add support for caller-provided buffers to setvbuf

diff --git a/src/stdio/setvbuf.c b/src/stdio/setvbuf.c
index 541a125ff124b1c1b5aa15949f0890a3d12c6f6b..0a0c139ff820e9fc7cc2d7a57dee0e9e14e4a7ab 100644 (file)
--- a/src/stdio/setvbuf.c
+++ b/src/stdio/setvbuf.c
@@ -1,22 +1,25 @@
 #include "stdio_impl.h"
 
-/* This function makes no attempt to protect the user from his/her own
- * stupidity. If called any time but when then ISO C standard specifically
- * allows it, all hell can and will break loose, especially with threads!
- *
- * This implementation ignores all arguments except the buffering type,
- * and uses the existing buffer allocated alongside the FILE object.
- * In the case of stderr where the preexisting buffer is length 1, it
- * is not possible to set line buffering or full buffering. */
+/* The behavior of this function is undefined except when it is the first
+ * operation on the stream, so the presence or absence of lockign is not
+ * observable in a program whose behavior is defined. Thus no locking is
+ * performed here. No allocation of buffers is performed, but a buffer
+ * provided by the caller is used as long as it is suitably sized. */
 
 int setvbuf(FILE *restrict f, char *restrict buf, int type, size_t size)
 {
        f->lbf = EOF;
 
-       if (type == _IONBF)
+       if (type == _IONBF) {
                f->buf_size = 0;
-       else if (type == _IOLBF)
-               f->lbf = '\n';
+       } else {
+               if (buf && size >= UNGET) {
+                       f->buf = (void *)buf;
+                       f->buf_size = size - UNGET;
+               }
+               if (type == _IOLBF && f->buf_size)
+                       f->lbf = '\n';
+       }
 
        f->flags |= F_SVB;