.\" $OpenBSD: imsg_init.3,v 1.36 2024/11/21 12:49:58 claudio Exp $
.\"
.\" Copyright (c) 2023 Claudio Jeker <claudio@openbsd.org>
.\" Copyright (c) 2010 Nicholas Marriott <nicm@openbsd.org>
.\"
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.\" purpose with or without fee is hereby granted, provided that the above
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.Dd $Mdocdate: November 21 2024 $
.Dt IMSG_INIT 3
.Os
.Sh NAME
.Nm imsg_init ,
.Nm imsg_read ,
.Nm imsg_write ,
.Nm imsg_flush ,
.Nm imsg_clear
.Nm imsg_get ,
.Nm imsg_get_ibuf ,
.Nm imsg_get_data ,
.Nm imsg_get_fd ,
.Nm imsg_get_id ,
.Nm imsg_get_len ,
.Nm imsg_get_pid ,
.Nm imsg_get_type ,
.Nm imsg_compose ,
.Nm imsg_composev ,
.Nm imsg_compose_ibuf ,
.Nm imsg_create ,
.Nm imsg_add ,
.Nm imsg_close ,
.Nm imsg_forward ,
.Nm imsg_free
.Nd IPC messaging functions
.Sh SYNOPSIS
.In sys/queue.h
.In imsg.h
.Ft void
.Fn imsg_init "struct imsgbuf *imsgbuf" "int fd"
.Ft ssize_t
.Fn imsg_read "struct imsgbuf *imsgbuf"
.Ft int
.Fn imsg_write "struct imsgbuf *imsgbuf"
.Ft int
.Fn imsg_flush "struct imsgbuf *imsgbuf"
.Ft void
.Fn imsg_clear "struct imsgbuf *imsgbuf"
.Ft ssize_t
.Fn imsg_get "struct imsgbuf *imsgbuf" "struct imsg *imsg"
.Ft int
.Fn imsg_get_ibuf "struct imsg *imsg" "struct ibuf *ibuf"
.Ft int
.Fn imsg_get_data "struct imsg *imsg" "void *data" "size_t len"
.Ft int
.Fn imsg_get_fd "struct imsg *imsg"
.Ft uint32_t
.Fn imsg_get_id "struct imsg *imsg"
.Ft size_t
.Fn imsg_get_len "struct imsg *imsg"
.Ft pid_t
.Fn imsg_get_pid "struct imsg *imsg"
.Ft uint32_t
.Fn imsg_get_type "struct imsg *imsg"
.Ft int
.Fn imsg_compose "struct imsgbuf *imsgbuf" "uint32_t type" "uint32_t id" \
    "pid_t pid" "int fd" "const void *data" "size_t datalen"
.Ft int
.Fn imsg_compose_ibuf "struct imsgbuf *imsgbuf" "uint32_t type" \
    "uint32_t id" "pid_t pid" "struct ibuf *buf"
.Ft "struct ibuf *"
.Fn imsg_create "struct imsgbuf *imsgbuf" "uint32_t type" "uint32_t id" \
    "pid_t pid" "size_t datalen"
.Ft int
.Fn imsg_add "struct ibuf *msg" "const void *data" "size_t datalen"
.Ft void
.Fn imsg_close "struct imsgbuf *imsgbuf" "struct ibuf *msg"
.Ft void
.Fn imsg_free "struct imsg *imsg"
.Ft int
.Fn imsg_forward "struct imsgbuf *imsgbuf" "struct imsg *msg"
.In sys/uio.h
.Ft int
.Fn imsg_composev "struct imsgbuf *imsgbuf" "uint32_t type" "uint32_t id" \
    "pid_t pid" "int fd" "const struct iovec *iov" "int iovcnt"
.Sh DESCRIPTION
The
.Nm imsg
functions provide a simple mechanism for communication between local processes
using sockets.
Each transmitted message is guaranteed to be presented to the receiving program
whole.
They are commonly used in privilege separated processes, where processes with
different rights are required to cooperate.
.Pp
A program using these functions should be linked with
.Em -lutil .
.Pp
The basic
.Nm
structure is the
.Em imsgbuf ,
which wraps a file descriptor and represents one side of a channel on which
messages are sent and received:
.Bd -literal -offset indent
struct imsgbuf {
	TAILQ_HEAD(, imsg_fd)	fds;
	struct ibuf_read	r;
	struct msgbuf		w;
	int			fd;
	pid_t			pid;
};
.Ed
.Pp
.Fn imsg_init
initializes
.Fa imsgbuf
as one side of a channel associated with
.Fa fd .
The file descriptor is used to send and receive messages,
but is not closed by any of the imsg functions.
An imsgbuf is initialized with the
.Em w
member as the output buffer queue,
.Em fd
with the file descriptor passed to
.Fn imsg_init
and the other members for internal use only.
.Pp
The
.Fn imsg_clear
function frees any data allocated as part of an imsgbuf.
.Pp
.Fn imsg_create ,
.Fn imsg_add
and
.Fn imsg_close
are generic construction routines for messages that are to be sent using an
imsgbuf.
.Pp
.Fn imsg_create
creates a new message with header specified by
.Fa type ,
.Fa id
and
.Fa pid .
A
.Fa pid
of zero uses the process ID returned by
.Xr getpid 2
when
.Fa imsgbuf
was initialized.
In addition to this common imsg header,
.Fa datalen
bytes of space may be reserved for attaching to this imsg.
This space is populated using
.Fn imsg_add .
