Add a warning against modifying this code without understanding it, and

an example of how not to make it more portable.  I've had this lying
around uncommitted since 2009...

diff --git a/libutil/flopen.c b/libutil/flopen.c
new file mode 100644 (file)
index 0000000..bc4119d
--- /dev/null
+++ b/libutil/flopen.c
@@ -0,0 +1,121 @@
+/*-
+ * Copyright (c) 2007-2009 Dag-Erling Coïdan Smørgrav
+ * 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
+ *    in this position and unchanged.
+ * 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.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/file.h>
+#include <sys/stat.h>
+
+#include <errno.h>
+#include <stdarg.h>
+#include <unistd.h>
+
+#include <libutil.h>
+
+/*
+ * Reliably open and lock a file.
+ *
+ * DO NOT, UNDER PAIN OF DEATH, modify this code without first reading the
+ * revision history and discussing your changes with <des@freebsd.org>.
+ * Don't be fooled by the code's apparent simplicity; there would be no
+ * need for this function if it was as easy to get right as you think.
+ */
+int
+flopen(const char *path, int flags, ...)
+{
+       int fd, operation, serrno, trunc;
+       struct stat sb, fsb;
+       mode_t mode;
+
+#ifdef O_EXLOCK
+       flags &= ~O_EXLOCK;
+#endif
+
+       mode = 0;
+       if (flags & O_CREAT) {
+               va_list ap;
+
+               va_start(ap, flags);
+               mode = (mode_t)va_arg(ap, int); /* mode_t promoted to int */
+               va_end(ap);
+       }
+
+        operation = LOCK_EX;
+        if (flags & O_NONBLOCK)
+                operation |= LOCK_NB;
+
+       trunc = (flags & O_TRUNC);
+       flags &= ~O_TRUNC;
+
+       for (;;) {
+               if ((fd = open(path, flags, mode)) == -1)
+                       /* non-existent or no access */
+                       return (-1);
+               if (flock(fd, operation) == -1) {
+                       /* unsupported or interrupted */
+                       serrno = errno;
+                       (void)close(fd);
+                       errno = serrno;
+                       return (-1);
+               }
+               if (stat(path, &sb) == -1) {
+                       /* disappeared from under our feet */
+                       (void)close(fd);
+                       continue;
+               }
+               if (fstat(fd, &fsb) == -1) {
+                       /* can't happen [tm] */
+                       serrno = errno;
+                       (void)close(fd);
+                       errno = serrno;
+                       return (-1);
+               }
+               if (sb.st_dev != fsb.st_dev ||
+                   sb.st_ino != fsb.st_ino) {
+                       /* changed under our feet */
+                       (void)close(fd);
+                       continue;
+               }
+               if (trunc && ftruncate(fd, 0) != 0) {
+                       /* can't happen [tm] */
+                       serrno = errno;
+                       (void)close(fd);
+                       errno = serrno;
+                       return (-1);
+               }
+#ifdef DONT_EVEN_THINK_ABOUT_IT
+               if (fcntl(fd, F_SETFD, FD_CLOEXEC) != 0) {
+                       serrno = errno;
+                       (void)close(fd);
+                       errno = serrno;
+                       return (-1);
+               }
+#endif
+               return (fd);
+       }
+}