## Module (chicken process signal)

This module offers procedures for dealing with POSIX process signals.

Please note that signals are very POSIX-specific. Windows only supports rudimentary in-process signals for dealing with user interrupts, segmentation violations, floating-point exceptions and the like. Inter-process signals are not supported. Therefore, most of the procedures here are not available on native Windows builds. If that's the case, the description contains a note.

### set-alarm!

set-alarm! SECONDSprocedure

  Sets an internal timer to raise the `signal/alrm` after SECONDS are elapsed. You can use the `set-signal-handler!` procedure to write a handler for this signal.

  NOTE: On native Windows builds (all except cygwin), this procedure is unimplemented and will raise an error.

### signal-handler

### set-signal-handler!

signal-handler SIGNUMprocedure

  Returns the signal handler for the code SIGNUM or `#f`.

set-signal-handler! SIGNUM PROCprocedure

  Establishes the procedure of one argument PROC as the handler for the signal with the code SIGNUM. PROC is called with the signal number as its sole argument. If the argument PROC is `#f` then any signal handler will be removed, and the corresponding signal set to `SIG_IGN`.

  Notes

  - it is unspecified in which thread of execution the signal handler will be invoked.
  - when signals arrive in quick succession (specifically, before the handler for a signal has been started), then signals will be queued (up to a certain limit); the order in which the queued signals will be handled is not specified
  - `(set! (signal-handler SIG) PROC)` can be used as an alternative to `(set-signal-handler! SIG PROC)`
  - Any signal handlers for the signals `signal/segv`, `signal/bus`, `signal/fpe` and `signal/ill` will be ignored and these signals will always trigger an exception, unless the executable was started with the `-:S` runtime option. This feature is only available on platforms that support the `sigprocmask(3)` POSIX API function.

### set-signal-mask!

set-signal-mask! SIGLISTprocedure

  Sets the signal mask of the current process to block all signals given in the list SIGLIST. Signals masked in that way will not be delivered to the current process.

  NOTE: On native Windows builds (all except cygwin), this procedure is unimplemented and will raise an error.

### signal-mask

signal-maskprocedure

  Returns the signal mask of the current process.

  NOTE: On native Windows builds (all except cygwin), this procedure is unimplemented and will raise an error.

### signal-masked?

signal-masked? SIGNUMprocedure

  Returns whether the signal for the code SIGNUM is currently masked.

  NOTE: On native Windows builds (all except cygwin), this procedure is unimplemented and will raise an error.

### signal-mask!

signal-mask! SIGNUMprocedure

  Masks (blocks) the signal for the code SIGNUM.

  NOTE: On native Windows builds (all except cygwin), this procedure is unimplemented and will raise an error.

### signal-unmask!

signal-unmask! SIGNUMprocedure

  Unmasks (unblocks) the signal for the code SIGNUM.

  NOTE: On native Windows builds (all except cygwin), this procedure is unimplemented and will raise an error.

### Signal codes

signal/termconstant
signal/killconstant
signal/intconstant
signal/hupconstant
signal/fpeconstant
signal/illconstant
signal/segvconstant
signal/abrtconstant
signal/trapconstant
signal/quitconstant
signal/alrmconstant
signal/vtalrmconstant
signal/profconstant
signal/ioconstant
signal/urgconstant
signal/chldconstant
signal/contconstant
signal/stopconstant
signal/tstpconstant
signal/pipeconstant
signal/xcpuconstant
signal/xfszconstant
signal/usr1constant
signal/usr2constant
signal/busconstant
signal/winchconstant
signal/breakconstant
signals-listconstant

  These variables contain signal codes for use with `process-signal`, `set-signal-handler!`, `signal-handler`, `signal-masked?`, `signal-mask!`, or `signal-unmask!`.

  NOTE: On native Windows builds (all except cygwin), only `signal/term`, `signal/int`, `signal/fpe`, `signal/ill`, `signal/segv`, `signal/abrt`, `signal/break` have an actual value. The others are all defined as zero, because those signals don't exist on Windows.

  NOTE: On UNIX builds and cygwin, `signal/break` is defined as zero because it only exists on Windows.

  To get a list of signals that are known to exist on the current platform, you can check `signals-list` which is a list of integers (signal numbers).

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