C言語: write(2)の正しい使い方

NAME
       write - write to a file descriptor

SYNOPSIS
       #include 

       ssize_t write(int fd, const void *buf, size_t count);

DESCRIPTION
       write writes up to count bytes to the file referenced by the file descriptor fd from the buffer
       starting at buf.  POSIX requires that a read() which can be proved to occur after a write() has
       returned returns the new data.  Note that not all file systems are POSIX conforming.

RETURN VALUE
       On  success, the number of bytes written are returned (zero indicates nothing was written).  On
       error, -1 is returned, and errno is set appropriately.  If count is zero and the file  descrip-
       tor  refers to a regular file, 0 will be returned without causing any other effect.  For a spe-
       cial file, the results are not portable.

(1) write(fd, buf, 1024) => 256
(2) write(fd, buf+256, 768) => 768

/* 注: 下部により簡潔なコードが有るのでこれは使わないで下さい */
int n = bytes_to_write;
int w = 0;
int r = 0;
while ((w < n) && ((r = write(fd, buf+w, n-w)) >= 0)) {
    w += r;
}

if (r < 0) {
    close(fd);
    perror("write failed");
    exit(EXIT_FAILURE);
}

const char *p = buf;
const char * const endp = p + size;

while (p < endp) {
  int num_bytes = write(fd, p, endp - p);
  if (num_bytes < 0) {
    perror("write failed");
    break;
  }

  p += num_bytes;
}

const char *p = buf;
const char * const endp = p + size;

while (p < endp) {
  const int num_bytes = write(fd, p, endp - p);
  if (num_bytes < 0) {
    if (errno == EINTR) continue;
    perror("write failed");
    break;
  }

  p += num_bytes;
}

/*
 * gauche.h - Gauche scheme system header
 *
 *   Copyright (c) 2000-2005 Shiro Kawai, All rights reserved.
 *
 *   Redistribution and use in source and binary forms, with or without
 *   modification, are permitted provided that the following conditions
 *   are met:
 *
 *   1. Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *
 *   2. Redistributions in binary form must reproduce the above copyright
 *      notice, this list of conditions and the following disclaimer in the
 *      documentation and/or other materials provided with the distribution.
 *
 *   3. Neither the name of the authors nor the names of its contributors
 *      may be used to endorse or promote products derived from this
 *      software without specific prior written permission.
 *
 *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 *   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 *   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 *   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 *   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 *   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 *   TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 *   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 *   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 *   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 *   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *  $Id: gauche.h,v 1.442 2005/10/13 08:14:13 shirok Exp $
 */

/* System call wrapper */
#define SCM_SYSCALL3(result, expr, check)       \
  do {	                                        \
    (result) = (expr);                          \
    if ((check) && errno == EINTR) {            \
      ScmVM *vm__ = Scm_VM();                   \
      errno = 0;                                \
      SCM_SIGCHECK(vm__);                       \
    } else {                                    \
      break;                                    \
    }                                           \
  } while (1)

#define SCM_SYSCALL(result, expr) \
  SCM_SYSCALL3(result, expr, (result < 0))

const char *p = buf;
const char * const endp = p + size;

while (p < endp) {
  int num_bytes;
  SCM_SYSCALL(num_bytes, write(fd, p, endp - p));
  if (num_bytes < 0) {
    perror("write failed");
    break;
  }

  p += num_bytes;
}