C stdlib.h Library Reference

01Introduction to stdlib.h

📚 What is stdlib.h?

<stdlib.h> (Standard Library) provides essential utilities: memory allocation, type conversions, random numbers, sorting, and program control.

📋 Key Functions Overview

Category	Functions
Memory	malloc, calloc, realloc, free
Conversion	atoi, atof, atol, strtol, strtod
Random	rand, srand
Sort/Search	qsort, bsearch
Program	exit, abort, atexit, system
Math	abs, labs, div, ldiv

02Memory Allocation Functions

malloc() - Allocate Memory

void *malloc(size_t size);

size: Number of bytes to allocate

Returns: Pointer to allocated memory, or NULL

main.c

int *arr = (int *)malloc(5 * sizeof(int));
if (arr == NULL) {
    printf("Allocation failed!\n");
    return 1;
}
// Memory is UNINITIALIZED (garbage values)

calloc() - Allocate and Zero-Initialize

void *calloc(size_t num, size_t size);

num: Number of elements

size: Size of each element

main.c

int *arr = (int *)calloc(5, sizeof(int));
// Memory is ZERO-INITIALIZED (all elements = 0)
printf("%d\n", arr[0]);  // Output: 0

realloc() - Resize Memory

void *realloc(void *ptr, size_t new_size);

ptr: Pointer to existing memory

new_size: New size in bytes

main.c

int *arr = malloc(5 * sizeof(int));
// Need more space? Resize!
int *temp = realloc(arr, 10 * sizeof(int));
if (temp != NULL) {
    arr = temp;  // Safe pattern
}

free() - Deallocate Memory

void free(void *ptr);

main.c

int *arr = malloc(5 * sizeof(int));
// Use the memory...
free(arr);      // Release memory
arr = NULL;     // Prevent dangling pointer

03String Conversion Functions

Function	Prototype	Description
atoi()	int atoi(const char *str)	String → int
atof()	double atof(const char *str)	String → double
atol()	long atol(const char *str)	String → long
strtol()	long strtol(const char str, char *end, int base)	String → long (with base)
strtod()	double strtod(const char str, char *end)	String → double (safer)

conversions.c

1#include <stdlib.h>
2#include <stdio.h>
3
4int main() {
5    // Basic conversions
6    int num = atoi("42");           // 42
7    double pi = atof("3.14159");    // 3.14159
8    long big = atol("1000000");     // 1000000
9    
10    // strtol with base (hex, binary, etc.)
11    char *end;
12    long hex = strtol("FF", &end, 16);   // 255 (hex)
13    long bin = strtol("1010", &end, 2);  // 10 (binary)
14    long dec = strtol("123abc", &end, 10); // 123, end points to "abc"
15    
16    printf("Hex FF = %ld\n", hex);
17    printf("Binary 1010 = %ld\n", bin);
18    
19    return 0;
20}

💡 strtol() vs atoi()

Use strtol() for better error handling — it tells you where parsing stopped and supports different bases (2, 8, 10, 16).

04Random Number Functions

rand()

int rand(void);

Returns pseudo-random number (0 to RAND_MAX)

srand()

void srand(unsigned int seed);

Seeds the random number generator

random.c

1#include <stdlib.h>
2#include <stdio.h>
3#include <time.h>
4
5int main() {
6    // Seed with current time (do once at program start!)
7    srand(time(NULL));
8    
9    // Random number 0 to RAND_MAX
10    int r1 = rand();
11    
12    // Random 0-99
13    int r2 = rand() % 100;
14    
15    // Random 1-6 (dice roll)
16    int dice = (rand() % 6) + 1;
17    
18    // Random 10-50
19    int r3 = (rand() % 41) + 10;
20    
21    printf("Dice: %d\n", dice);
22    printf("Range 10-50: %d\n", r3);
23    
24    return 0;
25}

⚠️ Important

Call srand() only once at program start. Calling it repeatedly with same seed produces same sequence!

05Sorting and Searching

qsort() - Quick Sort

void qsort(void *base, size_t num, size_t size, int (*compare)(const void*, const void*));

base: Pointer to array

num: Number of elements

size: Size of each element

compare: Comparison function

main.c

int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);  // Ascending
}
int main() {
    int arr[] = {5, 2, 8, 1, 9};
    int n = sizeof(arr) / sizeof(arr[0]);
    
    qsort(arr, n, sizeof(int), compare);
    // arr is now: {1, 2, 5, 8, 9}
    return 0;
}

bsearch() - Binary Search

void *bsearch(const void *key, const void *base, size_t num, size_t size, int (*compare)(...));

main.c

int arr[] = {1, 2, 5, 8, 9};  // Must be sorted!
int key = 5;
int *result = bsearch(&key, arr, 5, sizeof(int), compare);
if (result != NULL) {
    printf("Found: %d\n", *result);
} else {
    printf("Not found\n");
}

06Program Control Functions

Function	Prototype	Description
exit()	void exit(int status)	Terminate program normally
abort()	void abort(void)	Terminate abnormally
atexit()	int atexit(void (*func)(void))	Register exit handler
system()	int system(const char *cmd)	Execute shell command
getenv()	char getenv(const char name)	Get environment variable

program_control.c

1#include <stdlib.h>
2#include <stdio.h>
3
4void cleanup() {
5    printf("Cleanup called!\n");
6}
7
8int main() {
9    // Register cleanup function
10    atexit(cleanup);
11    
12    // Get environment variable
13    char *path = getenv("PATH");
14    printf("PATH: %.50s...\n", path);
15    
16    // Execute system command
17    system("echo Hello from shell!");
18    
19    // Normal exit (calls atexit handlers)
20    exit(EXIT_SUCCESS);  // or exit(0)
21    
22    // Never reached
23    return 0;
24}

07Integer Math Functions

Function	Prototype	Description
abs()	int abs(int n)	Absolute value of int
labs()	long labs(long n)	Absolute value of long
div()	div_t div(int num, int den)	Quotient and remainder
ldiv()	ldiv_t ldiv(long num, long den)	Long quotient and remainder

main.c

int x = abs(-42);    // 42
long y = labs(-100000L);  // 100000
div_t result = div(17, 5);
printf("17 / 5 = %d remainder %d\n", result.quot, result.rem);
// Output: 17 / 5 = 3 remainder 2

08Summary

🎯 Key Functions

Memory:

malloc, calloc, realloc, free

Conversion:

atoi, atof, strtol, strtod

Random:

rand, srand

Program:

exit, abort, system, getenv

09Next Steps

Learn about string manipulation functions:

Next: string.h→Review: stdio.h

What You Will Learn

01Introduction to stdlib.h

📚 What is stdlib.h?

📋 Key Functions Overview

02Memory Allocation Functions

malloc() - Allocate Memory

calloc() - Allocate and Zero-Initialize

realloc() - Resize Memory

free() - Deallocate Memory

03String Conversion Functions

💡 strtol() vs atoi()

04Random Number Functions

rand()

srand()

⚠️ Important

05Sorting and Searching

qsort() - Quick Sort

bsearch() - Binary Search

06Program Control Functions

07Integer Math Functions

08Summary

🎯 Key Functions

09Next Steps