Compile and run program without main() in C

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):

1. The arguments,
2. Return address,
3. Local variables, --> top of the stack

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.