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Other Standard C Functions
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abort stops the program
assert stops the program if an expression isn't true
atexit sets a function to be called when the program exits
bsearch perform a binary search
exit stop the program
getenv get enviornment information about a variable
longjmp start execution at a certain point in the program
qsort perform a quicksort
raise send a signal to the program
rand returns a pseudorandom number
setjmp set execution to start at a certain point
signal register a function as a signal handler
srand initialize the random number generator
system perform a system call
va_arg use variable length parameter lists
abort
Syntax:
#includevoid abort( void );
The function abort() terminates the current program. Depending on the implementation, the return value can indicate failure.
Related topics:
assert
atexit
exit
assert
Syntax:
#includeassert( exp );
The assert() macro is used to test for errors. If exp evaluates to zero, assert() writes information to stderr and exits the program. If the macro NDEBUG is defined, the assert() macros will be ignored.
Related topics:
abort
atexit
Syntax:
#includeint atexit( void (*func)(void) );
The function atexit() causes the function pointed to by func to be called when the program terminates. You can make multiple calls to atexit() (at least 32, depending on your compiler) and those functions will be called in reverse order of their establishment. The return value of atexit() is zero upon success, and non-zero on failure.
Related topics:
abort
exit
bsearch
Syntax:
#includevoid *bsearch( const void *key, const void *buf, size_t num, size_t size, int (*compare)(const void *, const void *) );
The bsearch() function searches buf[0] to buf[num-1] for an item that matches key, using a binary search. The function compare should return negative if its first argument is less than its second, zero if equal, and positive if greater. The items in the array buf should be in ascending order. The return value of bsearch() is a pointer to the matching item, or NULL if none is found.
Related topics:
qsort
exit
Syntax:
#includevoid exit( int exit_code );
The exit() function stops the program. exit_code is passed on to be the return value of the program, where usually zero indicates success and non-zero indicates an error.
Related topics:
abort
atexit
system
getenv
Syntax:
#includechar *getenv( const char *name );
The function getenv() returns environmental information associated with name, and is very implementation dependent. NULL is returned if no information about name is available.
Related topics:
system
longjmp
Syntax:
#includevoid longjmp( jmp_buf envbuf, int status );
The function longjmp() causes the program to start executing code at the point of the last call to setjmp(). envbuf is usually set through a call to setjmp(). status becomes the return value of setjmp() and can be used to figure out where longjmp() came from. status should not be set to zero.
Related topics:
setjmp
qsort
Syntax:
#includevoid qsort( void *buf, size_t num, size_t size, int (*compare)(const void *, const void *) );
The qsort() function sorts buf (which contains num items, each of size size) using Quicksort. The compare function is used to compare the items in buf. compare should return negative if the first argument is less than the second, zero if they are equal, and positive if the first argument is greater than the second. qsort() sorts buf in ascending order.
Example code:
For example, the following bit of code uses qsort() to sort an array of integers:
int compare_ints( const void* a, const void* b ) {
int* arg1 = (int*) a;
int* arg2 = (int*) b;
if( *arg1 < *arg2 ) return -1;
else if( *arg1 == *arg2 ) return 0;
else return 1;
}
int array[] = { -2, 99, 0, -743, 2, 3, 4 };
int array_size = 7;
...
printf( "Before sorting: " );
for( int i = 0; i < array_size; i++ ) {
printf( "%d ", array[i] );
}
printf( "\n" );
qsort( array, array_size, sizeof(int), compare_ints );
printf( "After sorting: " );
for( int i = 0; i < array_size; i++ ) {
printf( "%d ", array[i] );
}
printf( "\n" );
When run, this code displays the following output:
Before sorting: -2 99 0 -743 2 3 4
After sorting: -743 -2 0 2 3 4 99
Related topics:
bsearch
(C++ Algorithms) sort
raise
Syntax:
#includeint raise( int signal );
The raise() function sends the specified signal to the program. Some signals:
Signal
Meaning
SIGABRT
Termination error
SIGFPE
Floating pointer error
SIGILL
Bad instruction
SIGINT
User presed CTRL-C
SIGSEGV
Illegal memory access
SIGTERM
Terminate program
The return value is zero upon success, nonzero on failure.
Related topics:
signal
rand
Syntax:
#includeint rand( void );
The function rand() returns a pseudorandom integer between zero and RAND_MAX. An example:
srand( time(NULL) );
for( i = 0; i < 10; i++ )
printf( "Random number #%d: %d\n", i, rand() );
Related topics:
srand
setjmp
Syntax:
#includeint setjmp( jmp_buf envbuf );
The setjmp() function saves the system stack in envbuf for use by a later call to longjmp(). When you first call setjmp(), its return value is zero. Later, when you call longjmp(), the second argument of longjmp() is what the return value of setjmp() will be. Confused? Read about longjmp().
