Understanding the Linux Kernel, Second Edition

System Calls Related to Signal Handling

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, for historical reasons, several system calls exist that serve essentially the same purpose. As a result, some of these system calls are never invoked. For instance, sys_sigaction( ) and sys_rt_sigaction( ) are almost identical, so the sigaction( ) wrapper function included in the C library ends up invoking sys_rt_sigaction( ) instead of sys_sigaction( ). We shall describe some of the most significant POSIX system calls.

The kill( ) System Call

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 as 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 sets up a minimal siginfo_t table for the signal, and then invokes kill_something_info( ):

info.si_signo = sig; info.si_errno = 0; info.si_code = SI_USER; info._sifields._kill._pid = current->pid; info._sifields._kill._uid ...