stdlib.h

stdlib.h is the junk drawer of the C standard library. It covers dynamic memory allocation, process control, type conversions, random numbers, sorting, and searching — everything that did not fit cleanly into the other headers.

#include <stdlib.h>
 
// dynamic memory
void *p = malloc(1024);            // allocate 1024 bytes, uninitialized
void *z = calloc(16, sizeof(int)); // allocate and zero 16 ints
p = realloc(p, 2048);              // resize allocation
free(p);                           // release memory
 
// type conversion
int    i = atoi("42");
long   l = atol("123456");
double d = atof("3.14");
long   n = strtol("0xff", NULL, 16);   // base 16, returns 255
 
// process
exit(0);                               // flush buffers, terminate with status 0
abort();                               // immediate abnormal termination (SIGABRT)
int r = system("ls -l");              // run a shell command, return exit status
char *home = getenv("HOME");          // read environment variable
 
// random numbers
srand(42);
int x = rand();   // pseudo-random int in [0, RAND_MAX]
 
// sort and search
int cmp(const void *a, const void *b) {
    return *(int*)a - *(int*)b;
}
qsort(arr, n, sizeof(int), cmp);
int *found = bsearch(&key, arr, n, sizeof(int), cmp);

malloc returns NULL on failure; always check. Passing NULL to free is safe and does nothing. realloc returns NULL on failure without freeing the original pointer, so assign to a temporary to avoid leaking the old allocation.

qsort is an unstable sort (order of equal elements is not preserved). bsearch requires the array to already be sorted. Both use a comparator returning negative/zero/positive, following the same convention as strcmp.

atexit(fn) registers a function to be called when exit is reached, in LIFO order. Useful for cleanup that should always run on normal termination — close log files, flush custom buffers, release external resources.

// compile: gcc -o sortdemo sortdemo.c
// run: ./sortdemo
// description: sort an integer array with qsort, then binary-search it
 
#include <stdlib.h>
#include <stdio.h>
 
int cmp_int(const void *a, const void *b) {
    return *(int*)a - *(int*)b;
}
 
int main(void) {
    int arr[] = {5, 2, 8, 1, 9, 3};
    int n = sizeof(arr) / sizeof(arr[0]);
 
    qsort(arr, n, sizeof(int), cmp_int);
 
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    printf("\n");
 
    int key = 8;
    int *found = bsearch(&key, arr, n, sizeof(int), cmp_int);
    printf("search for 8: %s\n", found ? "found" : "not found");
 
    key = 7;
    found = bsearch(&key, arr, n, sizeof(int), cmp_int);
    printf("search for 7: %s\n", found ? "found" : "not found");
 
    return 0;
}

The comparator subtracts the two values, which works for integers that are far from INT_MIN/INT_MAX. For general safety, use (*(int*)a > *(int*)b) - (*(int*)a < *(int*)b) to avoid overflow on extreme values.