<stdlib.h> covers memory allocation (malloc, calloc, free), type conversion (atoi, strtol), sorting/searching (qsort, bsearch), and process control (exit, abort). It's the “junk drawer” of the C standard library.

Always check malloc for NULL; always check qsort comparators carefully for overflow.

This example sorts integers with qsort and binary-searches the result.

// compile: gcc -o stdlibexample stdlibexample.c
// run: ./stdlibexample
// description: sort array with qsort, then search it with bsearch
 
#include <stdlib.h>
#include <stdio.h>
 
int cmp_int(const void* a, const void* b) {
    return *(int*)a - *(int*)b;
}
 
int main() {
    int arr[] = {5, 2, 8, 1, 9, 3};
    int n = sizeof(arr) / sizeof(arr[0]);
 
    qsort(arr, n, sizeof(int), cmp_int);
 
    printf("sorted: ");
    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");
 
    return 0;
}

Memory allocation:

• malloc(size): allocate memory (returns NULL on failure)
• calloc(count, size): allocate zeroed memory
• realloc(ptr, size): resize allocated memory (can move pointer)
• free(ptr): deallocate memory (safe to free NULL)

String to number conversion:

• atoi(str): convert string to int (returns 0 on error, no error reporting)
• atol(str): convert string to long
• atoll(str): convert string to long long (C99)
• strtol(str, &endptr, base): convert to long (safer: reports errors, sets endptr)
• strtoll(str, &endptr, base): convert to long long
• strtoul(str, &endptr, base): convert to unsigned long
• strtoull(str, &endptr, base): convert to unsigned long long
• strtof(str, &endptr), strtod(str, &endptr), strtold(str, &endptr): floating-point

Sorting and searching:

• qsort(arr, count, size, cmp): quicksort in-place
• bsearch(key, arr, count, size, cmp): binary search (requires sorted array)

Pseudo-random numbers:

• rand(): pseudo-random integer [0, RAND_MAX]
• srand(seed): seed generator (call once before first rand call)

Environment and exit:

• exit(code): terminate program (flushes streams, calls atexit functions)
• abort(): terminate program (no cleanup, status ABORT)
• atexit(func): register function to call at exit
• getenv(name): get environment variable
• system(cmd): execute shell command

Math:

• abs(x): absolute value of int
• labs(x): absolute value of long
• llabs(x): absolute value of long long
• div(a, b): quotient and remainder (returns div_t struct)
• abs(x): absolute value
• exit(code): terminate program
• abort(): abnormal termination