signal(TCLX)

signal -- specify action to take when Tcl is signalled by UNIX

Syntax

signal action siglist [command]

Description

Specifies the action to take when a UNIX signal is received by Extended Tcl, or a program that embeds it. siglist is a list of either the symbolic or numeric UNIX signal (the ``SIG'' prefix is optional). action is one of the following actions to be performed on receipt of the signal. To specify all modifiable signals, use ``∗'' (this does not include SIGKILL or SIGSTOP as these cannot be modified).

default

Performs system default action when signal is received (see signal system call documentation).

ignore

Ignores the signal.

error

Generates a catchable Tcl error. It is as if the command that was running returned an error. The error code is in the form:

POSIX SIG signame

For the death of child signal, signame is always SIGCHLD, rather than SIGCLD, to allow writing portable code.

trap

When the signal occurs, executes command and continue execution if an error is not returned by command. The command will be executed in the global context. The command is edited before execution, replacing occurrences of ``%S'' with the signal name. Occurrences of ``%%'' result in a single ``%''. This editing occurs just before the trap command is evaluated. If an error is returned, then follow the standard Tcl error mechanism. Often command simply exits.

get

Retrieves the current settings of the specified signals. A keyed list is returned where the keys are one of the specified signals. The values are a list consisting of the action associated with the signal: a 0 value if the signal may be delivered (not blocked) and a 1 if it can be blocked. The actions may be one of default, ignore, error or trap. For example, if the action is trap, then the third element is the command associated with the action.

set

Sets the signals from a keyed list in the format returned by the get command. For this action, siglist is the keyed list of signal states. Signals with an action of unknown are not modified.

block

Blocks the specified signals from being received. (POSIX systems only).

unblock

Allows the specified signal to be received. Pending signals do not occur. (POSIX systems only.)

The signal action is enabled after the specified signal has occurred. The exception to this is SIGCHLD on systems without POSIX signals. For these systems, SIGCHLD is not automatically re-enabled. After a SIGCHLD signal is received, a call to wait must be performed to retrieve the exit status of the child process before issuing another signal SIGCHLD ... command. For code that is to be portable between both types of systems, use this approach.

Signals are not processed until after the completion of the Tcl command that is executing when the signal is received. If an interactive Tcl shell is running, then the SIGINT is set to error; non-interactive Tcl sessions leave SIGINT unchanged from when the process started (normally default for foreground processes and ignore for processes in the background).