Related topics:
longjmp
signal
Syntax:
#includevoid ( *signal( int signal, void (* func) (int)) ) (int);
The signal() function sets func to be called when signal is recieved by your program. func can be a custom signal handler, or one of these macros (defined in the csignal header file):
Macro
Explanation
SIG_DFL
default signal handling
SIG_IGN
ignore the signal
Some basic signals that you can attach a signal handler to are:
Signal
Description
SIGTERM
Generic stop signal that can be caught.
SIGINT
Interrupt program, normally ctrl-c.
SIGQUIT
Interrupt program, similar to SIGINT.
SIGKILL
Stops the program. Cannot be caught.
SIGHUP
Reports a disconnected terminal.
The return value of signal() is the address of the previously defined function for this signal, or SIG_ERR is there is an error.
Example code:
The following example uses the signal() function to call an arbitrary number of functions when the user aborts the program. The functions are stored in a vector, and a single "clean-up" function calls each function in that vector of functions when the program is aborted:
void f1() {
cout << "calling f1()..." << endl;
}
void f2() {
cout << "calling f2()..." << endl;
}
typedef void(*endFunc)(void);
vector endFuncs;
void cleanUp( int dummy ) {
for( unsigned int i = 0; i < endFuncs.size(); i++ ) {
endFunc f = endFuncs.at(i);
(*f)();
}
exit(-1);
}
int main() {
// connect various signals to our clean-up function
signal( SIGTERM, cleanUp );
signal( SIGINT, cleanUp );
signal( SIGQUIT, cleanUp );
signal( SIGHUP, cleanUp );
// add two specific clean-up functions to a list of functions
endFuncs.push_back( f1 );
endFuncs.push_back( f2 );
// loop until the user breaks
while( 1 );
return 0;
}
Related topics:
raise
srand
Syntax:
#includevoid srand( unsigned seed );
The function srand() is used to seed the random sequence generated by rand(). For any given seed, rand() will generate a specific "random" sequence over and over again.
srand( time(NULL) );
for( i = 0; i < 10; i++ )
printf( "Random number #%d: %d\n", i, rand() );
Related topics:
rand
(Standard C Date & Time) time
system
Syntax:
#includeint system( const char *command );
The system() function runs the given command by passing it to the default command interpreter.
The return value is usually zero if the command executed without errors. If command is NULL, system() will test to see if there is a command interpreter available. Non-zero will be returned if there is a command interpreter available, zero if not.
Related topics:
exit
getenv
va_arg
Syntax:
#includetype va_arg( va_list argptr, type ); void va_end( va_list argptr ); void va_start( va_list argptr, last_parm );
The va_arg() macros are used to pass a variable number of arguments to a function.
- First, you must have a call to va_start() passing a valid va_list and the mandatory first argument of the function. This first argument can be anything; one way to use it is to have it be an integer describing the number of parameters being passed.
- Next, you call va_arg() passing the va_list and the type of the argument to be returned. The return value of va_arg() is the current parameter.
- Repeat calls to va_arg() for however many arguments you have.
- Finally, a call to va_end() passing the va_list is necessary for proper cleanup.
For example:
int sum( int num, ... ) {
int answer = 0;
va_list argptr;
va_start( argptr, num );
for( ; num > 0; num-- ) {
answer += va_arg( argptr, int );
}
va_end( argptr );
return( answer );
}
int main( void ) {
int answer = sum( 4, 4, 3, 2, 1 );
printf( "The answer is %d\n", answer );
return( 0 );
}
This code displays 10, which is 4+3+2+1.
Here is another example of variable argument function, which is a simple printing function:
void my_printf( char *format, ... ) {
va_list argptr;
va_start( argptr, format );
while( *format != '\0' ) {
// string
if( *format == 's' ) {
char* s = va_arg( argptr, char * );
printf( "Printing a string: %s\n", s );
}
// character
else if( *format == 'c' ) {
char c = (char) va_arg( argptr, int );
printf( "Printing a character: %c\n", c );
break;
}
// integer
else if( *format == 'd' ) {
int d = va_arg( argptr, int );
printf( "Printing an integer: %d\n", d );
}
*format++;
}
va_end( argptr );
}
int main( void ) {
my_printf( "sdc", "This is a string", 29, 'X' );
return( 0 );
}
This code displays the following output when run:
Printing a string: This is a string
Printing an integer: 29
Printing a character: X