Question

When my program is disassembled in gdb, I can see the address of buf be pushed onto the stack, but I don’t see the format string pushed onto it. Any reason why this is? Is it a clever compiler optimization?

I’ve tried compiling a few different variations of printf statements to see if I could mimic the “%s” string (or address of it) not being pushed onto the stack, but I couldn’t do it.

This is the program code:

int main(int argc, char **argv) {

    char buf[128];
    if(argc < 2) return 1;

    strcpy(buf, argv[1]);

    printf("%s\n", buf);

    return 0;
}

compiled with gcc 4.5.2, 32 bit linux

Answer 1

Yes, it appears that gcc will throw away “printf (“%s\n”, buff)” and substitute “puts()” in its place:

vi tmp.c =>

#include <stdio.h>
#include <string.h>

int
main(int argc, char **argv)
{
    char buf[128];
    if(argc < 2)
      return 1;

    strcpy(buf, argv[1]);
    printf("%s\n", buf);

    return 0;
}

$ gcc -S -Wall -pedantic tmp.c
less tmp.s =>

        .file   "tmp.c"
        .text
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ecx
        subl    $148, %esp
        movl    %ecx, -140(%ebp)
        movl    -140(%ebp), %eax
        cmpl    $1, (%eax)
        jg      .L2
        movl    $1, -136(%ebp)
        jmp     .L4
.L2:
        movl    -140(%ebp), %edx
        movl    4(%edx), %eax
        addl    $4, %eax
        movl    (%eax), %eax
        movl    %eax, 4(%esp)
        leal    -132(%ebp), %eax
        movl    %eax, (%esp)
        call    strcpy
        leal    -132(%ebp), %eax
        movl    %eax, (%esp)
        call    puts
        movl    $0, -136(%ebp)
.L4:
        movl    -136(%ebp), %eax
        addl    $148, %esp
        popl    %ecx
        popl    %ebp
        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.1.2 20080704 (Red Hat 4.1.2-48)"
        .section        .note.GNU-stack,"",@progbits