CODEWITHSUNDEEP
cnetwork-programming
abyn
abyn

Reputation: 11

getsockname() can assign port to struct addr_in?

self._socket = socket(AF_INET, SOCK_DGRAM, 0);

// create addr
struct sockaddr_in addr;
bzero(&addr, sizeof(addr));
addr.sin_len = sizeof(addr);
addr.sin_family = AF_INET;
addr.sin_port = htons(0);
addr.sin_addr.s_addr = INADDR_ANY;

// bind socket
bind(self._socket, (struct sockaddr *)&addr, sizeof(addr));

printf("befor getsockname()->%d\n", ntohs(addr.sin_port));
socklen_t len = sizeof(addr);
getsockname(self._socket, (struct sockaddr *)&addr, &len); // if i comment this func, the last printf() will print 0; if not, it will print a real in use udp port(and it is correct!)
printf("after getsockname()->%d\n", ntohs(addr.sin_port));

So, is that when assign htons(0) to a port, the local socket must use getsockname() to assign a available port to itself? Or anything else? I think this is maybe because i just bind 0 to sin_port which mean to assign a random port but has not assigned by system yet, so just call getsockname() to make system really assign a port.

Upvotes: 1

Views: 855

Answers (2)

user207421
user207421

Reputation: 310869

So, is that when assign htons(0) to a port, the local socket must use getsockname() to assign a available port to itself? Or anything else?

No, you must use bind() to assign a port to the socket. getsockname() tells you what port was assigned if you specified zero.

Upvotes: 2

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215193

The whole point of the getsockname function is to get the sockaddr for the local side of the socket. For IPv4 sockets, this is an object of type sockaddr_in and it contains both the IP address and port.

Upvotes: 0

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