IMSG_ADD(3) | Library Functions Manual | IMSG_ADD(3)

# NAME

`imsg_add`, `imsg_close`, `imsg_compose`, `imsg_compose_ibuf`, `imsg_composev`, `imsg_create`, `imsg_forward`, `imsg_free`, `imsg_get_buf`, `imsg_get_data`, `imsg_get_fd`, `imsg_get_ibuf`, `imsg_get_id`, `imsg_get_len`, `imsg_get_pid`, `imsg_get_strbuf`, `imsg_get_type`, `imsg_ibufq_pop`, `imsg_ibufq_push`, `imsg_set_maxsize`, `imsgbuf_allow_fdpass`, `imsgbuf_clear`, `imsgbuf_flush`, `imsgbuf_get`, `imsgbuf_init`, `imsgbuf_queuelen`, `imsgbuf_read`, `imsgbuf_set_maxsize`, `imsgbuf_write` — IPC messaging functions

# SYNOPSIS

System Utilities Library (libutil, -lutil)
`#include <imsg.h>`

`#define IMSG_HEADER_SIZE sizeof(struct imsg_hdr)`
`#define MAX_IMSGSIZE 16384`

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

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

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

int
`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);

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

void
`imsg_free`(struct imsg *imsg);

int
`imsg_get_buf`(struct imsg *imsg, void *data, size_t len);

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

int
`imsg_get_fd`(struct imsg *imsg);

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

uint32_t
`imsg_get_id`(struct imsg *imsg);

size_t
`imsg_get_len`(struct imsg *imsg);

pid_t
`imsg_get_pid`(struct imsg *imsg);

int
`imsg_get_strbuf`(struct imsg *imsg, char *str, size_t len);

uint32_t
`imsg_get_type`(struct imsg *imsg);

int
`imsg_ibufq_pop`(struct ibufqueue *bufq, struct imsg *imsg);

void
`imsg_ibufq_push`(struct ibufqueue *bufq, struct imsg *imsg);

int
`imsg_set_maxsize`(struct ibuf *msg, size_t max);

void
`imsgbuf_allow_fdpass`(struct imsgbuf *imsgbuf);

void
`imsgbuf_clear`(struct imsgbuf *imsgbuf);

int
`imsgbuf_flush`(struct imsgbuf *imsgbuf);

int
`imsgbuf_get`(struct imsgbuf *imsgbuf, struct imsg *imsg);

int
`imsgbuf_init`(struct imsgbuf *imsgbuf, int fd);

uint32_t
`imsgbuf_queuelen`(struct imsgbuf *imsgbuf);

int
`imsgbuf_read`(struct imsgbuf *imsgbuf);

int
`imsgbuf_set_maxsize`(struct imsgbuf *imsgbuf, uint32_t max);

int
`imsgbuf_write`(struct imsgbuf *imsgbuf);

`#include <sys/uio.h>`

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

# DESCRIPTION

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.

`imsgbuf_init`() 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. It returns 0 if successful and -1 on failure.

`imsgbuf_allow_fdpass`() enables file descriptor passing in both directions for this imsgbuf.

`imsgbuf_set_maxsize`() changes the default maximum payload size to max. It returns 0 if successful and -1 on failure.

The `imsgbuf_clear`() function frees any data allocated as part of an imsgbuf. This function does not close the file descriptor used for communication.

The `imsgbuf_read`() routine reads pending data with recvmsg(2) and queues it as individual messages on imsgbuf. It returns 1 on success, 0 if the connection is closed, or -1 on error and the global variable errno is set to indicate the error. The errors `EINTR` and `EAGAIN` are treated as follows. `EINTR` will automatically retry the read operation while the other errors are ignored with a 1 return.

`imsgbuf_write`() writes out queued messages. It returns 0 if it succeeds, -1 on error and the global variable errno is set to indicate the error. The errors `EINTR`, `EAGAIN`, and `ENOBUFS` are treated as follows. `EINTR` will automatically retry the write operation while the other errors are ignored with a 0 return.

`imsgbuf_flush`() calls `imsgbuf_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 errno is set to indicate the error. `imsgbuf_flush`() should not be called on non-blocking sockets since it will busy loop if the socket is not available.

`imsgbuf_get`() fills in an individual imsg pending on imsgbuf into the structure pointed to by imsg. It returns 1 on success, 0 if no messages are ready, or -1 for an error. Received messages are returned as a struct imsg, which must be freed by `imsg_free`() when no longer required.

`imsgbuf_queuelen`() returns the number of messages ready to be sent. This function returns 0 if no messages are pending for transmission.

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

`imsg_create`() 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.

`imsg_add`() 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.

`imsg_set_maxsize`() reduces the maximum payload of msg to max. The routine returns 0 if it succeeds, -1 otherwise.

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

`imsg_compose`() 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.

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

`imsg_compose_ibuf`() 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.

`imsg_forward`() forwards a just received msg unaltered on imsgbuf. File descriptors are not forwarded by this function. It is possible to call `imsg_forward`() more than once per message.

The accessors `imsg_get_type`(), `imsg_get_pid`(), `imsg_get_id`(), and `imsg_get_len`(), 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.

`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 `imsg_free`().

`imsg_get_data`() and `imsg_get_ibuf`() 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 `ibuf_free`() on this stack based buffer. The function returns 0 on success, -1 otherwise.

`imsg_get_buf`() and `imsg_get_strbuf`() read len bytes from the imsg and copy them into buf or str, respectively. `imsg_get_strbuf`() ensures that str is NUL-terminated. The functions return 0 on success, -1 otherwise.

`imsg_set_maxsize`() reduces the maximum payload of msg to max. The routine returns 0 if it succeeds, -1 otherwise.

`imsg_ibufq_pop`() and `imsg_ibufq_push`() allow to requeue an imsg onto the ibufqueue bufq. `imsg_ibufq_pop`() returns 1 on success, 0 if no messages are ready, or -1 for an error. See ibufq_new(3) for more info.

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

# EXAMPLES

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 */
    	close(imsg_fds[0]);
    	if (imsgbuf_init(&child_ibuf, imsg_fds[1]) == -1)
    		err(1, NULL);
    	exit(child_main(&child_ibuf));
    }

    /* parent */
    close(imsg_fds[1]);
    if (imsgbuf_init(&parent_ibuf, imsg_fds[0]) == -1)
    	err(1, NULL);
    exit(parent_main(&parent_ibuf));

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

    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);
    	...
    }

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 `imsgbuf_write`():

    	if (imsgbuf_write(imsgbuf) == -1) {
    		if (errno == EPIPE)
    			/* handle closed connection */
    		else
    			/* handle write failure */
    	}

And when ready for reading, messages are first received using `imsgbuf_read`() and then extracted with `imsgbuf_get`():

    void
    dispatch_imsg(struct imsgbuf *imsgbuf)
    {
    	struct imsg	imsg;
    	int		n, idata;

    	switch (imsgbuf_read(imsgbuf)) {
    	case -1:
    		/* handle read error */
    		break;
    	case 0:
    		/* handle closed connection */
    		break;
    	}

    	for (;;) {
    		if ((n = imsgbuf_get(imsgbuf, &imsg)) == -1) {
    			/* handle read error */
    			break;
    		}
    		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);
    	}
    }

# SEE ALSO

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

June 13, 2025 | Debian