Thread: recvfrom getting : Interrupted system call

Many of the blocking IO calls in linux may be interrupted by signals. If I interpreted your question correctly, that is what happened. I guess usual way to deal with interrupted blocking calls is to continue waiting the remaining wait time. I assume you have read the man pages?

I guess errno will have EINTR set, if recfrom was interrupted by a signal. Usually you check if that is the case, and if it is, you'll perform the same I/O call again. (If call accepts timeout [can't remember if recvfrom does], you should decrement the timeout by already elapsed time.) If there was data coming in, next call will collect the remaining data.

Or then I misinterpreted your question.

System call interruption is not an error (although it's reported like one). You must restart the system call. The general idiom is:

Code:

int err;

while((err = syscall(...)) < 0 && errno == EINTR)
    ;
if(err < 0)
    handle_error();

In this specific case, you should probably change your timeout to account for the time already spent.

If you treat syscall interruption as an error, your code is semantically wrong. You could receive a harmless signal at any time. And debuggers use signals to control processes, so running your program under a debugger could break, if you do not handle this correctly.

Originally Posted by -EquinoX-

What if instead of checking the flag I also check for system call interruption, if thats the case then it shouldn't go inside the body?

You have to check the return value first. The value of errno is not valid unless a system call has failed. So you have to check both.

And just treating it as if the call succeeded isn't right, either. The call didn't fail or succeed, it just never FINISHED. You can't continue doing things until you know whether it failed or succeeded -- you must restart

Originally Posted by -EquinoX-

so I should check what recvfrom returns right? if it's -1 then I should also check for the errno... then what do I do? what do you mean by restart? won't restarting it have the same effect? i.e: will return the system call interrupted

You should do pretty much what I indicated in the example code.

Restarting the call will not be interrupted again unless a second signal is delivered. In theory, it could happen, but you just need to keep looping until it is not interrupted.

Originally Posted by -EquinoX-

what should be the ...

syscall(...) stands in for whatever system call you are performing. In this case, it's your call to recvfrom()

EDIT: Maybe this is a dumb question, but why are you using SIGALRM? Are you trying to implement a timeout for recvfrom()? Because if so, that changes everything.

Originally Posted by -EquinoX-

so it would be:

Code:

while((err = syscall(recvfrom(sock_fd, ACK, sizeof(Frame), 0,  (struct sockaddr *) 0, (socklen_t *) 0))) < 0 && errno == EINTR)

No... the "syscall" part was just a stand-in for your actual system call. Other than that, you're correct.