what’s the meaning of exit code of program with empty void main()?

I made a simple C program with empty main as given below and compiled it with gcc

void main(){
}

upon execution, it return exit code as 25, and if I add a printf statement,after including stdio.h, it returns exit code 5

what’s the meaning of 25 and 5 here and why it’s being returned by whatever code that is executed before or after main()? exit code is zero if I use int main() and don’t put a return statement.

Answer

(Adding on John Bollinger’s answer)

* void main

The old ISO C Standard (ISO/IEC 9899:1999) stated:

[main] shall be defined

• with a return type of int and
  • with no parameters […] or
  • with two parameters […] or equivalent; or
  • in some other implementation-defined manner.

^{§ 5.1.2.2.1 ¶ 1 of the C Standard}

If the return type is not compatible with int, the termination status returned to the host environment is unspecified.

^{§ 5.1.2.2.3 ¶ 1}

which indicates that allowing forms that didn’t return int was intentional.

Many compiler’s manuals (e.g. Watcom C/C++, IBM VisualAge C/C++, Microsoft Visual C/C++) stated that main may have the return type void so a program with void main() was a conforming program.

For a long time much code was written with a return type of void. gcc (probably) consider important to be compatible with legacy code and allows void main() but in that case:

• it gives a warning (warning: return type of ‘main’ is not ‘int’);
• the return value of the program is undefined.

References:

• http://web.archive.org/web/20160604095048/homepage.ntlworld.com/jonathan.deboynepollard/FGA/legality-of-void-main.html

* int main

int main() {}

This is undefined in C89/90 and well defined in following versions (where it returns 0).

* actual value

On x86 the EAX register is normally used for return values. So

int main() {}

is compiled to something like:

main:
        push    rbp
        mov     rbp, rsp
        mov     eax, 0
        pop     rbp
        ret

For

void main() {}

the simplest action is removing mov eax, 0:

main:
        push    rbp
        mov     rbp, rsp
        nop
        pop     rbp
        ret

If you add a printf statement:

#include <stdio.h>

void main()
{
  printf("1234");
}

you get:

.LC0:
        .string "1234"
main:
        push    rbp
        mov     rbp, rsp
        mov     edi, OFFSET FLAT:.LC0
        mov     eax, 0
        call    printf
        pop     rbp
        ret

The printf call alters the EAX register (returns the number of characters written to the stream, and EAX is used for the return value).