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 processes. (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 get-buffer-process. nil stands for the process associated with the current buffer. Except with stop-process and 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 is non-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 current-group is lambda, the signal is sent to the process-group that owns the terminal, but only if it is not 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 SIGQUIT to the process process. This signal is the one sent by the quit character (usually C-\) when you are not inside Emacs.
This function stops the specified process. If it is a real subprocess running a program, it sends the signal SIGTSTP to 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 execution.
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.
Sometimes, it is necessary to send a signal to a non-local asynchronous process. This is possible by writing an own interrupt-process implementation. This function must be added then to interrupt-process-functions.
This variable is a list of functions to be called for interrupt-process. The arguments of the functions are the same as for interrupt-process. These functions are called in the order of the list, until one of them returns non-nil. The default function, which shall always be the last in this list, is internal-default-interrupt-process.
This is the mechanism, how Tramp implements interrupt-process.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Signals-to-Processes.html