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, 2 insertions, 3 deletions

diff --git a/src/stdio/fread.c b/src/stdio/fread.c
index c461256c..33a65f58 100644
--- a/src/stdio/fread.c
+++ b/src/stdio/fread.c
@@ -8,11 +8,10 @@ size_t fread(void *restrict destv, size_t size, size_t nmemb, FILE *restrict f)
 	unsigned char *dest = destv;
 	size_t len = size*nmemb, l = len, k;
 
-	/* Never touch the file if length is zero.. */
-	if (!l) return 0;
-
 	FLOCK(f);
 
+	f->mode |= f->mode-1;
+
 	if (f->rend - f->rpos > 0) {
 		/* First exhaust the buffer. */
 		k = MIN(f->rend - f->rpos, l);