two strings are equal:

if(!strcmp(s1, s2))

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

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

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

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

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

17.7 Should I use symbolic names like TRUE and FALSE for Boolean constants, or plain 1 and 0?

17.8 What is ``Hungarian Notation''? Is it worthwhile?

17.9 Where can I get the ``Indian Hill Style Guide'' and other coding standards?

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

17.11 People always say that good style is important, but when they go out of their way to use clear techniques and make their programs readable, they seem to end up with less efficient programs. Since efficiency is so important, isn't it necessary to sacrifice some style and readability?

17.12 Which is correct,

char *p

char* p

18. Tools and Resources

18.1 I need some C development tools.

18.2 How can I track down these pesky malloc problems?

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

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

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

18.6 Can I declare main as void , to shut off these annoying ``main returns no value'' messages?

18.7 Where can I get an ANSI-compatible lint ?

18.8 Don't ANSI function prototypes render lint obsolete?

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

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

18.10 What's a good book for learning C? What about advanced books and references?

18.11 Where can I find answers to the exercises in K&R?

18.12 Does anyone know where the source code from books like Numerical Recipes in C, Plauger's The Standard C Library, or Kernighan and Pike's The UNIX Programming Environment is available on-line?

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

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

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

18.14 I need code to parse and evaluate expressions.

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

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

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

18.15d I need code for performing multiple precision arithmetic.

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

18.17 Where can I get extra copies of this list?

19. System Dependencies

19.1 How can I read a single character from the keyboard without waiting for the RETURN key? How can I stop characters from being echoed on the screen as they're typed?

19.2 How can I find out if there are characters available for reading (and if so, how many)? Alternatively, how can I do a read that will not block if there are no characters available?

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

19.4 How can I clear the screen?

How can I print text in color?

How can I move the cursor to a specific x, y position?

19.4b I'm compiling some test programs on a windows-based system, and the windows containing my program's output are closing so quickly after my program calls exit that I can't see the output. How can I make it pause before closing?

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

19.6 How do I read the mouse?

19.7 How can I do serial (``comm'') port I/O?

19.8 How can I direct output to the printer?

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

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

19.10 How can I do graphics?

19.10b How can I display GIF and JPEG images?

19.10c How can I load new fonts for display?

19.10d How can I send mail from within a C program?

19.11 How can I check whether a file exists? I want to warn the user if a requested input file is missing.

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

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

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

19.14 How can I insert or delete a line (or record) in the middle of a file?

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

19.16 How can I delete a file?

19.16b How do I copy files?

19.17 Why can't I open a file by its explicit path? The call

fopen("c:

ewdir\file.dat", "r")

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

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

19.18 I'm getting an error, ``Too many open files''. How can I increase the allowable number of simultaneously open files?

19.19 How can I find out how much free space is available on disk?

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

19.21 How do I create a directory?

How do I remove a directory (and its contents)?

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

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

19.24 What does the error message ``DGROUP data allocation exceeds 64K'' mean, and what can I do about it? I thought that using large model meant that I could use more than 64K of data!

19.25 How can I access memory (a memory-mapped device, or graphics memory) located at a certain address?

How can I do PEEK and POKE in C?

19.25b How can I determine whether a machine's byte order is big-endian or little-endian?

19.26 How can I access an interrupt vector located at the machine's location 0? If I set a pointer to 0 , the compiler might translate it to some nonzero internal null pointer value.

19.27 How can I invoke another program (a standalone executable, or an operating system command) from within a C program?

19.28 How can I call system when parameters (filenames, etc.) of the executed command aren't known until run time?

19.29 How do I get an accurate error status return from system on MS-DOS?

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

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

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

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

19.34 How can I open files mentioned on the command line, and parse option flags?

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

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

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

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

19.39 How can I handle floating-point exceptions gracefully?

19.39b How can I ensure that integer arithmetic doesn't overflow?

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

19.40a Can I combine .OBJ and .LIB files from Microsoft C and Turbo C?

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

19.40c I'm trying to compile this program, but the compiler is complaining that `` union REGS '' is undefined, and the linker is complaining that int86 is undefined.

19.40d What are ``near'' and ``far'' pointers?

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

19.42 Why isn't any of this standardized in C? Any real program has to do some of these things.

20. Miscellaneous

20.1 How can I return multiple values from a function?

20.2 What's a good data structure to use for storing lines of text? I started to use fixed-size arrays of arrays of char , but they're just too restrictive.

20.3 How can I open files mentioned on the command line, and parse option flags?

20.4 What's the right way to use errno ?

20.5 How can I write data files which can be read on other machines with different word size, byte order, or floating point formats?

20.6 If I have a char * variable pointing to the name of a function, how can I call that function? Code like

extern int func(int, int); char *funcname = "func"; int r = (*funcname)(1, 2);

r = (*(int (*)(int, int))funcname)(1, 2);

20.6b How can I ensure that integer arithmetic doesn't overflow?

20.7 How can I manipulate individual bits?

20.8 How can I implement sets or arrays of bits?

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

20.9b How do I swap bytes?

20.10 How can I convert integers to binary or hexadecimal?

20.11 Can I use base-2 constants (something like 0b101010 )?

Is there a printf format for binary?

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

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

20.14 Are pointers really faster than arrays? How much do function calls slow things down? Is ++i faster than i = i + 1 ?

20.15 I've been replacing multiplications and divisions with shift operators, because shifting is more efficient.

20.15b People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed, but my compiler can't even replace i/=2 with a shift.

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

20.16 Which is more efficient, a switch statement or an if / else chain?

20.17 Is there a way to switch on strings?

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

20.19 Are the outer parentheses in return statements really optional?

20.20 Why don't C comments nest? How am I supposed to comment out code containing comments? Are comments legal inside quoted strings?

20.20b Why isn't there a numbered, multi-level break statement to break out of several loops at once? What am I supposed to use instead, a goto ?

20.21 There seem to be a few missing operators, like ^^ , &&= , and ->= .

20.21a Does C have circular shift operators?

20.21b Is C a great language, or what? Where else could you write something like a+++++b ?

20.22 If the assignment operator were := , wouldn't it then be harder to accidentally write things like if(a = b) ?

20.23 Does C have an equivalent to Pascal's with statement?

20.24 Why doesn't C have nested functions?

20.24b What is assert() and when would I use it?

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

20.26 Does anyone know of a program for converting Pascal or FORTRAN (or LISP, Ada, awk, ``Old'' C, ...) to C?

20.27 Is C++ a superset of C? What are the differences between C and C++? Can I use a C++ compiler to compile C code?

20.28 I need a sort of an ``approximate'' strcmp routine, for comparing two strings for close, but not necessarily exact, equality.

20.29 What is hashing?

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

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

20.32 Is (year % 4 == 0) an accurate test for leap years? (Was 2000 a leap year?)

20.33 Why can tm_sec in the tm structure range from 0 to 61, suggesting that there can be 62 seconds in a minute?

20.34 Here's a good puzzle: how do you write a program which produces its own source code as output?

20.35 What is ``Duff's Device''?

20.36 When will the next International Obfuscated C Code Contest (IOCCC) be held? How do I submit contest entries? Who won this year's IOCCC? How can I get a copy of the current and previous winning entries?

20.37 What was the entry keyword mentioned in K&R1?

20.38 Where does the name ``C'' come from, anyway?

20.39 How do you pronounce `` char ''? What's that funny name for the `` # '' character?

20.39b What do ``lvalue'' and ``rvalue'' mean?

20.40 Where can I get extra copies of this list?

Glossary

Bibliography

Acknowledgements