Understanding the Linux Kernel

9.4. Real-Time Signals

The POSIX standard introduced a new class of signals denoted as real-time signals; the corresponding signal numbers range from 32 to 63. The main difference with respect to standard signals is that real-time signals of the same kind may be queued. This ensures that multiple signals sent will be received. Although the Linux kernel does not make use of real-time signals, it fully supports the POSIX standard by means of several specific system calls (see Section 9.5.6).

The queue of real-time signals is implemented as a list of signal_queue elements:

struct signal_queue { 
    struct signal_queue *next; 
    siginfo_t info; 
};

The info table of type siginfo_t was explained in Section 9.2.1; the next field points to the next element in the list.

Each process descriptor has two specific fields; sigqueue points to the first element of the queue of received real-time signals, while sigqueue_tail points to the next field of the last element of the queue.

When sending a signal, the send_sig_info( ) function checks whether its number is greater than 31; if so, it inserts the signal in the queue of real-time signals for the destination process.

Similarly, when receiving a signal, dequeue_signal( ) checks whether the signal number of the pending signal is greater than 31; if so, it extracts from the queue the element corresponding to the received signal. If the queue does not contain other signals of the same type, the function also clears the corresponding bit in current->signal ...