Apparently setvbuf() size can be nonzero with _IONBF. That makes no sense, but is necessary if setbuf() is to work as it is defined at OpenGroup.org.

libc/stdio/lib_setbuf.c

@@ -59,11 +59,7 @@
  *
  *     setvbuf(stream, buf, _IONBF, BUFSIZ)
  *
- *    if buf is a null pointer.
+ *   if buf is a null pointer.
- *
- * EXCEPTION:  Currently, the NuttX setvbuf() expects the size argument to
- * be zero if the mode is __INOBF.  That is a descrepancy!  What would a
- * non-zero size mean in that case?
  *
  * Input Parameters:
  *   stream - The stream whose buffer will be modified
@@ -77,16 +73,15 @@
 
 void setbuf(FAR FILE *stream, FAR char *buf)
 {
+#ifndef CONFIG_STDIO_DISABLE_BUFFERING
+  int mode;
+
   DEBUGASSERT(stream != NULL);
 
-#ifndef CONFIG_STDIO_DISABLE_BUFFERING
+  mode = (buf != NULL) ? _IOFBF : _IONBF;
-  if (buf != NULL)
+  (void)setvbuf(stream, buf, mode, BUFSIZ);
-    {
+
-      (void)setvbuf(stream, buf, _IOFBF, BUFSIZ);
+#else
-    }
+  DEBUGASSERT(stream != NULL);
-  else
-    {
-      (void)setvbuf(stream, NULL, _IONBF, 0);
-    }
 #endif
 }

libc/stdio/lib_setvbuf.c

@@ -113,16 +113,6 @@ int setvbuf(FAR FILE *stream, FAR char *buffer, int mode, size_t size)
       goto errout;
     }
 
-  /* A non-zero size (or a non-NULL buffer) with mode = _IONBF makes no
-   * sense.
-   */
-
-  if (mode == _IONBF && size > 0)
-    {
-      errcode = EINVAL;
-      goto errout;
-    }
-
   /* My assumption is that if size is zero for modes {_IOFBF, _IOLBF} the
    * caller is only attempting to change the buffering mode.  In this case,
    * the existing buffer should be re-used (if there is one).  If there is no
@@ -136,6 +126,17 @@ int setvbuf(FAR FILE *stream, FAR char *buffer, int mode, size_t size)
       size = BUFSIZ;
     }
 
+  /* A non-zero size (or a non-NULL buffer) with mode = _IONBF makes no
+   * sense but is, apparently, permissible. We simply force the buffer to
+   * NULL and size to zero in this case without complaining.
+   */
+
+  else if (mode == _IONBF)
+    {
+      buffer = NULL;
+      size   = 0;
+    }
+
   /* Make sure that we have exclusive access to the stream */
 
   lib_take_semaphore(stream);
@@ -229,7 +230,6 @@ int setvbuf(FAR FILE *stream, FAR char *buffer, int mode, size_t size)
       case _IONBF:
         /* No buffer needed... We must be performing unbuffered I/O */
 
-        DEBUGASSERT(size == 0);
         newbuf = NULL;
         break;