Signals

A signal is an asynchronous event that can happen in a program. The operating system defines the possible kinds of signals, and gives each kind a name and a number. For example, in Unix SIGINT is the signal a program gets when you type an interrupt character (often C-c); SIGSEGV is the signal a program gets from referencing a place in memory far away from all the areas in use; SIGALRM occurs when the alarm clock timer goes off (which happens only if your program has requested an alarm).

Some signals, including SIGALRM, are a normal part of the functioning of your program. Others, such as SIGSEGV, indicate errors; these signals are fatal (they kill your program immediately) if the program has not specified in advance some other way to handle the signal. SIGINT does not indicate an error in your program, but it is normally fatal so it can carry out the purpose of the interrupt: to kill the program.

has the ability to detect any occurrence of a signal in your program. You can tell in advance what to do for each kind of signal.

Normally, is set up to let the non-erroneous signals like SIGALRM be silently passed to your program (so as not to interfere with their role in the program's functioning) but to stop your program immediately whenever an error signal happens. You can change these settings with the handle command.

info signals

info handle

Print a table of all the kinds of signals and how has been told to handle each one. You can use this to see the signal numbers of all the defined types of signals. info handle is an alias for info signals.

handle signal keywords...

Change the way handles signal signal. signal can be the number of a signal or its name (with or without the `SIG' at the beginning); a list of signal numbers of the form `low-high'; or the word `all', meaning all the known signals. The keywords say what change to make.

The keywords allowed by the handle command can be abbreviated. Their full names are:

nostop

should not stop your program when this signal happens. It may still print a message telling you that the signal has come in.

stop

should stop your program when this signal happens. This implies the print keyword as well.

print

should print a message when this signal happens.

noprint

should not mention the occurrence of the signal at all. This implies the nostop keyword as well.

pass

noignore

should allow your program to see this signal; your program can handle the signal, or else it may terminate if the signal is fatal and not handled. pass and noignore are synonyms.

nopass

ignore

should not allow your program to see this signal. nopass and ignore are synonyms.

When a signal stops your program, the signal is not visible to the program until you continue. Your program sees the signal then, if pass is in effect for the signal in question at that time. In other words, after reports a signal, you can use the handle command with pass or nopass to control whether your program sees that signal when you continue.

The default is set to nostop, noprint, pass for non-erroneous signals such as SIGALRM, SIGWINCH and SIGCHLD, and to stop, print, pass for the erroneous signals.

You can also use the signal command to prevent your program from seeing a signal, or cause it to see a signal it normally would not see, or to give it any signal at any time. For example, if your program stopped due to some sort of memory reference error, you might store correct values into the erroneous variables and continue, hoping to see more execution; but your program would probably terminate immediately as a result of the fatal signal once it saw the signal. To prevent this, you can continue with `signal 0'. See section Giving your program a signal.

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