Signaling Processes

When events out of the ordinary occur, a process may receive a signal. Signals are asynchronous and are generated when an event occurs that requires attention. They can be thought of as a software version of a hardware interrupt and may be generated by various sources:

• Hardware Such as when a process attempts to access addresses outside its own address space or divides by zero.
• Kernel Notifying the process that an I/O device for which it has been waiting (say, input from the terminal) is available.
• Other processes A child process notifying its parent process that it has terminated .
• User Pressing keyboard sequences that generate a quit, interrupt, or stop signal.

Signals are numbered and historically were defined in the header file . In Linux signal definitions reside in . This file is included automatically when you include . The should not be directly included in your program. The process that receives a signal can take one of three courses of action:

1. Perform the system-specified default for the signal. For most signals the default action (what will be done by the process if nothing else has been specified) is to (a) notify the parent process that it is terminating, (b) generate a core file (a file containing the current memory image of the process), and (c) terminate.
2. Ignore the signal. A process can do this with all but two special signals: SIGSTOP (signal 23), a stop-processing signal that was not generated from the terminal, and SIGKILL (signal 9), which indicates the process is to be killed (terminated). The inability of a process to ignore these special signals ensures the operating system the ability to remove errant processes.
3. Catch the signal. As with ignoring signals, this can be done for all signals except the SIGSTOP and SIGKILL signals. When a process catches a signal, it invokes a special signal handling routine. After executing the code in the signal handling routine, the process, if appropriate, resumes where it was interrupted .

A child process inherits the actions associated with specific signals from its parent. However, should the child process overlay its process space with another executable image, such as with an exec system call (see Chapter 3, "Using Processes"), all signals that were associated with signal catching routines at specific addresses in the process are reset to their default action in the new process. This resetting to the default action is done by the system, as the address associated with the signal catching routine is no longer valid in the new process image. In most cases (except for I/O on slow devices such as the terminal) when a process is executing a system call and a signal is received, the interrupted system call generates an error (usually returning -1) and sets the global errno variable to the value EINTR . The process issuing the system call is responsible for re-executing the interrupted system call. As the responsibility for checking each system call for signal interrupts carries such a large overhead, it is rare that once a signal is caught the process resumes normal execution. More often than not, the process uses the signal catching routine to perform housekeeping duties (such as closing files, etc.) before exiting on its own. Signals sent to a process/session group leader are also passed to the members of the group. Signals and signal catching routines are covered in considerable detail in Chapter 4, "Primitive Communications."