How can I check if my wrapper function system call - read() is correct? [duplicate]

问题

Possible Duplicate:
Inline Assembler for wrapper function doesn’t work for some reason

I'm requested to write wrapper function for read , write , close , open & fork .

I've written 4 wrapper functions for read , write , close , open .

My questions are :

How can I write wrapper function for fork , using the 4 wrapper functions that I wrote for read , write , close & open ?
How can I check if the wrapper that I wrote is correct ? Here is the code for the wrapper function of read - called my_read :

ssize_t my_read(int fd, void *buf, size_t count)   
{    
      ssize_t res;

      __asm__ volatile(
        "int $0x80"        /* make the request to the OS */
        : "=a" (res),       /* return result in eax ("a") */
          "+b" (fd),     /* pass arg1 in ebx ("b") */
          "+c" (buf),     /* pass arg2 in ecx ("c") */
          "+d" (count)      /* pass arg3 in edx ("d") */
        : "a"  (5)          /* passing the system call for read to %eax , with call number 5  */
        : "memory", "cc"); 

      /* The operating system will return a negative value on error;
       * wrappers return -1 on error and set the errno global variable */

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

      return res;
}

Remarks : I'm not allowed to use directly the open ,close ,read , write & fork commands .

I can attach the rest of the code of the other 3 wrappers , if it is needed . The above is the wrapper for read.

Regards

Ron

回答1:

Fork should be system call 2, so

    __asm__ volatile ("int $0x80" : "=a" (res) : "0" (2));

should work. Remember, fork returns twice, with res being the pid of the child(in the parent) and 0(in the child).

来源：https://stackoverflow.com/questions/10252848/how-can-i-check-if-my-wrapper-function-system-call-read-is-correct

标签

Linux

system-calls