Wrapper function for write() doesn't work as expected

Question

Given the wrapper function for system call write :

ssize_t my_write(int fd, const void *buf, size_t count)
{
    long __res;
    __asm__ volatile
        ("int $0x80"
                : "=a" (__res)
                : "0" (4),"D" ((long)(fd)),"S" ((long)(buf)), "d" ((long)(count))
                : "ebx","memory");

      if (-125 <= __res && __res < 0)
      {
        errno = -__res;
        __res   = -1;
      }

      return __res;
}

I've tried it with the code (from int main()) :

int main() {


     my_write(2,"an read error occured\n",26);


     return 0;
 }

However it doesn't work . Any idea why ?

Thanks

Try reading this question: http://stackoverflow.com/questions/499188/how-is-the-system-call-in-linux-implemented — ugoren, Apr 22 '12 at 12:55
That is your fourth question in the same direction. what kind of project is that where you are forced to implement all low level IO with assembler calls into the kernel? — Jens Gustedt, Apr 22 '12 at 16:02
@JensGustedt: Friend,just making a conversation . As you can see I'm not asking you to solve my homework , just correct me when I'm wrong. — JAN, Apr 22 '12 at 16:47

score 3 · Accepted Answer · answered Apr 22 '12 at 12:54

3

Your constraints are off, the file descriptor needs to go in EBX, the buffer in ECX (not EDI/ESI respectively like you have).

Try:

__asm__ volatile
    ("int $0x80"
            : "=a" (__res)
            : "0" (4),"b" ((long)(fd)),"c" ((long)(buf)), "d" ((long)(count))
            : "memory");

answered Apr 22 '12 at 12:54

Mat

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thank you , can you please direct me to a manual that can tell me exactly which registers do I need for each system call ? – JAN Apr 22 '12 at 13:00
See http://www.win.tue.nl/~aeb/linux/lk/lk-4.html: " The system call number goes into %eax, the first parameter in %ebx, the second in %ecx, the third in %edx, the fourth in %esi, the fifth in %edi, the sixth in %ebp." – Mat Apr 22 '12 at 13:04
@R..: what makes you think it was for x86_64 anyway? The only register name in the question is EBX. – Mat Apr 22 '12 at 14:16
edi, esi, and edx are the argument registers for x86_64 – R.. GitHub STOP HELPING ICE Apr 22 '12 at 15:06
@R..: this is `int 0x80`, not `syscall` – Mat Apr 22 '12 at 15:17
Yes, I noticed. I don't claim the code *worked* on x86_64, just that OP was mistakenly doing some things in the x86_64 way. – R.. GitHub STOP HELPING ICE Apr 22 '12 at 15:18

Wrapper function for write() doesn't work as expected

1 Answers1