Fix race condition in concurrent printf and fopen.

newlib/libc/stdio/findfp.c

 /*
  * Copyright (c) 1990 The Regents of the University of California.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms are permitted
  * provided that the above copyright notice and this paragraph are
  * duplicated in all such forms and that any documentation,
  * advertising materials, and other materials related to such
  * distribution and use acknowledge that the software was developed
  * by the University of California, Berkeley.  The name of the
@@ skipping 89 matching lines @@
  * Find a free FILE for fopen et al.
  */
 
 FILE *
 _DEFUN(__sfp, (d),
        struct _reent *d)
 {
   FILE *fp;
   int n;
   struct _glue *g;
+  int found_fp = 0;
 
   CHECK_INIT(_GLOBAL_REENT, NULL);
 
   __sfp_lock_acquire ();
 
   for (g = &_GLOBAL_REENT->__sglue;; g = g->_next)
     {
       for (fp = g->_iobs, n = g->_niobs; --n >= 0; fp++)
-        if (fp->_flags == 0)
-          goto found;
+        {
+          _flockfile(fp);
+          found_fp = (fp->_flags == 0);
+          _funlockfile(fp);
+          if (found_fp)
+            goto found;
+        }
       if (g->_next == NULL &&
           (g->_next = __sfmoreglue (d, NDYNAMIC)) == NULL)
         break;
     }
   __sfp_lock_release ();
   d->_errno = ENOMEM;
   return NULL;
 
 found:
   fp->_file = -1;               /* no file */
@@ skipping 167 matching lines @@
 }
 
 _VOID
 _DEFUN_VOID(__fp_unlock_all)
 {
   _CAST_VOID _fwalk (_REENT, __fp_unlock);
 
   __sfp_lock_release ();
 }
 #endif