Standard Library: stdlib.h

The stdlib.h header provides general-purpose functions for memory management, type conversions, random numbers, and program control.

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Memory Management

int *arr = (int *)malloc(5 * sizeof(int));
int *zeros = (int *)calloc(5, sizeof(int));

arr = (int *)realloc(arr, 10 * sizeof(int));
free(arr);

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Type Conversion Functions

char str[] = "123";
int n = atoi(str);
printf("%d\n", n + 1);   /* 124 */

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Random Number Generation

#include <stdlib.h>
#include <time.h>

srand(time(NULL));          /* seed with current time */
int r = rand();             /* random int 0 to RAND_MAX */
int dice = rand() % 6 + 1; /* random 1 to 6 */

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Sorting and Searching

/* qsort */
int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);
}

int arr[] = {5, 2, 8, 1, 9};
qsort(arr, 5, sizeof(int), compare);
/* arr is now: 1 2 5 8 9 */

/* bsearch (array must be sorted) */
int key = 5;
int *result = (int *)bsearch(&key, arr, 5, sizeof(int), compare);
if (result) printf("Found: %d\n", *result);

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Program Control

#include <stdlib.h>

int main() {
    system("cls");           /* clear screen (Windows) */
    printf("Path: %s\n", getenv("PATH"));
    exit(0);
}