.Fn imsg_create
returns a pointer to a new message if it succeeds, NULL otherwise.
.Pp
.Fn imsg_add
appends to
.Fa msg
.Fa datalen
bytes of ancillary data pointed to by
.Fa data .
It returns
.Fa datalen
if it succeeds, otherwise
.Fa msg
is freed and \-1 is returned.
.Pp
.Fn imsg_close
completes creation of
.Fa msg
by adding it to
.Fa imsgbuf
output buffer.
.Pp
.Fn imsg_compose
is used to quickly create and queue an imsg.
It takes the same parameters as the
.Fn imsg_create ,
.Fn imsg_add
and
.Fn imsg_close
routines,
except that only one ancillary data buffer can be provided.
Additionally, the file descriptor
.Fa fd
may be passed over the socket to the other process.
If
.Fa fd
is given, it is closed in the sending program after the message is sent.
A value of \-1 indicates no file descriptor should be passed.
This routine returns 1 if it succeeds, \-1 otherwise.
.Pp
.Fn imsg_composev
is similar to
.Fn imsg_compose .
It takes the same parameters, except that the ancillary data buffer is specified
by
.Fa iovec .
.Pp
.Fn imsg_compose_ibuf
is similar to
.Fn imsg_compose .
It takes the same parameters, except that the ancillary data buffer is specified
by an ibuf
.Fa buf .
This routine returns 1 if it succeeds, \-1 otherwise.
In either case the buffer
.Fa buf
is consumed by the function.
.Pp
.Fn imsg_forward
forwards a just received
.Fa msg
unaltered on
.Fa imsgbuf .
File descriptors are not forwarded by this function.
It is possible to call
.Fn imsg_forward
more than once per message.
.Pp
.Fn imsg_flush
calls
.Fn imsg_write
in a loop until all imsgs in the output buffer are sent.
It returns 0 if it succeeds, \-1 otherwise and the global variable
.Va errno
is set to indicate the error.
.Pp
The
.Fn imsg_read
routine reads pending data with
.Xr recvmsg 2
and queues it as individual messages on
.Fa imsgbuf .
It returns the number of bytes read on success, or \-1 on error.
A return value of \-1 from
.Fn imsg_read
invalidates
.Fa imsgbuf ,
and renders it suitable only for passing to
.Fn imsg_clear .
.Pp
.Fn imsg_write
writes out queued messages.
It returns 0 if it succeeds, -1 on error and the global variable
.Va errno
is set to indicate the error.
The errors
.Er EINTR ,
.Er EAGAIN ,
and
.Er ENOBUFS
are treated as follows.
.Er EINTR
will automatically retry the write operation while the other errors are
ignored with a 0 return.
The caller will then retry the operation at a later stage.
.Fn imsg_get
fills in an individual imsg pending on
.Fa imsgbuf
into the structure pointed to by
.Fa imsg .
It returns the total size of the message, 0 if no messages are ready, or \-1
for an error.
Received messages are returned as a
.Em struct imsg ,
which must be freed by
.Fn imsg_free
when no longer required.
.Pp
The accessors
.Fn imsg_get_type ,
.Fn imsg_get_pid ,
.Fn imsg_get_id ,
and
.Fn imsg_get_len ,
return the
.Fa type ,
.Fa pid ,
.Fa id ,
and payload length used in
.Fn imsg_create
to build the
.Fa imsg .
If there is no payload
.Fn imsg_get_len
returns 0.
.Pp
.Fn imsg_get_fd
returns the file descriptor and passes the responsibility to track the
descriptor back to the program.
Unclaimed file descriptors are closed by
.Fn imsg_free .
.Pp
.Fn imsg_get_data
and
.Fn imsg_get_ibuf
are used to extract the payload of an
.Fa imsg .
.Fn imsg_get_data
can be used if the structure of the payload is known and can be extracted
in one go.
0 is returned on success and \-1 on failure.
.Fn imsg_get_ibuf
initializes the passed
.Fa ibuf
to hold the payload which can be read using
.Xr ibuf_get 3 .
The
.Fa ibuf
remains valid until
.Fn imsg_free
is called and there is no need to call
.Fn ibuf_free
on this stack based buffer.
The function returns 0 on success, \-1 otherwise.
.Pp
MAX_IMSGSIZE is defined as the maximum size of a single imsg, currently
16384 bytes.
.Sh EXAMPLES
In a typical program, a channel between two processes is created with
.Xr socketpair 2 ,
and an
.Em imsgbuf
created around one file descriptor in each process:
.Bd -literal -offset indent
struct imsgbuf	parent_ibuf, child_ibuf;
int		imsg_fds[2];

