Copyright (c) 2023 Claudio Jeker <claudio@openbsd.org>
Copyright (c) 2010 Nicholas Marriott <nicm@openbsd.org>
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.Dd $Mdocdate: December 13 2023 $ .Dt IMSG_INIT 3 .Os .Sh NAME .Nm imsg_init , .Nm imsg_read , .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 , .Nm imsg_flush , .Nm imsg_clear .Nd IPC messaging functions .Sh SYNOPSIS n sys/queue.h n imsg.h .Ft void .Fn imsg_init "struct imsgbuf *imsgbuf" "int fd" .Ft ssize_t .Fn imsg_read "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" .Ft int .Fn imsg_flush "struct imsgbuf *imsgbuf" .Ft void .Fn imsg_clear "struct imsgbuf *imsgbuf" n 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.
p A program using these functions should be linked with .Em -lutil .
p 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: d -literal -offset indent struct imsgbuf { TAILQ_HEAD(, imsg_fd) fds; struct ibuf_read r; struct msgbuf w; int fd; pid_t pid; }; .Ed
p .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.
p The .Fn imsg_clear function frees any data allocated as part of an imsgbuf.
p .Fn imsg_create , .Fn imsg_add and .Fn imsg_close are generic construction routines for messages that are to be sent using an imsgbuf.
p .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.
p .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.
p .Fn imsg_close completes creation of .Fa msg by adding it to .Fa imsgbuf output buffer.
p .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.
p .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 .
p .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.
p .Fn imsg_forward forwards a just received .Fa msg unaltered on .Fa imsgbuf . Any attached file descriptor is closed.
p .Fn imsg_flush calls .Fn msgbuf_write in a loop until all imsgs in the output buffer are sent. It returns 0 if it succeeds, -1 otherwise.
p 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 .
p .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.
p 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.
p .Fn imsg_get_fd returns the file descriptor and passes the responsibility to track the descriptor back to the program.
p .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.
p 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: d -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
p Messages may then be composed and queued on the .Em imsgbuf , for example using the .Fn imsg_compose function: d -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
p 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 msgbuf_write : d -literal -offset indent if ((n = msgbuf_write(&imsgbuf-\*(Gtw)) == -1 && errno != EAGAIN) { /* handle write failure */ } if (n == 0) { /* handle closed connection */ } .Ed
p And when ready for reading, messages are first received using .Fn imsg_read and then extracted with .Fn imsg_get : d -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