Wednesday, August 24, 2011

POSIX Signal Tutorial - 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. These names are all defined by positive integer constants (the signal number) in the header


Implementations actually define the individual signals in an alternate header file, but this header file is included by <signal.h>. It is considered bad form for the kernel to include header files meant for user-level applications, so if the applications and the kernel both need the same definitions, the information is placed in a kernel header file that is then included by the userlevel header file. Thus, both FreeBSD 5.2.1 and Mac OS X 10.3 define the signals in <sys/signal.h>. Linux 2.4.22 defines the signals in <bits/signum.h>, and Solaris 9 defines them in


No signal has a signal number of 0. but the kill function uses the signal number of 0 for a special case. POSIX.1 calls this value the null signal.

Numerous conditions can generate a signal.
  • The terminal-generated signals occur when users press certain terminal keys. Pressing the DELETE key on the terminal (or Control-C on many systems) normally causes the interrupt signal (SIGINT) to be generated. This is how to stop a runaway program.
  • Hardware exceptions generate signals: divide by 0, invalid memory reference, and the like. These conditions are usually detected by the hardware, and the kernel is notified. The kernel then generates the appropriate signal for the process that was running at the time the condition occurred. For example, SIGSEGV is generated for a process that executes an invalid memory reference.
  • The kill(2) function allows a process to send any signal to another process or process group. Naturally, there are limitations: we have to be the owner of the process that we're sending the signal to, or we have to be the superuser.
  • The kill(1) command allows us to send signals to other processes. This program is just an interface to the kill function. This command is often used to terminate a runaway background process.
  • Software conditions can generate signals when something happens about which the process should be notified. These aren't hardware-generated conditions (as is the divide-by-0 condition), but software conditions. Examples are SIGURG (generated when out-of-band data arrives over a network connection), SIGPIPE (generated when a process writes to a pipe after the reader of the pipe has terminated), and SIGALRM (generated when an alarm clock set by the process expires).

Signals are classic examples of asynchronous events. Signals occur at what appear to be random times to the process. The process can't simply test a variable (such as errno) to see whether a signal has occurred; instead, the process has to tell the kernel "if and when this signal occurs, do the following."

We can tell the kernel to do one of three things when a signal occurs. We call this the disposition of the signal, or the action associated with a signal.
  1. Ignore the signal. This works for most signals, but two signals can never be ignored: SIGKILL and SIGSTOP. The reason these two signals can't be ignored is to provide the kernel and the superuser with a surefire way of either killing or stopping any process. Also, if we ignore some of the signals that are generated by a hardware exception (such as illegal memory reference or divide by 0), the behavior of the process is undefined.
  2. Catch the signal. To do this, we tell the kernel to call a function of ours whenever the signal occurs. In our function, we can do whatever we want to handle the condition. If we're writing a command interpreter, for example, when the user generates the interrupt signal at the keyboard, we probably want to return to the main loop of the program, terminating whatever command we were executing for the user. If the SIGCHLD signal is caught, it means that a child process has terminated, so the signal-catching function can call waitpid to fetch the child's process ID and termination status. As another example, if the process has created temporary files, we may want to write a signal-catching function for the SIGTERM  ignal (the termination signal that is the default signal sent by the kill command) to clean up the temporary files. Note that the two signals SIGKILL and SIGSTOP can't be caught.
  3. Let the default action apply. Every signal has a default action, shown in Figure 2.1. Note that the default action for most signals is to terminate the process.

Figure 2.1 lists the names of all the signals, an indication of which systems support the signal, and the default action for the signal. The SUS column contains • if the signal is defined as part of the base POSIX.1 specification and XSI if it is defined as an XSI extension to the base.

When the default action is labeled "terminate+core," it means that a memory image of the process is left in the file named core of the current working directory of the process. (Because the file is named core, it shows how long this feature has been part of the UNIX System.) This file can be used with most UNIX System debuggers to examine the state of the process at the time it terminated.
  1. The generation of the core file is an implementation feature of most versions of the UNIX System. Although this feature is not part of POSIX.1, it is mentioned as a potential implementation-specific action in the Single UNIX Specification's XSI extension.
  2. The name of the core file varies among implementations. On FreeBSD 5.2.1, for example, the core file is named cmdname.core, where cmdname is the name of the command corresponding to the process that received the signal. On Mac OS X 10.3, the core file is named, where pid is the ID of the process that received the signal. (These systems allow the core filename to be configured via a sysctl parameter.)
  3. Most implementations leave the core file in the current working directory of the corresponding process; Mac OS X places all core files in /cores instead.

The core file will not be generated if (a) the process was set-user-ID and the current user is not the owner of the program file, or (b) the process was set-group-ID and the current user is not the group owner of the file, (c) the user does not have permission to write in the current working directory, (d) the file already exists and the user does not have permission to write to it, or (e) the file is too big. The permissions of the core file (assuming that the file doesn't already exist) are usually user-read and user-write, although Mac OS X sets only user-read. In Figure 2.1, the signals with a description "hardware fault" correspond to implementation-defined hardware faults. Many of these names are taken from the original PDP-11 implementation of the UNIX System. Check your system's manuals to determine exactly what type of error these signals correspond to.

We will now see the various types of the signal and its descriptions. Click here to read.

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