C sendto works sometimes...just stops after some time... NO ERRORS

Question

Ive been getting strange results with the sendto function in C.(LINUX) What I am trying to do is to implement a reliable UDP kind of scheme.

The snippet of code provided below is a part of the timer that expires when a packet is dropped and sends a message to a process to retransmit that packet.

The weird problem which I am having is that when transferring a large file say... >300KB The code given below works perfectly (i.e. THE ELSE PART IS EXECUTED) But after a certain number of packets are sent...it executes the then part.!!!

THIS IS STRANGE BECAUSE THE CODE WORKED FINE FOR ABOUT 250 PACKETS BUT WITH THE 251 PACKET kabooom!!!

 n =  sendto(sockfd, &(forwardPeer->id), sizeof(forwardPeer->id), 0, (struct sockaddr*)&tcpd_addr, sizeof(tcpd_addr));
 if(n<0)
    printf("\n error sending to tcpdc");
 else 
    printf("\n message sent to tcpdc");

PLEASE HELP!!!! THanks in advance

Output `strerror(errno)` in case of error, so you know what happened. — Peter G., Mar 10 '11 at 08:05
Error sending msg to tcpdc Unknown error 134514723 using strerror — Rachit Sood, Mar 10 '11 at 08:25
The problem might be on the other end - the receiving end. And the sending end fails because there is no process left to receive your message? — Jonathan Leffler, Mar 10 '11 at 08:34
hmmm....im using the select function on the receiving end...inside a while(1) loop....but then if the same code works 200 times why does it crash the next time.... — Rachit Sood, Mar 10 '11 at 08:36
@Rachit Sood: "Bad file descriptor" means that the `sockfd` file descriptor got closed before the failing `sendto()` call (or that variable got overwritten by junk due to a bounds overflow in your program). — caf, Mar 10 '11 at 08:54
@caf I believe it could be the second problem....because the code is pretty huge ....any ideas on how I could deal with this?????? — Rachit Sood, Mar 10 '11 at 08:58
I am using a link list on the side where this error is occurring and while deleting nodes from the list I was simply re directing the pointers...of the previous and the next nodes...I try to free it and check if the error happens again — Rachit Sood, Mar 10 '11 at 09:02
i freed all the nodes ....not working ... could use some help here — Rachit Sood, Mar 10 '11 at 10:24

score 1 · Answer 1 · answered Mar 11 '11 at 03:20

The Bad file descriptor error means that the passed sockfd value is incorrect. Either:

The file descriptor has been closed; or
The value of that variable has been overwritten by junk, probably due to a bounds overflow somewhere in your program.

To catch the second case, run the program under a debugger and set a watchpoint on the sockfd variable - this will break into the debugger whenver the value changes, which should let you see where it's being changed when it shouldn't be.

You could also try running the program under valgrind, and fixing any issues it reports.

C sendto works sometimes...just stops after some time... NO ERRORS

1 Answers1