linux: Implement tmpfile with O_TMPFILE (BZ#21530)

This patch adds O_TMPFILE support to tmpfile on Linux.  This is
similar to the previous suggestion by Andreas Schwab [1] with the
difference the file descriptor creation is parameterized to
compartmentalize Linux only open flags (O_TMPFILE) on sysdeps.

Checked on x86_64-linux-gnu.

	Adhemerval Zanella  <adhemerval.zanella@linaro.org>
	Andreas Schwab  <schwab@suse.de>

	[BZ #21530]
	* include/stdio.h (__gen_tempfd): New function.
	* stdio-common/Makefile (routines): Add gentempfd.
	* stdio-common/gentempfd.c: New file.
	* sysdeps/unix/sysv/linux/gentempfd.c: Likewise.
	* stdio-common/tmpfile.c (tmpfile): First try to use a system specific
	unnamed file first.

[1] https://sourceware.org/ml/libc-alpha/2017-06/msg01293.html

1 files changed, 17 insertions, 9 deletions

diff --git a/stdio-common/tmpfile.c b/stdio-common/tmpfile.c
index e6030be0af..c5cbb688f4 100644
--- a/stdio-common/tmpfile.c
+++ b/stdio-common/tmpfile.c
@@ -34,23 +34,31 @@
 FILE *
 tmpfile (void)
 {
-  char buf[FILENAME_MAX];
   int fd;
   FILE *f;
-
-  if (__path_search (buf, FILENAME_MAX, NULL, "tmpf", 0))
-    return NULL;
   int flags = 0;
 #ifdef FLAGS
   flags = FLAGS;
 #endif
-  fd = __gen_tempname (buf, 0, flags, __GT_FILE);
+
+  /* First try a system specific method.  */
+  fd = __gen_tempfd (flags);
+
   if (fd < 0)
-    return NULL;
+    {
+      char buf[FILENAME_MAX];
+
+      if (__path_search (buf, sizeof buf, NULL, "tmpf", 0))
+	return NULL;
+
+      fd = __gen_tempname (buf, 0, flags, __GT_FILE);
+      if (fd < 0)
+	return NULL;
 
-  /* Note that this relies on the Unix semantics that
-     a file is not really removed until it is closed.  */
-  (void) __unlink (buf);
+      /* Note that this relies on the Unix semantics that
+	 a file is not really removed until it is closed.  */
+      (void) __unlink (buf);
+    }
 
   if ((f = __fdopen (fd, "w+b")) == NULL)
     __close (fd);