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

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 <[email protected]>

---
 man7/sock_diag.7 | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

man7/sock_diag.7 +6 -3

index 91ccf3ac5..191e6ac49 100644
@@ -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)) {