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IMSG_INIT(3) Library Functions Manual IMSG_INIT(3)

imsg_init, imsg_read, imsg_get, imsg_get_ibuf, imsg_get_data, imsg_get_fd, imsg_get_id, imsg_get_len, imsg_get_pid, imsg_get_type, imsg_compose, imsg_composev, imsg_compose_ibuf, imsg_create, imsg_add, imsg_close, imsg_forward, imsg_free, imsg_flush, imsg_clearIPC messaging functions

#include <sys/queue.h>
#include <imsg.h>

imsg_init(struct imsgbuf *imsgbuf, int fd);

imsg_read(struct imsgbuf *imsgbuf);

imsg_get(struct imsgbuf *imsgbuf, struct imsg *imsg);

imsg_get_ibuf(struct imsg *imsg, struct ibuf *ibuf);

imsg_get_data(struct imsg *imsg, void *data, size_t len);

imsg_get_fd(struct imsg *imsg);

imsg_get_id(struct imsg *imsg);

imsg_get_len(struct imsg *imsg);

imsg_get_pid(struct imsg *imsg);

imsg_get_type(struct imsg *imsg);

imsg_compose(struct imsgbuf *imsgbuf, uint32_t type, uint32_t id, pid_t pid, int fd, const void *data, size_t datalen);

imsg_compose_ibuf(struct imsgbuf *imsgbuf, uint32_t type, uint32_t id, pid_t pid, struct ibuf *buf);

struct ibuf *
imsg_create(struct imsgbuf *imsgbuf, uint32_t type, uint32_t id, pid_t pid, size_t datalen);

imsg_add(struct ibuf *msg, const void *data, size_t datalen);

imsg_close(struct imsgbuf *imsgbuf, struct ibuf *msg);

imsg_free(struct imsg *imsg);

imsg_forward(struct imsgbuf *imsgbuf, struct imsg *msg);

imsg_flush(struct imsgbuf *imsgbuf);

imsg_clear(struct imsgbuf *imsgbuf);

#include <sys/uio.h>

imsg_composev(struct imsgbuf *imsgbuf, uint32_t type, uint32_t id, pid_t pid, int fd, const struct iovec *iov, int iovcnt);

The 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.

A program using these functions should be linked with .

The basic imsg_init structure is the imsgbuf, which wraps a file descriptor and represents one side of a channel on which messages are sent and received:

struct imsgbuf {
	TAILQ_HEAD(, imsg_fd)	fds;
	struct ibuf_read	r;
	struct msgbuf		w;
	int			fd;
	pid_t			pid;

() initializes imsgbuf as one side of a channel associated with 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 member as the output buffer queue, with the file descriptor passed to imsg_init() and the other members for internal use only.

The () function frees any data allocated as part of an imsgbuf.

(), imsg_add() and imsg_close() are generic construction routines for messages that are to be sent using an imsgbuf.

() creates a new message with header specified by type, id and pid. A pid of zero uses the process ID returned by getpid(2) when imsgbuf was initialized. In addition to this common imsg header, datalen bytes of space may be reserved for attaching to this imsg. This space is populated using imsg_add(). imsg_create() returns a pointer to a new message if it succeeds, NULL otherwise.

() appends to msg datalen bytes of ancillary data pointed to by data. It returns datalen if it succeeds, otherwise msg is freed and -1 is returned.

() completes creation of msg by adding it to imsgbuf output buffer.

() is used to quickly create and queue an imsg. It takes the same parameters as the imsg_create(), imsg_add() and imsg_close() routines, except that only one ancillary data buffer can be provided. Additionally, the file descriptor fd may be passed over the socket to the other process. If 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.

() is similar to imsg_compose(). It takes the same parameters, except that the ancillary data buffer is specified by iovec.

() is similar to imsg_compose(). It takes the same parameters, except that the ancillary data buffer is specified by an ibuf buf. This routine returns 1 if it succeeds, -1 otherwise. In either case the buffer buf is consumed by the function.

() forwards a just received msg unaltered on imsgbuf. Any attached file descriptor is closed.

() calls () in a loop until all imsgs in the output buffer are sent. It returns 0 if it succeeds, -1 otherwise.

The () routine reads pending data with recvmsg(2) and queues it as individual messages on imsgbuf. It returns the number of bytes read on success, or -1 on error. A return value of -1 from imsg_read() invalidates imsgbuf, and renders it suitable only for passing to imsg_clear().

() fills in an individual imsg pending on imsgbuf into the structure pointed to by 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 , which must be freed by () when no longer required.

The accessors (), (), (), and (), return the type, pid, id, and payload length used in imsg_create() to build the imsg. If there is no payload imsg_get_len() returns 0.

() returns the file descriptor and passes the responsibility to track the descriptor back to the program.

() and () are used to extract the payload of an imsg. 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. imsg_get_ibuf() initializes the passed ibuf to hold the payload which can be read using ibuf_get(3). The ibuf remains valid until imsg_free() is called and there is no need to call () on this stack based buffer. The function returns 0 on success, -1 otherwise.

MAX_IMSGSIZE is defined as the maximum size of a single imsg, currently 16384 bytes.

In a typical program, a channel between two processes is created with socketpair(2), and an imsgbuf created around one file descriptor in each process:

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 */
	imsg_init(&child_ibuf, imsg_fds[1]);

/* parent */
imsg_init(&parent_ibuf, imsg_fds[0]);

Messages may then be composed and queued on the imsgbuf, for example using the imsg_compose() function:

enum imsg_type {

child_main(struct imsgbuf *imsgbuf)
	int	idata;
	idata = 42;
	imsg_compose(imsgbuf, IMSG_A_MESSAGE,
	    0, 0, -1, &idata, sizeof idata);

A mechanism such as poll(2) or the event(3) library is used to monitor the socket file descriptor. When the socket is ready for writing, queued messages are transmitted with msgbuf_write():

	if ((n = msgbuf_write(&imsgbuf->w)) == -1 && errno != EAGAIN) {
		/* handle write failure */
	if (n == 0) {
		/* handle closed connection */

And when ready for reading, messages are first received using imsg_read() and then extracted with imsg_get():

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 */

		switch (imsg_get_type(&imsg)) {
			if (imsg_get_data(&imsg, &idata,
			    sizeof(idata)) == -1) {
				/* handle corrupt message */
			/* handle message received */


socketpair(2), ibuf_add(3), unix(4)

December 29, 2023 OpenBSD-current