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UNIX SYSTEM PROGRAMMING UNIT ­VI 6.1 Introduction Signals Objectives : Basically signals are software interrupts. All most all the application programs need to deal with signals. Signals provide a way of handling asynchronous events: a user at a terminal typing the interrupt key to stop a program or the next program in a pipeline terminating ahead of time. Signals have been provided since the early versions of the UNIX System, but the signal model provided with systems such as Version 7 was not reliable. Signals could get lost, and it was difficult for a process to turn off selected signals when executing critical regions of code. Both 4.3BSD and SVR3 made changes to the signal model, adding what are called reliable signals. In this chapter user start with an overview of signals and a description of what each signal is normally used for. Then we look at the problems with earlier implementations. 6.2 Introduction Signal Concepts First, every signal has a name. These names all begin with the three characters SIG. For example, SIGABRT is the abort signal that is generated when a process calls the abort function. SIGALRM is the alarm signal that is generated when the timer set by the alarm function goes off. Version 7 had 15 different signals; SVR4 and 4.4BSD both have 31 different signals. FreeBSD 5.2.1, Mac OS X 10.3, and Linux 2.4.22 support 31 different signals, whereas Solaris 9 supports 38 different signals. Both Linux and Solaris, however, support additional application-defined signals as real-time extensions (the real-time extensions in POSIX aren't covered in this book The below table show the default action for the most signals is to terminate the proces. Table 6.1 Unix System Signals Name SIGABRT SIGALRM SIGBUS SIGCANCEL SIGCHLD SIGCONT SIGEMT SIGFPE SIGFREEZE SIGHUP SIGILL SIGINFO SIGINT Description abnormal termination (abort) timer expired (alarm) hardware fault threads library internal use change in status of child continue stopped process hardware fault arithmetic exception checkpoint freeze hangup illegal instruction status request from keyboard terminal interrupt character Default action terminate+core terminate terminate+core ignore ignore continue/ignore terminate+core terminate+core ignore terminate terminate+core ignore terminate Page 1 UNIX SYSTEM PROGRAMMING SIGIO asynchronous I/O terminate/ignore SIGIOT hardware fault terminate+core SIGKILL termination terminate SIGLWP threads library internal use ignore SIGPIPE write to pipe with no readers terminate SIGPOLL poll able event (poll) terminate SIGPROF profiling time alarm (set timer) terminate SIGPWR power fail/restart terminate/ignore SIGQUIT terminal quit character terminate+core SIGSEGV invalid memory reference terminate+core SIGSTKFLT coprocessor stack fault terminate SIGSTOP stop stop process SIGSYS invalid system call terminate+core SIGTERM termination terminate SIGTHAW checkpoint thaw ignore SIGTRAP hardware fault terminate+core SIGTSTP terminal stop character stop