if (socketpair(AF_UNIX, SOCK_STREAM, PF_UNSPEC, imsg_fds) == -1)
	err(1, "socketpair");

switch (fork()) {
case -1:
	err(1, "fork");
case 0:
	/* child */
	close(imsg_fds[0]);
	imsg_init(&child_ibuf, imsg_fds[1]);
	exit(child_main(&child_ibuf));
}

/* parent */
close(imsg_fds[1]);
imsg_init(&parent_ibuf, imsg_fds[0]);
exit(parent_main(&parent_ibuf));
.Ed
.Pp
Messages may then be composed and queued on the
.Em imsgbuf ,
for example using the
.Fn imsg_compose
function:
.Bd -literal -offset indent
enum imsg_type {
	IMSG_A_MESSAGE,
	IMSG_MESSAGE2
};

int
child_main(struct imsgbuf *imsgbuf)
{
	int	idata;
	...
	idata = 42;
	imsg_compose(imsgbuf, IMSG_A_MESSAGE,
	    0, 0, -1, &idata, sizeof idata);
	...
}
.Ed
.Pp
A mechanism such as
.Xr poll 2
or the
.Xr event 3
library is used to monitor the socket file descriptor.
When the socket is ready for writing, queued messages are transmitted with
.Fn imsg_write :
.Bd -literal -offset indent
	if (imsg_write(imsgbuf) == -1) {
		if (errno == EPIPE)
			/* handle closed connection */
		else
			/* handle write failure */
	}
.Ed
.Pp
And when ready for reading, messages are first received using
.Fn imsg_read
and then extracted with
.Fn imsg_get :
.Bd -literal -offset indent
void
dispatch_imsg(struct imsgbuf *imsgbuf)
{
	struct imsg	imsg;
	ssize_t         n;
	int		idata;

	if ((n = imsg_read(imsgbuf)) == -1 && errno != EAGAIN) {
		/* handle read error */
	}
	if (n == 0) {
		/* handle closed connection */
	}

	for (;;) {
		if ((n = imsg_get(imsgbuf, &imsg)) == -1) {
			/* handle read error */
		}
		if (n == 0)	/* no more messages */
			return;

		switch (imsg_get_type(&imsg)) {
		case IMSG_A_MESSAGE:
			if (imsg_get_data(&imsg, &idata,
			    sizeof(idata)) == -1) {
				/* handle corrupt message */
			}
			/* handle message received */
			break;
		...
		}

		imsg_free(&imsg);
	}
}
.Ed
.Sh SEE ALSO
.Xr socketpair 2 ,
.Xr ibuf_add 3 ,
.Xr unix 4