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. An error
is signaled if process does not identify a process.
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 it 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.
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 signal
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 non-
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 signal
SIGKILL. This signal kills the subprocess immediately, and cannot be handled by the subprocess.
This function sends the signal
SIGQUITto the process process. This signal is the one sent by the “quit character” (usually C-b or C-\) when you are not inside Emacs.
This function stops the process process by sending the signal
continue-processto resume its execution.
Outside of Emacs, on systems with job control, the “stop character” (usually C-z) normally sends this signal. 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 by sending it the signal
SIGCONT. This presumes that process was stopped previously.
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.