Question

I’m trying to write a wrapper function for read() , under Linux .

Please go easy on me , since this is my first time using Wrappers 🙂

Given the code the my_wrappers.c file :

#include "my_wrappers.h"
#include <unistd.h>
#include <sys/types.h>

ssize_t my_read (int fd, void *buf, size_t count)
{
  long ret;
  extern int errno;

  __asm__ __volatile__ ("pushl %%ebx\n\t"
                        "movl %%esi,%%ebx\n\t"
                        "int $0x80\n\t"
                        "popl %%ebx"
                      : "=a" (ret)
                      : "0" (SYS_read), "S" ((long) fd),
                        "c" ((long) buf) "d" ((long) count): "bx");
  if (ret >= 0)
  {
    return (int) ret;
  }
  errno = -ret;
  return -1;
}

and my_wrappers.h file :

#ifndef __MY_WRAPPERS_H_
#define __MY_WRAPPERS_H_

#include <unistd.h>
#include <sys/types.h>


int my_open(const char *pathname, int flags, mode_t mode);
ssize_t my_write(int fd, const void *buf, size_t count);
ssize_t my_read(int fd, void *buf, size_t count);
int my_close(int fd);

pid_t my_fork(void);

#endif

I’m only allowed to use call the libc wrappers, i.e. , I’m not allowed to call open(), read() and etc.

What’s wrong with this code ? maybe because I didn’t use the number of the system call table ?

The error from Eclipse : – ‘SYS_read’ undeclared (first use in this function)

Regards

Ron

Answer 1

You need to #include <syscall.h> or possibly #include <sys/syscall.h>