C stdlib.h Library Reference

?Why is stdlib.h Essential?

stdlib.h is the Swiss Army knife of C programming. Almost every non-trivial program needs it for memory, conversions, or program control.

•

Memory

malloc, free

•

Convert

atoi, strtol

⚄

Random

rand, srand

•

Sort

qsort, bsearch

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

!Code Pitfalls: Common Mistakes & What to Watch For

Code copied from the internet often has stdlib.h code that works in simple cases but fails in production. Here are critical mistakes beginners make:

Watch out: Missing malloc() return value check

Code often looks like:

int *ptr = malloc(sizeof(int) * 1000);
// Immediately uses ptr without NULL check!

malloc() can return NULL if memory allocation fails. Always check!

Watch out: Using atoi() without error handling

atoi("abc") returns 0, which is indistinguishable from atoi("0"). This is often ignored and doesn't validate input. Use strtol() for robust parsing!

Watch out: Not seeding rand() properly

Without srand(time(NULL)), rand() produces the same sequence every run. Copied code often forgets this, leading to predictable "random" values.

This causes memory leaks with realloc()

Copied code often:

ptr = realloc(ptr, newSize);
// If realloc fails, original pointer is lost = memory leak!

Frequently Asked Questions

Q: How do I generate random numbers in a specific range?

Use modulo with offset: rand() % (max - min + 1) + min. For example, to get numbers between 10 and 20: rand() % 11 + 10. Note: this has slight bias for large ranges; for cryptographic use, use a proper library.

Q: What's the difference between malloc() and calloc()?

malloc() allocates memory without initializing it (contains garbage). calloc() allocates AND initializes all bytes to zero. Use calloc() when you need guaranteed zero-initialization, like for arrays or structs.

Q: Why does rand() give the same numbers every time?

rand() uses a pseudo-random algorithm that needs a "seed" value. Without seeding, it uses 1 by default. Call srand(time(NULL)) once at program start to get different sequences each run.

Q: When should I use exit() vs return?

Use return in main() for normal termination. Use exit()when terminating from any function, especially to trigger atexit() handlers. Use abort()only for abnormal/emergency termination (skips cleanup).

Q: What's the difference between exit() and _Exit()?

exit() performs cleanup: it calls atexit() handlers, flushes stdio buffers, and closes files properly. _Exit() (C99) terminates immediately without any cleanup. Use _Exit() only when you need to avoid running cleanup code, such as in a child process after fork().

Q: How do I safely reallocate memory?

Never assign realloc() directly to your original pointer. If realloc() fails, it returns NULL but doesn't free the original memory. Use a temporary: void *tmp = realloc(ptr, newSize); if (tmp) ptr = tmp;This prevents memory leaks when reallocation fails.

Q: Why does qsort need a comparison function?

qsort() is generic—it doesn't know what you're sorting. Your comparison function tells it how to compare two elements. Return negative if first is less, positive if greater, zero if equal. This design allows sorting any data type with the same function.

Q: Is system() safe to use?

system() is convenient but risky. Never pass user input directly to system() — it's a major security vulnerability (command injection). For production code, use specific system calls like fork()/exec() on Unix or CreateProcess() on Windows.

Q: Why use strtol() instead of atoi()?

atoi() can't detect errors — it returns 0 for both "0" and invalid input like "abc". strtol() provides error detection via the endptr parameter and errno, making it essential for validating user input.

Q: What's the difference between exit() and abort()?

exit() performs cleanup: flushes buffers, closes files, calls atexit() handlers.abort() terminates immediately without cleanup, typically generating a core dump for debugging. Use exit() for normal termination, abort() for fatal errors.

Q: How do I generate random numbers in a specific range?

Use modulo: rand() % (max - min + 1) + min for range [min, max]. For better distribution, especially with large ranges, use:(int)((double)rand() / RAND_MAX * (max - min + 1)) + min.

10Summary

Key Functions

Memory:

malloc, calloc, realloc, free

Conversion:

atoi, atof, strtol, strtod

Random:

rand, srand

Program:

exit, abort, system, getenv

11Memory Management Best Practices

Always Free What You Allocate

For every malloc(), there should be a corresponding free(). Track ownership of allocated memory clearly in your code design.

