[PATCH] sock_diag.7: Fix recvmsg() usage in the example

[Alejandro Colomar <alx.manpages@gmail.com>]

From: Pali Rohár <pali@kernel.org>

The msg_name field for recvmsg() call points to a caller-allocated buffer
nladdr that is used to return the source address of the (netlink) socket.

As recvmsg() does not read this buffer and fills it for a caller, do not
initialize it and instead check its value in the example.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
---
 man7/sock_diag.7 | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/man7/sock_diag.7 b/man7/sock_diag.7
index b732b2571..569106dd6 100644
--- a/man7/sock_diag.7
+++ b/man7/sock_diag.7
@@ -753,9 +753,7 @@ static int
 receive_responses(int fd)
 {
     long buf[8192 / sizeof(long)];
-    struct sockaddr_nl nladdr = {
-        .nl_family = AF_NETLINK
-    };
+    struct sockaddr_nl nladdr;
     struct iovec iov = {
         .iov_base = buf,
         .iov_len = sizeof(buf)
@@ -782,6 +780,11 @@ receive_responses(int fd)
         if (ret == 0)
             return 0;
 
+        if (nladdr.nl_family != AF_NETLINK) {
+            fputs("!AF_NETLINK\en", stderr);
+            return \-1;
+        }
+
         const struct nlmsghdr *h = (struct nlmsghdr *) buf;
 
         if (!NLMSG_OK(h, ret)) {
-- 
2.30.0