Getting command line parameters from an assembly program

Question

Reading through the "Professional Assembly Language Book"; it seems that it provides an erroneous code for reading command-line arguments. I corrected it a bit and now it went from segfaulting to reading argument count then segfaulting.

Here's the full code:

.data
    output1:
        .asciz "There are %d params:\n"
    output2:
        .asciz "%s\n"

.text
.globl main
main:
    movl 4(%esp), %ecx  /* Get argument count.  */
    pushl %ecx
    pushl $output1
    call printf
    addl $4, %esp       /* remove output1  */

    /* ECX was corrupted by the printf call,
       pop it off the stack so that we get it's original
       value.  */
    popl %ecx

    /* We don't want to corrupt the stack pointer
       as we move ebp to point to the next command-line
       argument.  */
    movl %esp, %ebp
    /* Remove argument count from EBP.  */
    addl $4, %ebp

pr_arg:
    pushl (%ebp)
    pushl $output2
    call printf
    addl $8, %esp       /* remove output2 and current argument.  */
    addl $4, %ebp       /* Jump to next argument.  */
    loop pr_arg

    /* Done.  */
    pushl $0
    call exit

Code from the book:

.section .data
output1:
    .asciz “There are %d parameters:\n”
output2:
    .asciz “%s\n”
.section .text
.globl _start
_start:
    movl (%esp), %ecx
    pushl %ecx
    pushl $output1
    call printf
    addl $4, %esp
    popl %ecx
    movl %esp, %ebp
    addl $4, %ebp
loop1:
    pushl %ecx
    pushl (%ebp)
    pushl $output2
    call printf
    addl $8, %esp
    popl %ecx
    addl $4, %ebp
    loop loop1
    pushl $0
    call exit

Compiled it with GCC (gcc cmd.S), maybe that's the problem? __libc_start_main modifies the stack in some way? Not quite sure...

Even worse, trying to debug it to look at the stack but GDB seems to throw a lot of printf-related stuff (one of them was printf.c: File not found or something similar).

Answer 1

With the help from @Michael, I was able to track down the problem.

Using %ebp as argv as @Michael suggested (he used %eax though). Another problem was that I needed to compare the value of (%ebp) with 0 (the null terminator) and end the program at that point.

Code:

    movl 8(%esp), %ebp  /* Get argv.  */

pr_arg:
    cmpl $0, (%ebp)
    je endit

    pushl %ecx
    pushl (%ebp)
    pushl $output2
    call printf
    addl $8, %esp       /* remove output2 and current argument.  */
    addl $4, %ebp

    popl %ecx
    loop pr_arg

    ret