Compile and run program without main() in C
I'm trying to compile and run following program without main() function in C . I have compiled my program using the following command.
gcc -nostartfiles nomain.c
And compiler gives warning
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000400340
Ok, No problem. then, I have run executable file(a.out), both printf statements print successfully, and then get segmentation fault .
So, my question is, Why segmentation fault after successfully execute print statements?
my code:
#include <stdio.h>
void nomain()
{
printf("Hello World...n");
printf("Successfully run without main...n");
}
output:
Hello World...
Successfully run without main...
Segmentation fault (core dumped)
Note:
Here, -nostartfiles gcc flag prevents the compiler from using standard startup files when linking
Let's have a look at the generated assembly of your program:
.LC0:
.string "Hello World..."
.LC1:
.string "Successfully run without main..."
nomain:
push rbp
mov rbp, rsp
mov edi, OFFSET FLAT:.LC0
call puts
mov edi, OFFSET FLAT:.LC1
call puts
nop
pop rbp
ret
Note the ret statement. Your program's entry point is determined to be nomain , all is fine with that. But once the function returns, it attempts to jump into an address on the call stack... that isn't populated. That's an illegal access and a segmentation fault follows.
A quick solution would be to call exit() at the end of your program (and assuming C11 we might as well mark the function as _Noreturn ):
#include <stdio.h>
#include <stdlib.h>
_Noreturn void nomain(void)
{
printf("Hello World...n");
printf("Successfully run without main...n");
exit(0);
}
In fact, now your function behaves pretty much like a regular main function, since after returning from main , the exit function is called with main 's return value.
In C, when functions/subroutines are called the stack is populated as (in the order):
main() being the start point, ELF structures the program in such a way that whatever instructions comes first would get pushed first, in this case printfs are.
Now, program is sort of truncated without return-address OR __end__ and infact it assumes that whatever is there on the stack at that( __end__ ) location is the return-address, but unfortunately its not and hence it crashes.
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