Assembly - mmap2 called from c

I'm using AT&T syntax. I wrote code, which calls function mmap2 from c.

This is my C file:

#include <stdio.h>
int* callloc(int length);
int main(){
int *adres = callloc(1000);
printf("%u\n",adres);
return 0;
}

And this is my assembly function:

#define PROT_READ       0x1             /* Page can be read.  */
#define PROT_WRITE      0x2             /* Page can be written.  */
#define PROT_EXEC       0x4             /* Page can be executed.  */
#define PROT_NONE       0x0             /* Page can not be accessed.  */

#define __NR_mmap                90
#define __NR_munmap              91
#define __NR_mmap2              192


.data
MMAP2 = 192
MUNMAP = 91
PROT_READ = 0x1
MAP_ANONYMOUS = 0x20

.global callloc
callloc:
    #mmap2 function takes 6 arguments when called
    #void *addr - it must be equal 0
    #sieze_t length - this is variable from call in C
    #int prot - access level, I guess it should be equal 0x1
    #int flags - map_anonymous, then fd i offset are ignored
    #int fd, off_t offset - those parameters are ignored

    push %ebp
    mov %esp,%ebp       #esp has address of beginning of function
    xor %ebx,%ebx
    mov 8(%ebp),%ecx    #length of memory to allocate is kept in stack since 8 byte 
    mov $PROT_READ,%edx
    mov $MAP_ANONYMOUS,%esi

    mov $MMAP2,%eax
    int $0x80
    mov %ebp,%esp
    pop %ebp

    ret

My problem is - how could I check if this code is good? I guess that answer is in eax register, but - how can i access this value? I don't know how to use gdb in this case.

Solution

The first check should be to see if the returned value is a small negative number or not. Next, you can of course try to use it, as per your passed in flags. The simplest way is try to read the first byte. Furthermore, in gdb when stopped after the interrupt, you can compare the value in eax to the process memory map accessible using info proc mappings (or examining /proc directly in a separate terminal) to see whether it points to the beginning of an appropriate region. You can also use strace or ltrace to see the details about the performed system call.

Note that your code has various issues:

You put your code into the .data section and that won't work. You should change that to .text.
Your #define constants conflict with the asm versions, keep one or the other. It doesn't even assemble as-is.
fd may be ignored, but it's best to set it to -1
One of MAP_SHARED or MAP_PRIVATE must be set
C calling convention mandates you preserve some registers including ebx and esi but you destroy them. You should save them on the stack using push and pop them back after the system call.