Recall from our earlier discussions in the first chapter that Linux supports multi-threaded applications through lightweight processes. All LWPs of a threaded application are part of a process group and share signal handlers; each LWP (thread) maintains its own pending, and blocked signal queues.

The signal pointer of the task structure refers to the instance of type signal_struct, which is the signal descriptor. This structure is shared by all LWPs of a thread group and maintains elements such as a shared pending signal queue (for signals queued to a thread group), which is common to all threads in a process group.

The following figure represents the data structures involved in maintaining shared pending signals:

Following are a few important fields of signal_struct:

struct signal_struct {
 atomic_t sigcnt;
 atomic_t live;
 int nr_threads;
 struct list_head thread_head;

 wait_queue_head_t wait_chldexit; /* for wait4() */

 /* current thread group signal load-balancing target: */
 struct task_struct *curr_target;

 /* shared signal handling: */
 struct sigpending shared_pending;

 /* thread group exit support */
 int group_exit_code;
 /* overloaded:
 * - notify group_exit_task when ->count is equal to notify_count
 * - everyone except group_exit_task is stopped during signal delivery
 * of fatal signals, group_exit_task processes the signal.
 */
 int notify_count;
 struct task_struct *group_exit_task;

 /* thread group stop support, overloads group_exit_code too */
 int group_stop_count;
 unsigned int flags; /* see SIGNAL_* flags below */