document that fileno(3) returns -1 for some kinds of FILE * objects;

triggerd by but simpler than a similar patch sent in
by Simon Branch <simonmbranch at gmail dot com>;
OK millert@ jmc@

diff --git a/lib/libc/stdio/ferror.3 b/lib/libc/stdio/ferror.3
index 526c07e..0f0bdf2 100644 (file)
--- a/lib/libc/stdio/ferror.3
+++ b/lib/libc/stdio/ferror.3
@@ -1,4 +1,4 @@
-.\"    $OpenBSD: ferror.3,v 1.10 2013/07/17 05:42:11 schwarze Exp $
+.\"    $OpenBSD: ferror.3,v 1.11 2021/10/31 16:56:47 schwarze Exp $
 .\"
 .\" Copyright (c) 1990, 1991, 1993
 .\"    The Regents of the University of California.  All rights reserved.
@@ -31,7 +31,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: July 17 2013 $
+.Dd $Mdocdate: October 31 2021 $
 .Dt FERROR 3
 .Os
 .Sh NAME
@@ -73,9 +73,14 @@ function.
 .Pp
 The function
 .Fn fileno
-examines the argument
+returns the file descriptor associated with the given
 .Fa stream
-and returns its integer file descriptor.
+or \-1 if it is not associated with any file descriptor,
+for example if it was created with
+.Xr fmemopen 3 ,
+.Xr open_memstream 3 ,
+or
+.Xr funopen 3 .
 .Sh ERRORS
 These functions should not fail and do not set the external
 variable