Sending a signal to a subprocess is a way of interrupting its
activities. There are several different signals, each with its own
meaning. The set of signals and their names is defined by the operating
system. For example, the signal
SIGINT means that the user has
typed C-c, or that some analogous thing has happened.
Each signal has a standard effect on the subprocess. Most signals kill the subprocess, but some stop (or resume) execution instead. Most signals can optionally be handled by programs; if the program handles the signal, then we can say nothing in general about its effects.
You can send signals explicitly by calling the functions in this
section. Emacs also sends signals automatically at certain times:
killing a buffer sends a
SIGHUP signal to all its associated
processes; killing Emacs sends a
SIGHUP signal to all remaining
SIGHUP is a signal that usually indicates that the
user “hung up the phone”, i.e., disconnected.)
Each of the signal-sending functions takes two optional arguments: process and current-group.
The argument process must be either a process, a process
name, a buffer, a buffer name, or
nil. A buffer or buffer name
stands for a process through
stands for the process associated with the current buffer. Except
continue-process, an error is
signaled if process does not identify an active process, or if
it represents a network, serial, or pipe connection.
The argument current-group is a flag that makes a difference
when you are running a job-control shell as an Emacs subprocess. If it
nil, then the signal is sent to the current process-group
of the terminal that Emacs uses to communicate with the subprocess. If
the process is a job-control shell, this means the shell’s current
subjob. If current-group is
nil, the signal is
sent to the process group of the immediate subprocess of Emacs. If
the subprocess is a job-control shell, this is the shell itself. If
lambda, the signal is sent to the
process-group that owns the terminal, but only if it is not the shell
The flag current-group has no effect when a pipe is used to
communicate with the subprocess, because the operating system does not
support the distinction in the case of pipes. For the same reason,
job-control shells won’t work when a pipe is used. See
process-connection-type in Asynchronous Processes.
This function interrupts the process process by sending the
SIGINT. Outside of Emacs, typing the interrupt
character (normally C-c on some systems, and DEL on
others) sends this signal. When the argument current-group is
nil, you can think of this function as typing C-c
on the terminal by which Emacs talks to the subprocess.
This function kills the process process by sending the
SIGKILL. This signal kills the subprocess immediately,
and cannot be handled by the subprocess.
This function sends the signal
SIGQUIT to the process
process. This signal is the one sent by the quit
character (usually C-\) when you are not inside
This function stops the specified process. If it is a real
subprocess running a program, it sends the signal
that subprocess. If process represents a network, serial, or
pipe connection, this function inhibits handling of the incoming data
from the connection; for a network server, this means not accepting
new connections. Use
continue-process to resume normal
Outside of Emacs, on systems with job control, the stop character
(usually C-z) normally sends the
SIGTSTP signal to a
subprocess. When current-group is non-
nil, you can think
of this function as typing C-z on the terminal Emacs uses to
communicate with the subprocess.
This function resumes execution of the process process. If it
is a real subprocess running a program, it sends the signal
SIGCONT to that subprocess; this presumes that process
was stopped previously. If process represents a network,
serial, or pipe connection, this function resumes handling of the
incoming data from the connection. For serial connections, data that
arrived during the time the process was stopped might be lost.
This function sends a signal to process process. The argument signal specifies which signal to send; it should be an integer, or a symbol whose name is a signal.
The process argument can be a system process ID (an integer); that allows you to send signals to processes that are not children of Emacs. See System Processes.