handle UB for invalid fflush() operations and mixing input and output without intervening fflush() or repositioning

1 files changed, 8 insertions, 0 deletions

diff --git a/src/stdio/fflush.c b/src/stdio/fflush.c
index e13f8979..cb5f49c6 100644
--- a/src/stdio/fflush.c
+++ b/src/stdio/fflush.c
@@ -27,6 +27,14 @@ int fflush(FILE *stream)
 		return 0;
 	}
 
+	if (!stream->write) {
+		UNDEFINED("attempt to fflush() an input stream");
+	}
+
+	if (stream->read && stream->operation == OP_INPUT) {
+		UNDEFINED("attempt to fflush() an update stream after input");
+	}
+
 	if (stream->bpos == 0) {
 		return 0;
 	}