sigurg signal

The receiving process needs to be notified when out-of-band data has arrived. This is particularly true if it must be read separately from the normal data stream. One such method for doing this is to have the Linux kernel send your process the SIGURG signal when out-of-band data has arrived.

There are two requirements for using SIGURG signal notification:

• You must establish ownership of the socket. 
• You must establish a signal handler for SIGURG.

To receive the SIGURG signal, you must establish your process (or process group) as the owner of the socket. To establish this ownership, you use the fcntl(2) function. Its function prototype as it applies to us here is as follows:

#include <unistd.h>

#include <fcntl.h>

int fcntl(int fd, int cmd, long arg);

The arguments for this function are as follows:

1. The file descriptor fd (or socket) to apply a control function to. 
2. The control function cmd to apply. 
3. The value arg to set (if any).

The return value depends upon the control function being exercised by fcntl(2). The Linux man page for fcntl(2) describes the cmd operation F_SETOWN in some detail, for those who are interested in additional reading. To establish your process (or process group) as the owner of a socket, the receiving program could use the following code:

Example

int z; /* Status */

int s; /* Socket */

z = fcntl(s, F_SETOWN, getpid());

if ( z == -1 ) {

   perror("fcntl(2)");

   exit(1);

}

The F_SETOWN operation causes the fcntl(2) function to return zero if successful, or -1 if it fails (errno indicates the cause of the failure). One additional requirement is that the program must be prepared to receive the signal SIGURG, which is done by establishing a signal handler for the signal. You'll see an example of this shortly.