libc: Allow setting close-on-exec in fopen/freopen/fdopen.

This commit adds a new mode option 'e' that must follow any 'b', '+' and/or
'x' options. C11 is clear about the 'x' needing to follow 'b' and/or '+' and
that is what we implement; therefore, require a strict position for 'e' as
well.

For freopen() with a non-NULL path argument and fopen(), the close-on-exec
flag is set iff the 'e' mode option is specified. For freopen() with a NULL
path argument and fdopen(), the close-on-exec flag is turned on if the 'e'
mode option is specified and remains unchanged otherwise.

Although the same behaviour for fopen() can be obtained by open(O_CLOEXEC)
and fdopen(), this needlessly complicates the calling code.

Apart from the ordering requirement, the new option matches glibc.

PR:		kern/169320

4 files changed, 33 insertions, 2 deletions

diff --git a/lib/libc/stdio/fdopen.c b/lib/libc/stdio/fdopen.c
index 26e2cd7..8fc90a4 100644
--- a/lib/libc/stdio/fdopen.c
+++ b/lib/libc/stdio/fdopen.c
@@ -80,6 +80,12 @@ fdopen(fd, mode)
 
 	if ((fp = __sfp()) == NULL)
 		return (NULL);
+
+	if ((oflags & O_CLOEXEC) && _fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
+		fp->_flags = 0;
+		return (NULL);
+	}
+
 	fp->_flags = flags;
 	/*
 	 * If opened for appending, but underlying descriptor does not have
diff --git a/lib/libc/stdio/flags.c b/lib/libc/stdio/flags.c
index 0b6b075..e445fed 100644
--- a/lib/libc/stdio/flags.c
+++ b/lib/libc/stdio/flags.c
@@ -97,6 +97,7 @@ __sflags(mode, optr)
 
 	/* 'x' means exclusive (fail if the file exists) */
 	if (*mode == 'x') {
+		mode++;
 		if (m == O_RDONLY) {
 			errno = EINVAL;
 			return (0);
@@ -104,6 +105,10 @@ __sflags(mode, optr)
 		o |= O_EXCL;
 	}
 
+	/* set close-on-exec */
+	if (*mode == 'e')
+		o |= O_CLOEXEC;
+
 	*optr = m | o;
 	return (ret);
 }
diff --git a/lib/libc/stdio/fopen.3 b/lib/libc/stdio/fopen.3
index 08438e7..da89ed4 100644
--- a/lib/libc/stdio/fopen.3
+++ b/lib/libc/stdio/fopen.3
@@ -32,7 +32,7 @@
 .\"     @(#)fopen.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd October 17, 2011
+.Dd November 30, 2012
 .Dt FOPEN 3
 .Os
 .Sh NAME
@@ -97,6 +97,14 @@ or
 causes the
 .Fn fopen
 call to fail if the file already exists.
+An optional
+.Dq Li e
+following the above
+causes the
+.Fn fopen
+call to set the
+.Dv FD_CLOEXEC
+flag on the underlying file descriptor.
 .Pp
 The
 .Fa mode
@@ -144,6 +152,11 @@ of the stream must be compatible with the mode of the file descriptor.
 The
 .Dq Li x
 mode option is ignored.
+If the
+.Dq Li e
+mode option is present, the
+.Dv FD_CLOEXEC
+flag is set, otherwise it remains unchanged.
 When the stream is closed via
 .Xr fclose 3 ,
 .Fa fildes
@@ -277,3 +290,7 @@ The
 function
 conforms to
 .St -p1003.1-88 .
+The
+.Dq Li e
+mode option does not conform to any standard
+but is also supported by glibc.
diff --git a/lib/libc/stdio/freopen.c b/lib/libc/stdio/freopen.c
index be7bc8a..8d1ec91 100644
--- a/lib/libc/stdio/freopen.c
+++ b/lib/libc/stdio/freopen.c
@@ -118,6 +118,8 @@ freopen(file, mode, fp)
 			(void) ftruncate(fp->_file, (off_t)0);
 		if (!(oflags & O_APPEND))
 			(void) _sseek(fp, (fpos_t)0, SEEK_SET);
+		if (oflags & O_CLOEXEC)
+			(void) _fcntl(fp->_file, F_SETFD, FD_CLOEXEC);
 		f = fp->_file;
 		isopen = 0;
 		wantfd = -1;
@@ -194,7 +196,8 @@ finish:
 	 * assume stderr is always fd STDERR_FILENO, even if being freopen'd.
 	 */
 	if (wantfd >= 0) {
-		if (_dup2(f, wantfd) >= 0) {
+		if ((oflags & O_CLOEXEC ? _fcntl(f, F_DUP2FD_CLOEXEC, wantfd) :
+		    _dup2(f, wantfd)) >= 0) {
 			(void)_close(f);
 			f = wantfd;
 		} else