What are the four execution steps before entering main?

Question

Take the following 5-line file I have:

#include <stdio.h>
int main() {
    printf("Hello");
    return 0;
}

It corresponds to the following assembly:

`main:
    0x100000f60 <+0>:  pushq  %rbp
    0x100000f61 <+1>:  movq   %rsp, %rbp
    0x100000f64 <+4>:  subq   $0x10, %rsp
    0x100000f68 <+8>:  movl   $0x0, -0x4(%rbp)
->  0x100000f6f <+15>: leaq   0x34(%rip), %rdi          ; "Hello"

We can notice the first line in main which prints "Hello" corresponds to the fifth instruction. What are the four preceding instructions: what do they do?

It is called stack frame. Maintained with ebp in 32 bit and rbp in 64 bit Intel ASM. bp means "base pointer". You can search for usages of ebp/rbp and what is a stack frame. — unlut, Oct 06 '19 at 03:29
In combination it is what is call a *Function Prolog* where control is passed from the calling process to your executable. Before control is transferred the caller must set up a stackframe for your process and save the address where control will return when your program exits. At the end of your program the *Function Epilogue* returns control back where it left off. — David C. Rankin, Oct 06 '19 at 03:33
The *block* that `printf("Hello");` compiles to *starts* at the `lea` of the first (and only) arg. That instruction alone just puts the address of the string constant in a register. Nothing happens until `call printf`. — Peter Cordes, Oct 06 '19 at 04:30
It's an instruction mnemonic you can look up in the manual. It calculates an addressing mode and puts the *address* in a register. — Peter Cordes, Oct 07 '19 at 07:29

score 1 · Accepted Answer · answered Oct 06 '19 at 03:29

0x100000f60 <+0>:  pushq  %rbp

Push the caller's base pointer.

0x100000f61 <+1>:  movq   %rsp, %rbp

Copy the stack pointer into the base pointer (set up this function's stack frame)

0x100000f64 <+4>:  subq   $0x10, %rsp

Reserve stack space (presumably for the return value - you probably didn't compile this program with any optimizations enabled)

0x100000f68 <+8>:  movl   $0x0, -0x4(%rbp)

Put the return value (zero) on the stack.

->  0x100000f6f <+15>: leaq   0x34(%rip), %rdi          ; "Hello"

Load a pointer to the "Hello" string literal into rdi register.

What are the four execution steps before entering main?

1 Answers1