What's the difference between puts and printf in C compiled into Assembly language

Question

This is my C program using puts():

#include <stdio.h>
int main(void){
puts("testing");
}

After using gcc -S -o sample.s sample.cto compiled it into Assembly, this is what I got:

        .file   "sample.c"
        .section        .rodata
.LC0:
        .string "testing"
        .text
.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        andl    $-16, %esp
        subl    $16, %esp
        movl    $.LC0, (%esp)
        call    puts
        leave
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.4.5 20110214 (Red Hat 4.4.5-6)"
        .section        .note.GNU-stack,"",@progbits

I did the same way, this time I was using printf() instead of puts and this is what I got:

    .file   "sample.c"
        .section        .rodata
.LC0:
        .string "testing"
        .text
.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        andl    $-16, %esp
        subl    $16, %esp
        movl    $.LC0, %eax        //this is the difference
        movl    %eax, (%esp)
        call    printf
        leave
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.4.5 20110214 (Red Hat 4.4.5-6)"
        .section        .note.GNU-stack,"",@progbits

Here is what I don't understand, the printf() function mov $.LC0 to %eax, then mov %eax to (%esp) while the puts() function mov %.LC0 directly to (%esp). I don't know why is that.

Answer 1

The big difference between the two functions, at the assembly level, is that the puts() function will just take one argument (a pointer to the string to display) and the printf() function will take one argument (a pointer to the format string) and, then, an arbitrary number of arguments in the stack (printf() is a variadic function).

Note that, there is absolutely no check of the number of arguments, it is only depending of the number of time the character % is encountered in the format string. For example, this specificity is used in format string format bug exploitation method to interactively explore the content of the stack of a process.

So, basically, the difference is that puts() has only one argument and printf() is a variadic function.

If you want to better understand this difference, try to compile:

#include <stdio.h>

int main(void) {
    printf("testing %d", 10);
}