writev.c

glibc 库, 不仅可以学习使用库函数,还可以学习函数的具体实现,是提高功力的好资料

/* writev supports all Linux kernels >= 2.0.
   Copyright (C) 1997, 1998, 2000, 2002, 2003 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <errno.h>
#include <stddef.h>
#include <sys/param.h>
#include <sys/uio.h>

#include <sysdep-cancel.h>
#include <sys/syscall.h>
#include <bp-checks.h>

static ssize_t __atomic_writev_replacement (int, const struct iovec *,
					    int) internal_function;


/* Not all versions of the kernel support the large number of records.  */
#ifndef UIO_FASTIOV
# define UIO_FASTIOV	8	/* 8 is a safe number.  */
#endif


/* We should deal with kernel which have a smaller UIO_FASTIOV as well
   as a very big count.  */
static ssize_t
do_writev (int fd, const struct iovec *vector, int count)
{
  ssize_t bytes_written;

  bytes_written = INLINE_SYSCALL (writev, 3, fd, CHECK_N (vector, count), count);

  if (bytes_written >= 0 || errno != EINVAL || count <= UIO_FASTIOV)
    return bytes_written;

  return __atomic_writev_replacement (fd, vector, count);
}

ssize_t
__libc_writev (fd, vector, count)
     int fd;
     const struct iovec *vector;
     int count;
{
  if (SINGLE_THREAD_P)
    return do_writev (fd, vector, count);

  int oldtype = LIBC_CANCEL_ASYNC ();

  ssize_t result = do_writev (fd, vector, count);

  LIBC_CANCEL_RESET (oldtype);

  return result;
}
strong_alias (__libc_writev, __writev)
weak_alias (__libc_writev, writev)

#define __libc_writev static internal_function __atomic_writev_replacement
#include <sysdeps/posix/writev.c>