Introduction

In C programming, there are two ways to terminate a program from the main function: using return and using exit().

int main() { printf("Hello, World!"); return 0; // Method 1: Normal termination } int main() { printf("Hello, World!"); exit(0); // Method 2：Normal termination } 

Why can both methods terminate the program correctly, even though they appear completely different?
In this article, we'll unravel this mystery by understanding how C programs actually start and terminate.
Note that this article focuses on the implementation in GNU/Linux environments, specifically using glibc.

How exit() Works

First, let's examine how the exit function works to understand the program termination mechanism.
The exit function is a standard library function that properly terminates a program.
Internally, the _exit function, which is called by exit, is implemented in glibc as follows:

void _exit (int status) { while (1) { INLINE_SYSCALL (exit_group, 1, status); #ifdef ABORT_INSTRUCTION  ABORT_INSTRUCTION; #endif  } } 

Looking at this implementation, we can see that the _exit function receives an exit status as its argument and calls exit_group (system call number 231).

This system call performs the following operations:

1. Sends a program termination notification to the kernel
2. The kernel performs cleanup operations:
   • Releases resources used by the process
   • Updates the process table
   • Performs additional cleanup procedures

Through these operations, the program terminates properly.

So, why does returning from main() also properly terminate the program?

C Program's Hidden Entry Point

To understand this, we need to know an important fact: C programs don't actually start from main.

Let's check the default settings of the linker (ld) to see the actual entry point:

$ ld --verbose | grep "ENTRY" ENTRY(_start) 

As this output shows, the actual entry point of a C program is the _start function. main is called after _start.
The _start function is implemented in the standard library, and in glibc, it looks like this:

_start: # Initialize stack pointer xorl %ebp, %ebp popq %rsi # Get argc movq %rsp, %rdx # Get argv # Setup arguments for main pushq %rsi # Push argc pushq %rdx # Push argv # Call __libc_start_main call __libc_start_main 

The _start function has two main roles:

1. Initializes the stack frame required for program execution
2. Sets up command-line arguments (argc, argv) for the main function

After these initializations are complete, __libc_start_main is called.
This function is responsible for calling the main function.

Now, let's examine how __libc_start_main works in detail.

How __libc_start_main Makes return Work

__libc_start_call_main, which is called by __libc_start_main, is implemented as follows:

_Noreturn static void __libc_start_call_main (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL), int argc, char **argv #ifdef LIBC_START_MAIN_AUXVEC_ARG  , ElfW(auxv_t) *auxvec #endif  ) { int result; /* Memory for the cancellation buffer. */ struct pthread_unwind_buf unwind_buf; int not_first_call; DIAG_PUSH_NEEDS_COMMENT; #if __GNUC_PREREQ (7, 0)  /* This call results in a -Wstringop-overflow warning because struct pthread_unwind_buf is smaller than jmp_buf. setjmp and longjmp do not use anything beyond the common prefix (they never access the saved signal mask), so that is a false positive. */ DIAG_IGNORE_NEEDS_COMMENT (11, "-Wstringop-overflow="); #endif  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf); DIAG_POP_NEEDS_COMMENT; if (__glibc_likely (! not_first_call)) { struct pthread *self = THREAD_SELF; /* Store old info. */ unwind_buf.priv.data.prev = THREAD_GETMEM (self, cleanup_jmp_buf); unwind_buf.priv.data.cleanup = THREAD_GETMEM (self, cleanup); /* Store the new cleanup handler info. */ THREAD_SETMEM (self, cleanup_jmp_buf, &unwind_buf); /* Run the program. */ result = main (argc, argv, __environ MAIN_AUXVEC_PARAM); } else { /* Remove the thread-local data. */ __nptl_deallocate_tsd (); /* One less thread. Decrement the counter. If it is zero we terminate the entire process. */ result = 0; if (atomic_fetch_add_relaxed (&__nptl_nthreads, -1) != 1) /* Not much left to do but to exit the thread, not the process. */ while (1) INTERNAL_SYSCALL_CALL (exit, 0); } exit (result); } 

In this implementation, the key parts to focus on are as follows:

result = main (argc, argv, __environ MAIN_AUXVEC_PARAM); exit(result); 

Here, the important point is how the main function is executed and its return value is handled:

1. Executes the main function and stores its return value in result
2. Uses the return value from main as an argument for exit

Through this mechanism:

• When using return in main → The return value is passed to __libc_start_main, which then passes it to exit
• When exit() is called directly in main → The program terminates immediately

In either case, exit is ultimately called, ensuring proper program termination.

Conclusion

C programs have the following mechanism in place:

1. The program starts from _start
2. _start prepares for main's execution
3. main is executed through __libc_start_main
4. Receives main's return value and uses it as an argument for exit

Through this mechanism:

• Even when using return in main, the return value is automatically passed to exit
• As a result, both return and exit() terminate the program properly

Note that this mechanism is not limited to GNU/Linux; similar implementations exist in other operating systems (like Windows and macOS) and different C standard libraries.