As stated in the introduction of this chapter, programs running in User Mode are allowed to send and receive signals. This means that a set of system calls must be defined to allow these kinds of operations. Unfortunately, due to historical reasons, several noncompatible system calls exist that serve essentially the same purpose. In order to ensure full compatibility with older Unix versions, Linux supports both older system calls and newer ones introduced in the POSIX standard. We shall describe some of the most significant POSIX system calls.
The kill(pid,sig) system call is commonly used to send signals; its corresponding service routine is the sys_kill( ) function. The integer pid parameter has several meanings, depending on its numerical value:
pid > 0
The sig signal is sent to the process whose PID is equal to pid.
pid = 0
The sig signal is sent to all processes in the same group of the calling process.
pid = -1
The signal is sent to all processes, except swapper (PID 0), init (PID 1), and current.
pid < -1
The signal is sent to all processes in the process group -pid.
The sys_kill( ) function invokes kill_something_info( ). This in turn invokes either send_sig_info( ), to send the signal to a single process, or kill_pg_info( ), to scan all processes and invoke send_sig_info( ) for each process in the destination group.
System V and BSD Unix variants also have a killpg( ) system call, which ...