6 files changed, 36 insertions, 16 deletions
diff --git a/src/stdio/fclose.c b/src/stdio/fclose.c
index 8fdc3f7d..92bf7ff8 100644
--- a/src/stdio/fclose.c
+++ b/src/stdio/fclose.c
@@ -3,9 +3,12 @@
 int fclose(FILE *f)
 {
 	int r;
-	int perm = f->flags & F_PERM;
+	int perm;
+	
+	/* This lock is not paired with any unlock. */
+	FLOCK(f);
 
-	if (!perm) {
+	if (!(perm = f->flags & F_PERM)) {
 		OFLLOCK();
 		if (f->prev) f->prev->next = f->next;
 		if (f->next) f->next->prev = f->prev;
diff --git a/src/stdio/feof.c b/src/stdio/feof.c
index 5d7f4b02..56da6b91 100644
--- a/src/stdio/feof.c
+++ b/src/stdio/feof.c
@@ -4,7 +4,10 @@
 
 int feof(FILE *f)
 {
-	return !!(f->flags & F_EOF);
+	FLOCK(f);
+	int ret = !!(f->flags & F_EOF);
+	FUNLOCK(f);
+	return ret;
 }
 
 weak_alias(feof, feof_unlocked);
diff --git a/src/stdio/ferror.c b/src/stdio/ferror.c
index 8288a93d..d692eed9 100644
--- a/src/stdio/ferror.c
+++ b/src/stdio/ferror.c
@@ -4,7 +4,10 @@
 
 int ferror(FILE *f)
 {
-	return !!(f->flags & F_ERR);
+	FLOCK(f);
+	int ret = !!(f->flags & F_ERR);
+	FUNLOCK(f);
+	return ret;
 }
 
 weak_alias(ferror, ferror_unlocked);
diff --git a/src/stdio/fileno.c b/src/stdio/fileno.c
index 9ffb26d5..ba7f9391 100644
--- a/src/stdio/fileno.c
+++ b/src/stdio/fileno.c
@@ -2,6 +2,11 @@
 
 int fileno(FILE *f)
 {
+	/* f->fd never changes, but the lock must be obtained and released
+	 * anyway since this function cannot return while another thread
+	 * holds the lock. */
+	FLOCK(f);
+	FUNLOCK(f);
 	return f->fd;
 }
 
diff --git a/src/stdio/freopen.c b/src/stdio/freopen.c
index c80ce3b4..7ae116d8 100644
--- a/src/stdio/freopen.c
+++ b/src/stdio/freopen.c
@@ -4,8 +4,9 @@
  * hack the necessary parts of the new FILE into the old one, then
  * close the new FILE. */
 
-/* Locking is not necessary because, in the event of failure, the stream
- * passed to freopen is invalid as soon as freopen is called. */
+/* Locking IS necessary because another thread may provably hold the
+ * lock, via flockfile or otherwise, when freopen is called, and in that
+ * case, freopen cannot act until the lock is released. */
 
 int __dup3(int, int, int);
 
@@ -14,6 +15,8 @@ FILE *freopen(const char *restrict filename, const char *restrict mode, FILE *re
 	int fl = __fmodeflags(mode);
 	FILE *f2;
 
+	FLOCK(f);
+
 	fflush(f);
 
 	if (!filename) {
@@ -22,21 +25,22 @@ FILE *freopen(const char *restrict filename, const char *restrict mode, FILE *re
 		fl &= ~(O_CREAT|O_EXCL|O_CLOEXEC);
 		if (syscall(SYS_fcntl, f->fd, F_SETFL, fl) < 0)
 			goto fail;
-		return f;
 	} else {
 		f2 = fopen(filename, mode);
 		if (!f2) goto fail;
 		if (f2->fd == f->fd) f2->fd = -1; /* avoid closing in fclose */
 		else if (__dup3(f2->fd, f->fd, fl&O_CLOEXEC)<0) goto fail2;
-	}
 
-	f->flags = (f->flags & F_PERM) | f2->flags;
-	f->read = f2->read;
-	f->write = f2->write;
-	f->seek = f2->seek;
-	f->close = f2->close;
+		f->flags = (f->flags & F_PERM) | f2->flags;
+		f->read = f2->read;
+		f->write = f2->write;
+		f->seek = f2->seek;
+		f->close = f2->close;
 
-	fclose(f2);
+		fclose(f2);
+	}
+
+	FUNLOCK(f);
 	return f;
 
 fail2:
diff --git a/src/stdio/fwide.c b/src/stdio/fwide.c
index f4da47f6..48480685 100644
--- a/src/stdio/fwide.c
+++ b/src/stdio/fwide.c
@@ -5,6 +5,8 @@
 
 int fwide(FILE *f, int mode)
 {
-	if (!f->mode) f->mode = NORMALIZE(mode);
-	return f->mode;
+	FLOCK(f);
+	if (!f->mode) mode = f->mode = NORMALIZE(mode);
+	FUNLOCK(f);
+	return mode;
 }