1 signals

Signal is notification to a process/thread from the system. During generation of a signal it is determined whether it is addressed to a process or a thread.

hardware exceptions
actions by processes: kill(), alarm(), etc

Some popular signals:

Nr	Name	Meaning	Action
1	SIGHUP	Hangup	Exit
2	SIGINT	tty interrupt (typically: ^C)	Exit
9	SIGKILL	Unconditional process termination	Exit
11	SIGSEGV	Segmentation Fault	Core dump + exit

1.1 signals in C

int kill(pid_t pid, int sig)

Where pid is the process ID, and sig is the signal number.

The signal is sent to:

pid > 0: a process with given PID
pid == 0: all processes that belong to the process group of the sender
pid == -1: all processes in the system except init
pid < -1: to all processes that belong to the process group where the process group id is -pid

1.2 handling signals

Each process has a defined set of actions to take when a signal is received. In C, these actions can be overwritten with custom behavior. When handling a signal there is no information where did the signal come from, just the number.

The main program is stopped when handling a signal, therefore it is not advised to handle signals for too long (ex. don’t do IO).