The following procedures raise, handle and wait for signals.
Scheme code signal handlers are run via a system async (see System asyncs), so they’re called in the handler’s thread at the next safe opportunity. Generally this is after any currently executing primitive procedure finishes (which could be a long time for primitives that wait for an external event).
Sends a signal to the specified process or group of processes.
pid specifies the processes to which the signal is sent:
The process whose identifier is pid.
All processes in the current process group.
The process group whose identifier is -pid
If the process is privileged, all processes except for some special system processes. Otherwise, all processes with the current effective user ID.
sig should be specified using a variable corresponding to the Unix symbolic name, e.g.,
A full list of signals on the GNU system may be found in Standard Signals in The GNU C Library Reference Manual.
Sends a specified signal sig to the current process, where
sig is as described for the
Install or report the signal handler for a specified signal.
signum is the signal number, which can be specified using the value
of variables such as
If handler is omitted,
sigaction returns a pair: the
CAR is the current signal hander, which will be either an
integer with the value
SIG_DFL (default action) or
SIG_IGN (ignore), or the Scheme procedure which handles the
#f if a non-Scheme procedure handles the signal.
The CDR contains the current
sigaction flags for the
If handler is provided, it is installed as the new handler for
signum. handler can be a Scheme procedure taking one
argument, or the value of
SIG_DFL (default action) or
SIG_IGN (ignore), or
#f to restore whatever signal handler
was installed before
sigaction was first used. When a scheme
procedure has been specified, that procedure will run in the given
thread. When no thread has been given, the thread that made this
sigaction is used.
flags is a
logior (see Bitwise Operations) of the
following (where provided by the system), or
0 for none.
SIGCHLD is signalled when a child process stops
SIGSTOP), and when a child process terminates.
SIGCHLD is only signalled
for termination, not stopping.
SA_NOCLDSTOP has no effect on signals other than
If a signal occurs while in a system call, deliver the signal then
restart the system call (as opposed to returning an
from that call).
Guile handles signals asynchronously. When it receives a signal, the
synchronous signal handler just records the fact that a signal was
received and sets a flag to tell the relevant Guile thread that it has a
pending signal. When the Guile thread checks the pending-interrupt
flag, it will arrange to run the asynchronous part of the signal
handler, which is the handler attached by
This strategy has some perhaps-unexpected interactions with the
SA_RESTART flag, though: because the synchronous handler doesn’t
do very much, and notably it doesn’t run the Guile handler, it’s
impossible to interrupt a thread stuck in a long-running system call via
a signal handler that is installed with
synchronous handler just records the pending interrupt, but then the
system call resumes and Guile doesn’t have a chance to actually check
the flag and run the asynchronous handler. That’s just how it is.
The return value is a pair with information about the old handler as described above.
This interface does not provide access to the “signal blocking” facility. Maybe this is not needed, since the thread support may provide solutions to the problem of consistent access to data structures.
Return all signal handlers to the values they had before any call to
sigaction was made. The return value is unspecified.
Set a timer to raise a
SIGALRM signal after the specified
number of seconds (an integer). It’s advisable to install a signal
SIGALRM beforehand, since the default action is to terminate
The return value indicates the time remaining for the previous alarm, if any. The new value replaces the previous alarm. If there was no previous alarm, the return value is zero.
Pause the current process (thread?) until a signal arrives whose action is to either terminate the current process or invoke a handler procedure. The return value is unspecified.
Wait the given period secs seconds or usecs microseconds (both integers). If a signal arrives the wait stops and the return value is the time remaining, in seconds or microseconds respectively. If the period elapses with no signal the return is zero.
On most systems the process scheduler is not microsecond accurate and
the actual period slept by
usleep might be rounded to a system
clock tick boundary, which might be 10 milliseconds for instance.
scm_std_usleep for equivalents at
the C level (see Blocking).
Get or set the periods programmed in certain system timers.
These timers have two settings. The first setting, the interval, is the value at which the timer will be reset when the current timer expires. The second is the current value of the timer, indicating when the next expiry will be signalled.
which_timer is one of the following values:
A real-time timer, counting down elapsed real time. At zero it raises
SIGALRM. This is like
alarm above, but with a higher
A virtual-time timer, counting down while the current process is
actually using CPU. At zero it raises
A profiling timer, counting down while the process is running (like
ITIMER_VIRTUAL) and also while system calls are running on the
process’s behalf. At zero it raises a
This timer is intended for profiling where a program is spending its time (by looking where it is when the timer goes off).
getitimer returns the restart timer value and its current value,
as a list containing two pairs. Each pair is a time in seconds and
((interval_secs . interval_usecs)
(value_secs . value_usecs)).
setitimer sets the timer values similarly, in seconds and
microseconds (which must be integers). The interval value can be zero
to have the timer run down just once. The return value is the timer’s
previous setting, in the same form as
(setitimer ITIMER_REAL 5 500000 ;; Raise SIGALRM every 5.5 seconds 2 0) ;; with the first SIGALRM in 2 seconds
Although the timers are programmed in microseconds, the actual accuracy might not be that high.