Setting standard signal SIGIO in GNU - c

Question

I found SIGIO signal documented in GNU library. It is stated that there is a possibility the system to send a signal whenever I have input (particularly in socket).
According to documentation to create such signal I should set O_ASYNC flag to corresponding filedescritor.
My problem is that my GNU version (gcc 6.3.0) doesn't recognize such a keyword:
error: ‘O_ASYNC’ undeclared (first use in this function)
I used following block to set a flag:
/* Set socket status async */
int status = fcntl(sockfd, F_SETFL, O_ASYNC);;
if (status < 0) error("Can't set async mode");
else printf("Async is set for signal %d\n", SIGIO);
I use Cigwin GCC 6.3.0
The code is below:
static void OnTimer(int sig)
{
/* write request to socket BIO */
}
static void OnTick(int sig)
{
char read[BUFSIZE] = {};
int received;
received = SSL_read(ssl, read, sizeof(read));
/* Handle errors here */
/* print the server's reply */
printf("%s\n\n", read);
}
void connectSSL()
{
/* do all socket set-up and connection */
/* Establish handler for I/O signal */
saction.sa_flags = 0;
saction.sa_handler = OnTick;
sigemptyset(&saction.sa_mask);
sigaddset(&saction.sa_mask, SIGALRM);
if (sigaction(SIGIO, &saction, NULL) == -1) error("sigaction");
else printf("OnTick handler created\n");
/* Set socket status async */
int status = fcntl(sockfd, F_SETFL, fcntl(sockfd, F_GETFL, 0) | FASYNC);
if (status < 0) error("Can't set async mode");
else printf("Async is set for signal %d\n", SIGIO);
/* Select the process to receive the signal */
int process = fcntl(sockfd, F_SETOWN, getpid());
if (process < 0) error("Can't set address process");
else printf("Process %d is set\n", getpid());
/* do the rest SSL staff here */
}
int main(int argc, char argv[])
{
sigset_t mask, oldmask;
/* Wait for OnTimer event*/
sigemptyset(&oldmask);
sigaddset(&oldmask, SIGALRM);
sigemptyset(&mask);
sigaddset(&mask, SIGALRM);
sigaddset(&mask, SIGIO);
sigprocmask(SIG_BLOCK, &mask, &oldmask);
connectSSL();
createTimer(500000000);
while(1)
{
printf("\nWaiting OnTimer %d\n", count + 1);
sigsuspend(&oldmask);
}
return 0;
}

Answer 1

Most probably you forgot to include sys/fcntl.h, either indirectly or directly. This is the place that defines the flags.
In case O_ASYNC is not defined, you can try and use the functionally equivalent predecessor of that flag, FASYNC.
Also make sure you have executed a
int flags;
/* Install signal handler here... */
/* Set flags to receive SIGIO (and SIGURG) signals */
flags = fcntl (F_GETFL, 0);
flags = fcntl (F_SETFL, flags | FASYNC);
/* Bind our process as receiver of SIGIO signals */
flags = fcntl (F_SETOWN, getpid ());