creating a substring on Linux IA-32 assembler (gas)

Question

I wanna create a substring (ministring) of 3 asciz chars out of my original (thestring). The thing ain't printing when being run so I don't know what the hell I'm I doing. Why it ain't printing? Am I creating the ministring correctly?

.section .data

thestring: .asciz "111010101"

ministring: .asciz ""

formatd:    .asciz "%d"
formats:    .asciz "%s"
formatc:    .asciz "%c"




.section .text

.globl _start

_start:

xorl %ecx, %ecx

ciclo:movb thestring(%ecx,1), %al
movzbl %al, %eax
movl %eax, ministring(%ecx,1)
incl %ecx
cmpl $3, %ecx
jl ciclo


movl thestring, %eax
pushl %eax
pushl $formats
call printf
addl $4, %esp


movl $1, %eax
movl $0, %ebx
int $0x80

Answer 1

You haven't reserved enough memory space to contain the null-terminated ministring which you're creating ... therefore, when you write to this memory, you're overwriting the value of formatd and formats (and so you're eventually passing something other than "%s" to printf).

Instead of your definition of the ministring memory location, try using the following :

ministring: .asciz "   "

---

Also, instead of this:

movl %eax, ministring(%ecx,1)

I don't understand why you aren't using this instead:

movb %al, ministring(%ecx,1)

---

Also, if you want to print the ministring, then instead of this:

movl thestring, %eax

Do this:

movl ministring, %eax

---

Also instead of this:

addl $4, %esp

Why not this:

addl $8, %esp

---

ALso I suggest that you use a debugger to:

• Step through the code
• Watch the values contained in registers and in memory as you step through
• Know the location of any segmentation fault