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

Question

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.

The question headline says you have void main(), but the question body says you're executing a program with int main(). Please clarify. — John Bollinger
If you compiled in C99 mode or later, the exit status must be zero. In C90 mode, it is indeterminate. — Jonathan Leffler
Probably return code will be different with different length printf's (printf("five\n"); or printf("2\n");). — pmg

manlio manlio · Accepted Answer · 2020-04-04T15:24:51

(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).

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

2 Answers

* void main

* int main

* actual value