Write byte to file with x86 assembler - c

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

Answer 1

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