sysvinit/man/initctl.5

'\" -*- coding: UTF-8 -*-
.\" Copyright (C) 2018 Jesse Smith
.\"
.\" This program is free software; you can redistribute it and/or modify
.\" it under the terms of the GNU General Public License as published by
.\" the Free Software Foundation; either version 2 of the License.
.\"
.\" This program is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\"
.\" You should have received a copy of the GNU General Public License
.\" along with this program; if not, write to the Free Software
.\" Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
.\"
.TH INITCTL 5 "April 13, 2018" "sysvinit " "File Formats"
.SH NAME
initctl \- /run/initctl is a named pipe which passes commands to SysV init
.SH SYNOPSIS
/run/initctl
.SH DESCRIPTION

This document describes the communication pipe set up by SysV \fBinit\fR
at \fI/run/initctl\fR. This named pipe allows programs with the proper
permissions (typically programs run by root have read+write access to
the pipe) to send signals to the \fBinit\fR program (PID 1).

The \fBinit\fR manual page has, up until recently, simply stated
that people wishing to understand how to send messages to \fBinit\fR
should read the init program's source code, but that is not usually practical.

Messages sent to the pipe to talk to \fBinit\fR must have a special format.
This format is defined as a C structure and the technical break-down
is presented here:

/*
 *      Because of legacy interfaces, "runlevel" and "sleeptime"
 *      aren't in a separate struct in the union.
 *
 *      The weird sizes are because init expects the whole
 *      struct to be 384 bytes.
 */

struct init_request {
        int     magic;                  /* Magic number                 */
        int     cmd;                    /* What kind of request         */
        int     runlevel;               /* Runlevel to change to        */
        int     sleeptime;              /* Time between TERM and KILL   */
        union {
                struct init_request_bsd bsd;
                char                    data[368];
        } i;
};


Let's go through the init_request structure one line at a time. The
first variable, the "magic" number must be of the value 0x03091969.
The \fBinit\fR program then knows that only programs with root access which send
this magic number are authorized to communicate with init.

The \fIcmd\fR variable is a value in the range of 0-8 (currently). This \fIcmd\fR
variable tells init what we want it to do. Here are the possible options:

1 - Set the current runlevel, specified by the runlevel variable.

2 - The power will fail soon (probably low battery) prepare to shutdown.

3 - The power is failing, do shutdown immediately.

4 - The power is okay, cancel shutdown.

6 - Set an environment variable to a value to be specified in 
    the \fIdata\fR variable of this structure.

Other \fIcmd\fR options may be added to \fBinit\fR later. For example, command values
0, 5 and 7 are defined but currently not implemented.

The \fIrunlevel\fR variable will specify the runlevel to switch to (0-6).

The \fIsleeptime\fR variable is to be used when we want to tell \fBinit\fR to change
the time spent waiting between sending \fBSIGTERM\fR and \fBSIGKILL\fR during the
shutdown process. Changing this at run time is not yet implemented.

The \fIdata\fR variable (in the union) can be used to pass misc data which init
might need to process our request. For example, when setting environment
variables.

When setting an environment variable through \fBinit\fR's \fI/run/initctl\fR pipe,
the data variable should have the format \fIVARIABLE\fR=\fIVALUE\fR. The string
should be terminated with a NULL character.

.SH EXAMPLES

The following C code example shows how to send a set environment variable
request to the \fBinit\fR process using the \fI/run/initctl\fR pipe. This example
is simplified and skips the error checking. A more complete example can be
found in the shutdown.c program's \fBinit_setnv\fR() function.

.nf
struct init_request     request;           /* structure defined above */
int                     fd;                /* file descriptor for pipe */

memset(&request, 0, sizeof(request));      /* initialize structure */
request.magic = 0x03091969;                /* magic number required */
request.cmd = 6;                           /* 6 is to set a variable */
sprintf(request.data, "VARIABLE=VALUE");   /* set VAR to VALUE in init */

if ((fd = open(INIT_FIFO, O_WRONLY)) >= 0) /* open pipe for writing */
{ 
    size_t s  = sizeof(request);           /* size of structure to write */
    void *ptr = &request;                  /* temporary pointer */
    write(fd, ptr, s);                     /* send structure to the pipe */
    close(fd);                             /* close the pipe when done */
}
.fi

.sp
.SH NOTES
Usually the \fI/run/initctl\fR pipe would only be used by low-level programs to
request a power-related shutdown or change the runlevel, like \fBtelinit\fR
would do. Most of the time there is no need to talk to \fBinit\fR directly, but
this gives us an extendable approach so \fBinit\fR can be taught how to learn
more commands.
.PP
The commands passed through the \fI/run/initctl\fR pipe must be sent in a specific
binary format and be of a specific length. Larger data structures or ones
not using the proper format will be ignored. Typically, only root has the
ability to write to the initctl pipe for security reasons.
.PP
The \fI/run/initctl\fR pipe can be closed by sending init (PID 1) the \fBSIGUSR2\fR
signal. This closes the pipe and leaves it closed. This may be useful
for making sure \fBinit\fR is not keeping any files open. However, when the
pipe is closed, \fBinit\fR no longer receives signals, such as those sent by
\fBshutdown\fR(8) or \fBtelinit\fR(8). In other words if we close the pipe, \fBinit\fR cannot
change its runlevel directly. The pipe may be re-opened by sending \fBinit\fR (PID 1)
the \fBSIGUSR1\fR signal.
.PP
If the \fI/run/initctl\fR pipe is closed then it may still be possible to bring
down the system using the \fBshutdown\fR(8) command's \fB-n\fR flag, but this is not
always clean and not recommended.

.SH FILES
/run/initctl
/sbin/init

.SH AUTHOR
.MT jsmith@\:resonatingmedia\:.com 
Jesse Smith
.ME
.SH "SEE ALSO"
.BR init (8)