diff --git a/include/string.h b/include/string.h
index 3844d41738ec..efc8aecec081 100644
--- a/include/string.h
+++ b/include/string.h
@@ -108,8 +108,19 @@ char	*strtok_r(char *, const char *, char **);
 #endif
 size_t	 strxfrm(char * __restrict, const char * __restrict, size_t);
 #if __BSD_VISIBLE
-void	 swab(const void *, void *, size_t);
-#endif
+
+#ifndef _SWAB_DECLARED
+#define _SWAB_DECLARED
+
+#ifndef _SSIZE_T_DECLARED
+typedef	__ssize_t	ssize_t;
+#define	_SSIZE_T_DECLARED
+#endif /* _SIZE_T_DECLARED */
+
+void	 swab(const void * __restrict, void * __restrict, ssize_t);
+#endif /* _SWAB_DECLARED */
+
+#endif /* __BSD_VISIBLE */
 __END_DECLS
 
 #endif /* _STRING_H_ */
diff --git a/include/unistd.h b/include/unistd.h
index 56551f8cd40c..fa7346298c08 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -436,7 +436,12 @@ ssize_t	 pwrite(int, const void *, size_t, off_t);
 int	 setpgrp(pid_t _pid, pid_t _pgrp); /* obsoleted by setpgid() */
 int	 setregid(gid_t, gid_t);
 int	 setreuid(uid_t, uid_t);
-/* void	 swab(const void * __restrict, void * __restrict, ssize_t); */
+
+#ifndef _SWAB_DECLARED
+#define _SWAB_DECLARED
+void	 swab(const void * __restrict, void * __restrict, ssize_t);
+#endif /* _SWAB_DECLARED */
+
 void	 sync(void);
 useconds_t	 ualarm(useconds_t, useconds_t);
 int	 usleep(useconds_t);
diff --git a/lib/libc/string/swab.3 b/lib/libc/string/swab.3
index 4d27e17d5b3f..93d879d2a35f 100644
--- a/lib/libc/string/swab.3
+++ b/lib/libc/string/swab.3
@@ -41,9 +41,9 @@
 .Sh LIBRARY
 .Lb libc
 .Sh SYNOPSIS
-.In string.h
+.In unistd.h
 .Ft void
-.Fn swab "const void * restrict src" "void * restrict dst" "size_t len"
+.Fn swab "const void * restrict src" "void * restrict dst" "ssize_t len"
 .Sh DESCRIPTION
 The function
 .Fn swab
diff --git a/lib/libc/string/swab.c b/lib/libc/string/swab.c
index aa6f2682c01b..8c0c43febfde 100644
--- a/lib/libc/string/swab.c
+++ b/lib/libc/string/swab.c
@@ -40,10 +40,10 @@ static char sccsid[] = "@(#)swab.c	8.1 (Berkeley) 6/4/93";
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include <string.h>
+#include <unistd.h>
 
 void
-swab(const void * __restrict from, void * __restrict to, size_t len)
+swab(const void * __restrict from, void * __restrict to, ssize_t len)
 {
 	unsigned long temp;
 	int n;