Document memset_s(3). memset_s(3) is defined in

C11 standard (ISO/IEC 9899:2011) K.3.7.4.1 The memset_s function
(p: 621-622)

Fix memset(3) portion of the man page by replacing the first argument
(destination) "b" with "dest", which is more descriptive than "b".
This also makes it consistent with the term used in the memset_s()
portion of the man page.

See also http://en.cppreference.com/w/c/string/byte/memset.

Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D13682

lib/libc/string/Makefile.inc

@@ -50,6 +50,7 @@ MLINKS+=ffs.3 ffsl.3 \
 	ffs.3 flsll.3
 MLINKS+=index.3 rindex.3
 MLINKS+=memchr.3 memrchr.3
+MLINKS+=memset.3 memset_s.3
 MLINKS+=strcasecmp.3 strncasecmp.3 \
 	strcasecmp.3 strcasecmp_l.3 \
 	strcasecmp.3 strncasecmp_l.3

lib/libc/string/memset.3

@@ -32,7 +32,7 @@
 .\"     @(#)memset.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd June 4, 1993
+.Dd February 15, 2018
 .Dt MEMSET 3
 .Os
 .Sh NAME
@@ -43,7 +43,9 @@
 .Sh SYNOPSIS
 .In string.h
 .Ft void *
-.Fn memset "void *b" "int c" "size_t len"
+.Fn memset "void *dest" "int c" "size_t len"
+.Ft errno_t
+.Fn memset_s "void *dest" "rsize_t destsz" "int c" "rsize_t len"
 .Sh DESCRIPTION
 The
 .Fn memset
@@ -55,13 +57,66 @@ bytes of value
 (converted to an
 .Vt "unsigned char" )
 to the string
-.Fa b .
+.Fa dest .
+Undefined behaviour from
+.Fn memset ,
+resulting from storage overflow, will occur if
+.Fa len
+is greater than the the length of buffer
+.Fa dest .
+The behaviour is also undefined if
+.Fa dest
+is an invalid pointer.
+.Pp
+The
+.Fn memset_s
+function behaves the same as
+.Fn memset
+except that an error is returned and the currently registered
+runtime-constraint handler is called if
+.Fa dest
+is a null pointer,
+.Fa destsz
+or
+.Fa len
+is greater than
+.Dv RSIZE_MAX ,
+or
+.Sp
+.Fa len
+is greater than
+.Fa destsz
+(buffer overflow would occur).
+The runtime-constraint handler is called first and may not return.
+If it does return, an error is returned to the caller.
+Like
+.Xr explicit_bzero 3 ,
+.Fn memset_s
+is not removed through Dead Store Elimination (DSE), making it useful for
+clearing sensitve data.
+In contrast
+.Fn memset
+function 
+may be optimized away if the object modified by the function is not accessed
+again.
+To clear memory that will not subsequently be accessed it is advised to use
+.Fn memset_s
+instead of
+.Fn memset .
+For instance, a buffer containing a password should be cleared with
+.Fn memset_s
+before
+.Xr free 3 .
 .Sh RETURN VALUES
 The
 .Fn memset
 function returns its first argument.
+The
+.Fn memset_s
+function returns zero on success, non-zero on error.
 .Sh SEE ALSO
 .Xr bzero 3 ,
+.Xr explicit_bzero 3 ,
 .Xr swab 3 ,
 .Xr wmemset 3
 .Sh STANDARDS
@@ -70,3 +125,7 @@ The
 function
 conforms to
 .St -isoC .
+.Fn memset_s
+conforms to:
+.St -isoC-2011
+K.3.7.4.1.