Write byte to file with x86 assembler

Question

I want to make a function in assembler to be called from c that will write a byte(char) to the file. Here is how function should look in c:

void writebyte (FILE *f, char b)
{
    fwrite(&b, 1, 1, f);
}

And here is the code that will call it:

#include <stdio.h>

extern void writebyte(FILE *, char);

int main(void) {
    FILE *f  = fopen("test.txt", "w");    
    writebyte(f, 1);
    fclose(f);
    return 0;
}

So far I came up with following assembler code:

    .global     writebyte  
writebyte:
    pushl   %ebp
    movl    %esp, %ebp  #standard params
    pushl   12(%ebp)    # pushing byte to the stack
    pushl   $1
    pushl   $1
    pushl   8(%ebp)     #file to write
    call    fwrite
    popl    %ebp
    ret

I keep getting from gdb:

Program terminated with signal SIGSEGV, Segmentation fault.
#0  0xffa9702c in ?? ()

How do I write such a function in assembly?

EDIT: I am using Ubuntu 16.04

As you don't have have std library with assembler, then the code will be OS-dependent. — Matt
– Matt, Commented May 4, 2016 at 13:48
The first parameter in calling fwrite is an address that points to the byte, not the byte itself; assuming that you have linked the program correctly, you are most likely getting a seg fault because fwrite is trying to use your byte as an address, and access memory at addresses that aren't mapped in your virtual address space. — Matt Jordan
– Matt Jordan, Commented May 4, 2016 at 13:53

Anton Angelov · Accepted Answer · 2016-05-06 07:50:36Z

3

According to cdecl convention, you should push the arguments in reverse order. So f should go first and b should go last. Also the stack should be cleaned up by the caller after calling fwrite().

As noted in the comments, b will be received as value, but we need to pass it to fwrite() as pointer. The pointer will be equal to the value of ebp + 12.

This seems to work for me:

    .global writebyte
writebyte:
    //create new stack frame
    pushl   %ebp
    movl    %esp, %ebp

    //push the four arguments to stack (in reverse order)
    pushl   8(%ebp)
    pushl   $1
    pushl   $1

    //get pointer of "b" argument (%ebp+12) and move it to %eax
    leal    12(%ebp), %eax
    pushl   %eax

    //call fwrite()
    call    fwrite

    //remove arguments from stack and pop %ebp
    leave

    ret

edited May 6, 2016 at 7:50

answered May 4, 2016 at 15:30

Anton Angelov

1,2447 silver badges19 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

polyx Over a year ago

why do you use underscore in call and function name?

Anton Angelov Over a year ago

Because I used Windows to compile this example and GCC on Windows requires the symbol names to begin with underscore, and actually prepends underscores to all the C functions. You can see that if you disassemble a compiled object files. But anyway, on Linux the underscores shoudn't be required, so I'll edit the post to remove them.

Peter Cordes Over a year ago

Why do you pusha/popa? You even comment the line with the fact that it's our callers problem if they're clobbered. That would let you use leave for the epilogue instead of add $16, %esp / pop %ebp. (And of course, the compiler wouldn't even bother making a stack frame, but gcc is dumb and copies the char into a local, rather than passing the address of its location on the stack. Also, gcc keeps the stack 16B-aligned for fwrite(). This tends not to matter, since stdio functions don't spill SSE vectors to the stack.)

Anton Angelov Over a year ago

You are right, the pusha/popa are not neccessary at all. Perhaps I wasn't paying attention when I was writing this. I'll edit the post to remove them. And it's interesting to see how GCC behaves.

Collectives™ on Stack Overflow

Write byte to file with x86 assembler

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related