C atexit() Function | Register Functions to Execute on Program Exit

Introduction

The atexit() function allows you to register a function that will be called automatically when the program terminates normally. This is useful for performing cleanup tasks that need to be done before the program exits, such as closing files, freeing memory, or saving data.

atexit() Function Syntax

The syntax for the atexit() function is as follows:

int atexit(void (*func)(void));

Parameters:

• func: A pointer to the function to be called upon program termination. This function must take no arguments and return no value.

Returns:

• The function returns 0 on success and a non-zero value on failure.

Understanding atexit() Function

The atexit() function registers the specified function to be called when the program terminates normally. Multiple functions can be registered with atexit(), and they will be called in the reverse order of their registration. The registered functions are called automatically when the exit() function is called or when the program returns from the main() function.

Examples

Registering a Single Function

To demonstrate how to use atexit() to register a function to be called upon program termination, we will write a simple program.

Example

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

void cleanup_function(void) {
    printf("Cleanup function called.\n");
}

int main() {
    // Register the cleanup function
    if (atexit(cleanup_function) != 0) {
        printf("Failed to register cleanup function.\n");
        return 1;
    }

    printf("Main function executing.\n");

    // Program will exit normally
    return 0;
}

Output:

Main function executing.
Cleanup function called.

Registering Multiple Functions

This example shows how to register multiple functions with atexit().

Example

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

void cleanup_function1(void) {
    printf("Cleanup function 1 called.\n");
}

void cleanup_function2(void) {
    printf("Cleanup function 2 called.\n");
}

int main() {
    // Register the first cleanup function
    if (atexit(cleanup_function1) != 0) {
        printf("Failed to register cleanup function 1.\n");
        return 1;
    }

    // Register the second cleanup function
    if (atexit(cleanup_function2) != 0) {
        printf("Failed to register cleanup function 2.\n");
        return 1;
    }

    printf("Main function executing.\n");

    // Program will exit normally
    return 0;
}

Output:

Main function executing.
Cleanup function 2 called.
Cleanup function 1 called.

Real-World Use Case

Cleaning Up Resources

In real-world applications, the atexit() function can be used to ensure that resources are properly released when the program exits. For example, you can register functions to close open files, free dynamically allocated memory, or save the program's state.

Example: Closing a File on Exit

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

FILE *file;

void close_file(void) {
    if (file != NULL) {
        fclose(file);
        printf("File closed.\n");
    }
}

int main() {
    // Open a file
    file = fopen("example.txt", "w");
    if (file == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }

    // Register the close_file function
    if (atexit(close_file) != 0) {
        printf("Failed to register close_file function.\n");
        return 1;
    }

    printf("Main function executing.\n");

    // Write to the file
    fprintf(file, "Hello, World!\n");

    // Program will exit normally
    return 0;
}

Output:

Main function executing.
File closed.

Conclusion

The atexit() function in C is a standard library function that registers a function to be called upon normal program termination. It is part of the C standard library (stdlib.h). This function is useful for performing cleanup operations, such as releasing resources or saving state, before the program exits.