recv.2: Rework discussion of 'src_addr' and 'addrlen' for recvfrom()

The existing text was a bit opaque.

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

man2/recv.2

@@ -242,16 +242,12 @@ However, the call may still return less data than requested if
 a signal is caught, an error or disconnect occurs, or the next data to be
 received is of a different type than that returned.
 .SS recvfrom()
-.PP
 If
 .I src_addr
-is not NULL, and the underlying protocol provides the source address,
-this source address is filled in.
-When
-.I src_addr
-is NULL, nothing is filled in; in this case,
-.I addrlen
-is not used, and should also be 0.
+is not NULL,
+and the underlying protocol provides the source address of the message,
+that source address is placed in the buffer pointed to by
+.IR src_addr .
 .\" (Note: for datagram sockets in both the UNIX and Internet domains,
 .\" .I src_addr
 .\" is filled in.
@@ -260,17 +256,25 @@ is not used, and should also be 0.
 .\" filled in for stream sockets in the Internet domain.)
 .\" [The above notes on AF_UNIX and AF_INET sockets apply as at
 .\" Kernel 2.4.18. (MTK, 22 Jul 02)]
-The argument
+In this case,
 .I addrlen
-is a value-result argument,
-which the caller should initialize before the call to
-the size of the buffer associated with
-.IR src_addr ,
-and modified on return to indicate the actual size of the source address.
+is a value-result argument.
+Before the call,
+it should be initialized to the size of the buffer associated with
+.IR src_addr .
+Upon return,
+.I addrlen
+is updated to contain the actual size of the source address.
 The returned address is truncated if the buffer provided is too small;
 in this case,
 .I addrlen
 will return a value greater than was supplied to the call.
+
+If the caller is not interested in the source address,
+.I src_addr
+should be specified as NULL and
+.I addrlen
+should be specified as 0.
 .SS recv()
 The
 .BR recv ()