Make FILE*s less usable after fclose(3).

BSD doesn't invalidate the fd stored in struct FILE, which can make
it possible (via fileno(3), for example), to perform operations on
an fd you didn't intend to (rather than just failing with EBADF).

Fixing this makes the code slightly simpler anyway, and might help
catch bad code before it ships.

Bug: http://stackoverflow.com/questions/10816837/fclose-works-differently-on-android-and-linux
Change-Id: I9db74584038229499197a2695c70b58ed0372a87

1 files changed, 8 insertions, 1 deletions

diff --git a/libc/stdio/stdio_ext.cpp b/libc/stdio/stdio_ext.cpp
index f273d45fe..88e59510d 100644
--- a/libc/stdio/stdio_ext.cpp
+++ b/libc/stdio/stdio_ext.cpp
@@ -27,6 +27,8 @@
  */
 
 #include <stdio_ext.h>
+
+#include <errno.h>
 #include <stdlib.h>
 
 #include "local.h"
@@ -101,5 +103,10 @@ int ferror_unlocked(FILE* fp) {
 }
 
 int fileno_unlocked(FILE* fp) {
-  return __sfileno(fp);
+  int fd = fp->_file;
+  if (fd == -1) {
+    errno = EBADF;
+    return -1;
+  }
+  return fd;
 }