Set O_APPEND mode for stdout/stderr and output-sync temporary files.

POSIX does not guarantee that writes will be atomic if a file is
opened for normal (non-append) output.  That means if multiple processes
are writing to the same file, output could be lost.  I can't think of
a real use-case where we would NOT want append for stdout/stderr, so
force it if we can.

1 files changed, 20 insertions, 0 deletions

diff --git a/misc.c b/misc.c
index 1b1441b..d3a1da5 100644
--- a/misc.c
+++ b/misc.c
@@ -22,6 +22,11 @@ this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include <stdarg.h>
 
+#ifdef HAVE_FCNTL_H
+# include <fcntl.h>
+#else
+# include <sys/file.h>
+#endif
 
 /* Compare strings *S1 and *S2.
    Return negative if the first is less, positive if it is greater,
@@ -897,6 +902,19 @@ get_path_max (void)
 #endif
 
 
+/* Set a file descriptor to be in O_APPEND mode.
+   If it fails, just ignore it.  */
+
+void
+set_append_mode (int fd)
+{
+#if defined(F_GETFL) && defined(F_SETFL) && defined(O_APPEND)
+  int flags = fcntl (fd, F_GETFL, 0);
+  if (flags >= 0)
+    fcntl (fd, F_SETFL, flags | O_APPEND);
+#endif
+}
+
 /* Provide support for temporary files.  */
 
 #ifndef HAVE_STDLIB_H
@@ -928,6 +946,8 @@ open_tmpfd ()
 
   fclose (tfile);
 
+  set_append_mode (fd);
+
   return fd;
 }