CODEWITHSUNDEEP
cnetwork-programming
misteryes
misteryes

Reputation: 2287

when the first datagram is received, recvfrom() doesn't fill the structur `sockaddr`

I have two machines A and B in LAN I have a UDP client on A and a UDP server on B, like 

for(;;){
    n = recvfrom(sockfd, mesg, 10000, 0, (struct sockaddr *)&cliaddr, &len);
    ....}

I notice when the UDP client sends the first datagram
I can get the datagram data payload through mesg correctly
but the structure cliaddr is not filled, it is with the original value
e.g, if I use bzero(&cliaddr, sizeof(cliaddr));,
in gdb, I got 

$1 = {sin_family = 0, sin_port = 0, sin_addr = {s_addr = 0}, sin_zero = 
 "\000\000\000\000\000\000\000"}

what is the reason when the first datagram is received, recvfrom() doesn't fill the structure cliaddr?

for the sebsequent datagram, the valid info can be obtained.

Upvotes: 3

Views: 781

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409176

Before calling recvfrom you must properly initialize the len argument.

E.g.

len = sizeof(struct sockaddr_in);
n = recvfrom(..., &len);

The recvfrom function uses the length to help determine what kind of structure the sockaddr pointer actually points to.

Upvotes: 6

Related Questions