C.3 Signals

Signals are a simple UNIX mechanism for controlling processes. A signal is a 5-bit message to a process that requires immediate attention. Each signal has associated with it a default action; for some signals, you can change this default action. Signals are generated by exceptions, which include:

• Attempts to use illegal instructions

• Certain kinds of mathematical operations

• Window resize events

• Predefined alarms

• The user pressing an interrupt key on a terminal

• Another program using the kill() l or killpg() system calls

• A program running in the background attempting to read from or write to its controlling terminal

• A child process calling exit or terminating abnormally

The system default may be to ignore the signal, to terminate the process receiving the signal (and, optionally, generate a core file), or to suspend the process until it receives a continuation signal. Some signals can be caught - that is, a program can specify a particular function that should be run when the signal is received. By design, UNIX supports exactly 31 signals. They are listed in the files /usr/include/signal.h and /usr/include/sys/signal.h. Table 27.4 contains a summary.

[7] The signal number varies on some systems.

[8] The default action for most signals is to terminate.

Key:

Signals are normally used between processes for process control. They are also used within a process to indicate exceptional conditions that should be handled immediately (for example, floating-point overflows).