libutil/flopen.c

   1 /*-
   2  * Copyright (c) 2007-2009 Dag-Erling Coïdan Smørgrav
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer
  10  *    in this position and unchanged.
  11  * 2. Redistributions in binary form must reproduce the above copyright
  12  *    notice, this list of conditions and the following disclaimer in the
  13  *    documentation and/or other materials provided with the distribution.
  14  *
  15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  17  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  19  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  21  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  22  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  23  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  24  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  25  * SUCH DAMAGE.
  26  */
  27
  28 #include <sys/cdefs.h>
  29 __FBSDID("$FreeBSD$");
  30
  31 #include <sys/file.h>
  32 #include <sys/stat.h>
  33
  34 #include <errno.h>
  35 #include <stdarg.h>
  36 #include <unistd.h>
  37
  38 #include <libutil.h>
  39
  40 /*
  41  * Reliably open and lock a file.
  42  *
  43  * Please do not modify this code without first reading the revision history
  44  * and discussing your changes with <des@freebsd.org>.  Don't be fooled by the
  45  * code's apparent simplicity; there would be no need for this function if it
  46  * was easy to get right.
  47  */
  48 int
  49 flopen(const char *path, int flags, ...)
  50 {
  51         int fd, operation, serrno, trunc;
  52         struct stat sb, fsb;
  53         mode_t mode;
  54
  55 #ifdef O_EXLOCK
  56         flags &= ~O_EXLOCK;
  57 #endif
  58
  59         mode = 0;
  60         if (flags & O_CREAT) {
  61                 va_list ap;
  62
  63                 va_start(ap, flags);
  64                 mode = (mode_t)va_arg(ap, int); /* mode_t promoted to int */
  65                 va_end(ap);
  66         }
  67
  68         operation = LOCK_EX;
  69         if (flags & O_NONBLOCK)
  70                 operation |= LOCK_NB;
  71
  72         trunc = (flags & O_TRUNC);
  73         flags &= ~O_TRUNC;
  74
  75         for (;;) {
  76                 if ((fd = open(path, flags, mode)) == -1)
  77                         /* non-existent or no access */
  78                         return (-1);
  79                 if (flock(fd, operation) == -1) {
  80                         /* unsupported or interrupted */
  81                         serrno = errno;
  82                         (void)close(fd);
  83                         errno = serrno;
  84                         return (-1);
  85                 }
  86                 if (stat(path, &sb) == -1) {
  87                         /* disappeared from under our feet */
  88                         (void)close(fd);
  89                         continue;
  90                 }
  91                 if (fstat(fd, &fsb) == -1) {
  92                         /* can't happen [tm] */
  93                         serrno = errno;
  94                         (void)close(fd);
  95                         errno = serrno;
  96                         return (-1);
  97                 }
  98                 if (sb.st_dev != fsb.st_dev ||
  99                     sb.st_ino != fsb.st_ino) {
 100                         /* changed under our feet */
 101                         (void)close(fd);
 102                         continue;
 103                 }
 104                 if (trunc && ftruncate(fd, 0) != 0) {
 105                         /* can't happen [tm] */
 106                         serrno = errno;
 107                         (void)close(fd);
 108                         errno = serrno;
 109                         return (-1);
 110                 }
 111                 /*
 112                  * The following change is provided as a specific example to
 113                  * avoid.
 114                  */
 115 #if 0
 116                 if (fcntl(fd, F_SETFD, FD_CLOEXEC) != 0) {
 117                         serrno = errno;
 118                         (void)close(fd);
 119                         errno = serrno;
 120                         return (-1);
 121                 }
 122 #endif
 123                 return (fd);
 124         }
 125 }