CODEWITHSUNDEEP
cassemblyx86-64stack-frame
user12976036
user12976036

Reputation:

Why are frame pointers saved in the beginning of the main function

Assume this C code:

int main(){
   return 0;
}

Would look like this in assembly:

main:
    pushq   %rbp
    movq    %rsp, %rbp
    movl    $0, %eax
    popq    %rbp
    ret

I know that Frame pointer fp needs to be saved in the start of functions by pushq %rbp since it needs to be restored when returnning to the caller function.

My question is why do so in main? what's the parent caller of main? Isn't fp pointing to a virtual address, meaning when main terminates the address doesn't mean anything anymore to the next program, correct?

Are fp (or even sp) values persistent between different programs and their address space?

Upvotes: 3

Views: 514

Answers (1)

anastaciu
anastaciu

Reputation: 23802

what's the parent caller of main?

In linux main is called by __libc_start_main witch in term is called by _start, in windows I'm not so sure but there is also a _start.

In fact a neat trick is to start a C program without main:

#include <stdio.h> 
#include <stdlib.h>

void _start() 
{  
    printf("No main function!\n");
    exit(0); 
}

compile with:

gcc main.c -nostartfiles

For Windows(10, gcc 8.1.0) and Ubuntu(18.04, gcc 9.2.0)

clang -Wl,-e,-Wl,__start main.c

For MacOS (10.14.6, Xcode 11.3)

Here is an article that talks about Linux x86 Program Start Up

Upvotes: 4

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