What is the difference between exit() and abort()?

In C++ programming, both the exit() and abort() functions are used to terminate the current program, but they have important differences in their purposes and behaviors:

1. Function Definitions:

   • The exit(int status) function is defined in the <stdlib.h> header file and is used for normal program termination, returning an exit status to the caller. This status is typically used to indicate whether the program succeeded or failed.
   • The abort() function is also defined in the <stdlib.h> header file and is used for abnormal program termination; it does not return any status.

2. Resource Cleanup:

   • When exit() is called, the program performs cleanup operations, such as invoking all functions registered with atexit(), closing all I/O streams (e.g., files and database connections), and clearing standard I/O buffers.
   • abort() terminates the program directly without performing any cleanup operations or invoking atexit() or similar registered functions. This may result in resources not being properly released, such as unclosed files.

3. Signal Handling:

   • The abort() function sends a SIGABRT signal to the current process, which typically causes abnormal termination and may generate a core dump file for subsequent debugging.
   • exit() does not send any signals; it simply terminates the program with the specified status code.

4. Usage Scenarios:

   • exit() is typically used for normal termination, such as when the program completes all tasks or detects an error during command-line argument parsing. For example, a program may call exit() to terminate after failing to open a file.
   • abort() is typically used for abnormal situations, such as when a serious internal error occurs (e.g., violating a logical assertion). Developers may choose to call abort() to terminate immediately for problem analysis using the core dump file.

Example:

Suppose we are developing a file processing program that needs to close all opened files and return a status code.

An example using exit() might be:

cpp
#include <iostream>
#include <fstream>
#include <stdlib.h>

void cleanUp() {
    std::cout << "Performing cleanup tasks..." << std::endl;
}

int main() {
    atexit(cleanUp); // Register cleanup function
    std::ifstream file("example.txt");
    if (!file.is_open()) {
        std::cout << "Failed to open file!" << std::endl;
        exit(EXIT_FAILURE); // Terminate using exit
    }
    // File processing logic...
    file.close();
    exit(EXIT_SUCCESS); // Normal termination
}

Whereas if the program detects a serious error that cannot guarantee safe continuation, using abort() might look like:

cpp
#include <iostream>
#include <cassert>

int main() {
    int x = 0;
    // After some logic, x should not be zero
    assert(x != 0); // Assert x is not zero
    if (x == 0) {
        std::cerr << "Critical error: x should not be zero here." << std::endl;
        abort(); // Terminate immediately due to serious error
    }
    return 0;
}

In this example, if x is zero, it violates the program's expected logic, likely due to a prior serious error, so abort() is chosen to terminate the program immediately.