fix multiple stdio functions' behavior on zero-length operations

previously, fgets, fputs, fread, and fwrite completely omitted locking
and access to the FILE object when their arguments yielded a zero
length read or write operation independent of the FILE state. this
optimization was invalid; it wrongly skipped marking the stream as
byte-oriented (a C conformance bug) and exposed observably missing
synchronization (a POSIX conformance bug) where one of these functions
could wrongly complete despite another thread provably holding the
lock.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/stdio/fgets.c b/src/stdio/fgets.c
index cf5b1039..d3f9819e 100644
--- a/src/stdio/fgets.c
+++ b/src/stdio/fgets.c
@@ -10,14 +10,16 @@ char *fgets(char *restrict s, int n, FILE *restrict f)
 	size_t k;
 	int c;
 
+	FLOCK(f);
+
 	if (n--<=1) {
+		f->mode |= f->mode-1;
+		FUNLOCK(f);
 		if (n) return 0;
 		*s = 0;
 		return s;
 	}
 
-	FLOCK(f);
-
 	while (n) {
 		z = memchr(f->rpos, '\n', f->rend - f->rpos);
 		k = z ? z - f->rpos + 1 : f->rend - f->rpos;