3.9 - How can I determine the local port number?

[Loco]

3.9 - How can I determine the local port number?

From: Fajun Shi (fajun@cs.msstate.edu):

Hi, my question is: When I write a client, how can I know the port number that the socket bound in my machine?

From: Jörg Jensch

Hi!

Use getsockname(3N) to find out the local port
number your client application uses. Here is a
simple example:
len = sizeof ( server );
if ( getsockname ( sock, &server, &len ) < 0 )
perror ( "getsockname" );
else
fprintf ( stderr, "local port number before connect: %d\n", ntohs ( ((struct sockaddr_in *)&server)->sin_port ));

connected = connect(sock, (struct sockaddr *) &address, sizeof(address));

len = sizeof ( server );
if ( getsockname ( sock, &server, &len ) < 0 )
perror ( "getsockname" );
else
fprintf ( stderr, "local port number after connect : %d\n", ntohs ( ((struct sockaddr_in *)&server)->sin_port ));

---------------------------------------

And here is the output I retrieve:

local port number before connect: 0
local port number after connect : 34994

As can be seen, the system do not bind any local port number for a socket before the client calls connect().

Greetings

Jayjay