Index: lib/libc/string/swab.3
===================================================================
--- lib/libc/string/swab.3	(revision 211522)
+++ lib/libc/string/swab.3	(working copy)
@@ -39,7 +39,7 @@
 .Sh SYNOPSIS
 .In unistd.h
 .Ft void
-.Fn swab "const void * restrict src" "void * restrict dst" "ssize_t len"
+.Fn swab "const void * restrict src" "void * restrict dst" "size_t len"
 .Sh DESCRIPTION
 The function
 .Fn swab
@@ -53,7 +53,8 @@
 .Pp
 The argument
 .Fa len
-must be an even number.
+must be an even number. For POSIX compatibility, if it's less than zero,
+nothing will be done.
 .Sh SEE ALSO
 .Xr bzero 3 ,
 .Xr memset 3
Index: lib/libc/string/swab.c
===================================================================
--- lib/libc/string/swab.c	(revision 211522)
+++ lib/libc/string/swab.c	(working copy)
@@ -45,6 +45,8 @@
 	int n;
 	char *fp, *tp;
 
+	if (len <= 0)
+		return;
 	n = len >> 1;
 	fp = (char *)from;
 	tp = (char *)to;