C stdlib.h Library

C stdlib.h Library

In C stdlib.h Library stands for Standard Library. It provides general-purpose functions for:

Memory management
Program control
Conversions
Random numbers
Searching and sorting

It is one of the most commonly used standard libraries in C.

📌 Common Categories & Functions

1️⃣ Memory Management

Function	Description
`malloc(size_t size)`	Allocates memory dynamically, returns pointer
`calloc(size_t n, size_t size)`	Allocates zero-initialized memory
`realloc(void *ptr, size_t size)`	Resizes previously allocated memory
`free(void *ptr)`	Frees allocated memory

Example:

#include <stdio.h>
#include <stdlib.h>int main() {
int *arr;
int n = 5;

arr = (int*)malloc(n * sizeof(int));
if(arr == NULL) {
printf("Memory allocation failed\n");
return 1;
}

for(int i=0; i<n; i++) arr[i] = i+1;

for(int i=0; i<n; i++) printf("%d ", arr[i]);

free(arr); // free memory
return 0;
}

#include <stdio.h>

#include <stdlib.h>int main() {

int *arr;

int n = 5;

arr = (int*)malloc(n * sizeof(int));

if(arr == NULL) {

printf("Memory allocation failed\n");

return 1;

}

for(int i=0; i<n; i++) arr[i] = i+1;

for(int i=0; i<n; i++) printf("%d ", arr[i]);

free(arr); // free memory

return 0;

}

2️⃣ Program Control

Function	Description
`exit(int status)`	Terminates program immediately
`abort()`	Aborts program
`atexit(void (*func)(void))`	Registers a function to run at program exit

Example:

3️⃣ Random Numbers

Function	Description
`rand()`	Returns pseudo-random number
`srand(unsigned int seed)`	Sets seed for `rand()`

Example:

4️⃣ String Conversion

Function	Description
`atoi(const char *str)`	Convert string to integer
`atof(const char *str)`	Convert string to float
`atol(const char *str)`	Convert string to long
`strtol`, `strtod`	Advanced conversion with error checking

Example:

5️⃣ Searching and Sorting

Function	Description
`qsort(void base, size_t nitems, size_t size, int (compar)(const void,const void))`	Sorts an array
`bsearch(const void key, const void base, size_t nitems, size_t size, int (compar)(const void,const void*))`	Binary search in sorted array

Example: Sorting integers

#include <stdio.h>
#include <stdlib.h>int compare(const void *a, const void *b) {
return (*(int*)a - *(int*)b);
}

int main() {
int arr[] = {5, 2, 8, 1, 9};
int n = sizeof(arr)/sizeof(arr[0]);

qsort(arr, n, sizeof(int), compare);

for(int i=0;i<n;i++) printf("%d ", arr[i]);
return 0;
}

#include <stdio.h>

#include <stdlib.h>int compare(const void *a, const void *b) {

return (*(int*)a - *(int*)b);

}

int main() {

int arr[] = {5, 2, 8, 1, 9};

int n = sizeof(arr)/sizeof(arr[0]);

qsort(arr, n, sizeof(int), compare);

for(int i=0;i<n;i++) printf("%d ", arr[i]);

return 0;

}

6️⃣ Dynamic Memory + Struct Example

#include <stdio.h>
#include <stdlib.h>struct Student {
char name[50];
int marks;
};

int main() {
int n = 2;
struct Student *students = malloc(n * sizeof(struct Student));

for(int i=0;i<n;i++) {
printf("Enter name and marks: ");
scanf("%s %d", students[i].name, &students[i].marks);
}

for(int i=0;i<n;i++) {
printf("%s - %d\n", students[i].name, students[i].marks);
}

free(students);
return 0;
}

#include <stdio.h>

#include <stdlib.h>struct Student {

char name[50];

int marks;

};

int main() {

int n = 2;

struct Student *students = malloc(n * sizeof(struct Student));

for(int i=0;i<n;i++) {

printf("Enter name and marks: ");

scanf("%s %d", students[i].name, &students[i].marks);

}

for(int i=0;i<n;i++) {

printf("%s - %d\n", students[i].name, students[i].marks);

}

free(students);

return 0;

}

📌 Summary Table

Category	Functions
Memory Management	`malloc`, `calloc`, `realloc`, `free`
Program Control	`exit`, `abort`, `atexit`
Random Numbers	`rand`, `srand`
String Conversion	`atoi`, `atof`, `atol`, `strtol`, `strtod`
Searching & Sorting	`qsort`, `bsearch`

✅ Notes

Always check the return of malloc/calloc/realloc for NULL.
Use free() to avoid memory leaks.
rand() generates pseudo-random numbers, use srand() to seed.
qsort() and bsearch() are powerful for arrays of structs or complex data.

C stdlib.h Library