Non blocking read never returns

Question

I am trying to do a non blocking read but the function never returns. Can someone suggest something? Here is my code to set nonblocking fd.

from_ap = open(FFS_GBEMU_OUT, O_RDWR|O_NONBLOCK);
if (from_ap < 0)
    return from_ap;

I have also tried this with similar results

from_ap = open(FFS_GBEMU_OUT, O_RDWR);
int status = fcntl(from_ap, F_SETFL, fcntl(from_ap, F_GETFL, 0) | O_NONBLOCK);

if (status == -1){
perror("calling fcntl");

Here is where I call my read function:

rsize = read(from_ap, cport_rbuf, ES1_MSG_SIZE);
if (rsize < 0) {
    printf("error %zd receiving from AP\n", rsize);
    return NULL;
}

I have also tried this with similar results:

fd_set readset;
struct timeval tv;
FD_ZERO(&readset);
FD_SET(from_ap, &readset);
tv.tv_sec = 0;
tv.tv_usec = 100;

result = select(from_ap+1, &readset, NULL, NULL, &tv);

if (result > 0 && FD_ISSET(from_ap, &readset)){
    printf("there was something to read\n");
    rsize=read(from_ap,cport_rbuf,ES1_MSG_SIZE);
}

The last message received is "there was something to read" and code does not progress further. What I am doing wrong? This is not a multithreaded program so no one can change flags but I have anyways confirmed them with printing back the flags before reading.

Answer 1

Does the device support O_NONBLOCK? This appears to be code from GitHub for gbsim . Read up on gbsim, it's entirely possible the driver does not support non-blocking calls.