system_data_types.7: Add 'socklen_t'

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/system_data_types.7

@@ -1251,6 +1251,34 @@ POSIX.1-2001 and later.
 .BR getsockname (2),
 .BR socket (2)
 .RE
+.\"------------------------------------- socklen_t --------------------/
+.TP
+.I socklen_t
+.RS
+.IR Include :
+.IR <sys/socket.h> .
+Alternatively,
+.IR <netdb.h> .
+.PP
+Describes the length of a socket address.
+According to POSIX,
+this shall be an integer type of at least 32 bits.
+.PP
+.IR "Conforming to" :
+POSIX.1-2001 and later.
+.PP
+.IR "See also" :
+.BR accept (2),
+.BR bind (2),
+.BR connect (2),
+.BR gethostbyaddr (2),
+.BR getnameinfo (2),
+.BR socket (2)
+.PP
+See also the
+.I sockaddr
+structure in this page.
+.RE
 .\"------------------------------------- ssize_t ----------------------/
 .TP
 .I ssize_t