Use calloc for Zero-Initialized Memory

When you need zero-initialized memory, use calloc() instead ofmalloc() followed by memset(). It's cleaner and often optimized by the OS to provide zero pages efficiently.

?Why is stdlib.h Essential?

stdlib.h is the Swiss Army knife of C programming. Almost every non-trivial program needs it for memory, conversions, or program control.

•

Memory

malloc, free

•

Convert

atoi, strtol

⚄

Random

rand, srand

•

Sort

qsort, bsearch

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

!Code Pitfalls: Common Mistakes & What to Watch For

Code copied from the internet often has stdlib.h code that works in simple cases but fails in production. Here are critical mistakes beginners make:

Watch out: Missing malloc() return value check

Code often looks like:

int *ptr = malloc(sizeof(int) * 1000);
// Immediately uses ptr without NULL check!

malloc() can return NULL if memory allocation fails. Always check!

Watch out: Using atoi() without error handling

atoi("abc") returns 0, which is indistinguishable from atoi("0"). This is often ignored and doesn't validate input. Use strtol() for robust parsing!

Watch out: Not seeding rand() properly

Without srand(time(NULL)), rand() produces the same sequence every run. Copied code often forgets this, leading to predictable "random" values.

This causes memory leaks with realloc()

Copied code often:

ptr = realloc(ptr, newSize);
// If realloc fails, original pointer is lost = memory leak!

Frequently Asked Questions

Q: How do I generate random numbers in a specific range?

Q: What's the difference between malloc() and calloc()?

Q: Why does rand() give the same numbers every time?

rand() uses a pseudo-random algorithm that needs a "seed" value. Without seeding, it uses 1 by default. Call srand(time(NULL)) once at program start to get different sequences each run.

Q: When should I use exit() vs return?

Q: What's the difference between exit() and _Exit()?

Q: How do I safely reallocate memory?

Q: Why does qsort need a comparison function?

Q: Is system() safe to use?

Q: Why use strtol() instead of atoi()?

Q: What's the difference between exit() and abort()?

Q: How do I generate random numbers in a specific range?

Use modulo: rand() % (max - min + 1) + min for range [min, max]. For better distribution, especially with large ranges, use:(int)((double)rand() / RAND_MAX * (max - min + 1)) + min.

10Summary

Key Functions

Memory:

malloc, calloc, realloc, free

Conversion:

atoi, atof, strtol, strtod

Random:

rand, srand

Program:

exit, abort, system, getenv

11Memory Management Best Practices

Always Free What You Allocate

For every malloc(), there should be a corresponding free(). Track ownership of allocated memory clearly in your code design.

Use calloc for Zero-Initialized Memory

When you need zero-initialized memory, use calloc() instead ofmalloc() followed by memset(). It's cleaner and often optimized by the OS to provide zero pages efficiently.

What You Will Learn

?Why is stdlib.h Essential?

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

!Code Pitfalls: Common Mistakes & What to Watch For

Watch out: Missing malloc() return value check

Watch out: Using atoi() without error handling

Watch out: Not seeding rand() properly

This causes memory leaks with realloc()

Frequently Asked Questions

Q: How do I generate random numbers in a specific range?

Q: What's the difference between malloc() and calloc()?

Q: Why does rand() give the same numbers every time?

Q: When should I use exit() vs return?

Q: What's the difference between exit() and _Exit()?

Q: How do I safely reallocate memory?

Q: Why does qsort need a comparison function?

Q: Is system() safe to use?

Q: Why use strtol() instead of atoi()?

Q: What's the difference between exit() and abort()?

Q: How do I generate random numbers in a specific range?

10Summary

Key Functions

11Memory Management Best Practices

Always Free What You Allocate

Use calloc for Zero-Initialized Memory

Test Your Knowledge

Related Tutorials

C stdio.h Library Reference

C string.h Library Reference

Dynamic Memory Allocation

Have Feedback?

What You Will Learn

?Why is stdlib.h Essential?

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

!Code Pitfalls: Common Mistakes & What to Watch For

Watch out: Missing malloc() return value check

Watch out: Using atoi() without error handling

Watch out: Not seeding rand() properly

This causes memory leaks with realloc()

Frequently Asked Questions

Q: How do I generate random numbers in a specific range?

Q: What's the difference between malloc() and calloc()?

Q: Why does rand() give the same numbers every time?

Q: When should I use exit() vs return?