IMSG_ADD(3)

NAME
imsg_add, imsg_close, imsg_compose, imsg_compose_ibuf, imsg_composev, imsg_create, imsg_forward, imsg_free, imsg_get, imsg_get_data, imsg_get_fd, imsg_get_ibuf, imsg_get_id, imsg_get_ibuf, imsg_get_len, imsg_get_pid, imsg_get_type, imsgbuf_allow_fdpass, imsgbuf_clear, imsgbuf_flush, imsgbuf_init, imsgbuf_queuelen, imsgbuf_read, imsgbuf_set_maxsize, imsgbuf_write - IPC messaging functions

SYNOPSIS
#include <sys/queue.h>
#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);
ssize_t
imsg_get(struct imsgbuf *imsgbuf, struct imsg *imsg);
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);
uint32_t
imsg_get_type(struct imsg *imsg);
void
imsgbuf_allow_fdpass(struct imsgbuf *imsgbuf);
void
imsgbuf_clear(struct imsgbuf *imsgbuf);
int
imsgbuf_flush(struct imsgbuf *imsgbuf);
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 maxsize);
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.

A program using these functions should be linked with -lutil.

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 imsg size from MAX_IMSGSIZE to maxsize. maxsize must be bigger than IMSG_HEADER_SIZE. 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_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_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.

imsg_get() 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 struct imsg, which must be freed by imsg_free() when no longer required.

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.

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 imsg_get():

    void
    dispatch_imsg(struct imsgbuf *imsgbuf)
    {
    	struct imsg	imsg;
    	ssize_t         n;
    	int		idata;
    	switch (imsgbuf_read(imsgbuf)) {
    	case -1:
    		/* handle read error */
    		break;
    	case 0:
    		/* handle closed connection */
    		break;
    	}
    	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);
    	}
    }

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