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

1 files changed, 6 insertions, 0 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