带有空 void main() 的程序的退出代码是什么意思？

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.

Answer 1

what's the meaning of 25 and 5 here

The C language specifications do not define behavior for a program whose main() is declared to return void . Implementations that nevertheless accept such code may or may not define behavior for it. In a general sense, then, no exit statuses have any meaning or significance for a program with void main() .

Apparently, GCC in particular does accept the code, but I find no definition for its behavior among GCC's documented C extensions , so neither in that particular case do the exit statuses you observe have any meaning.

and why it's being returned by whatever code that is executed before or after main()?

No particular reason. The behavior is undefined.

exit code is zero if I use int main() and don't put a return statement.

That case is well defined. C specifies that if execution reaches the end of the initial call to main() (presumed declared with one of the two C-standard signatures, both of which return int ) then the behavior is as if exit(0) were called. This is a special case. The behavior is otherwise undefined if execution reaches the closing brace of a non- void function, and its caller does anything with the return value.

Answer 2

(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 (eg 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).