file_syscalls.c

#include <types.h>
#include <kern/errno.h>
#include <kern/unistd.h>
#include <lib.h>
#include <uio.h>
#include <syscall.h>
#include <vnode.h>
#include <vfs.h>
#include <current.h>
#include <proc.h>

/* handler for write() system call                  */
/*
 * n.b.
 * This implementation handles only writes to standard output 
 * and standard error, both of which go to the console.
 * Also, it does not provide any synchronization, so writes
 * are not atomic.
 *
 * You will need to improve this implementation
 */

int
sys_write(int fdesc,userptr_t ubuf,unsigned int nbytes,int *retval)
{
  struct iovec iov;
  struct uio u;
  int res;

  DEBUG(DB_SYSCALL,"Syscall: write(%d,%x,%d)\n",fdesc,(unsigned int)ubuf,nbytes);
  
  /* only stdout and stderr writes are currently implemented */
  if (!((fdesc==STDOUT_FILENO)||(fdesc==STDERR_FILENO))) {
    return EUNIMP;
  }
  KASSERT(curproc != NULL);
  KASSERT(curproc->console != NULL);
  KASSERT(curproc->p_addrspace != NULL);

  /* set up a uio structure to refer to the user program's buffer (ubuf) */
  iov.iov_ubase = ubuf;
  iov.iov_len = nbytes;
  u.uio_iov = &iov;
  u.uio_iovcnt = 1;
  u.uio_offset = 0;  /* not needed for the console */
  u.uio_resid = nbytes;
  u.uio_segflg = UIO_USERSPACE;
  u.uio_rw = UIO_WRITE;
  u.uio_space = curproc->p_addrspace;

  res = VOP_WRITE(curproc->console,&u);
  if (res) {
    return res;
  }

  /* pass back the number of bytes actually written */
  *retval = nbytes - u.uio_resid;
  KASSERT(*retval >= 0);
  return 0;
}