C <stdlib.h>

C <stdlib.h>: Standard Library Functions

The <stdlib.h> (Standard Library) header file defines several general-purpose functions, including dynamic memory management, random number generation, communication with the environment, integer arithmetic, and searching/sorting utilities.

1. Dynamic Memory Allocation

Unlike standard arrays where the size is fixed at compile time, <stdlib.h> allows you to allocate memory dynamically during program execution (at runtime) on the Heap.

• malloc(size): Allocates a block of memory of the specified size (in bytes). Does not initialize the memory.
• calloc(n, size): Allocates memory for an array of n elements, initializing all bytes to zero.
• realloc(ptr, new_size): Resizes a previously allocated memory block.
• free(ptr): Deallocates previously allocated memory to prevent memory leaks.

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Allocate memory for an array of 5 integers
    int *arr = (int *) malloc(5 * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed!\n");
        return 1;
    }
    arr[0] = 10;
    printf("First element: %d\n", arr[0]);
    // Always free dynamically allocated memory!
    free(arr);
    return 0;
}

2. Process Control (Exit)

Sometimes you need to terminate a program immediately, especially if a critical error occurs.

• exit(status): Terminates the program. exit(0) generally signifies successful termination, while non-zero values (like exit(1)) indicate an error.

3. String Conversion Functions

<stdlib.h> provides useful functions for converting numeric strings into actual numbers:

• atoi(): Converts a string to an int.
• atof(): Converts a string to a double.
• atol(): Converts a string to a long int.

#include <stdio.h>
#include <stdlib.h>

int main() {
    char str[] = "2024";
    int year = atoi(str);
    printf("Next year is: %d\n", year + 1);
    return 0;
}