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imsg provides functions for communication between processes using sockets. imsg is used by programs in OpenBSD such as ypldap.
550 lines
12 KiB
Groff
550 lines
12 KiB
Groff
.\" $OpenBSD: imsg_init.3,v 1.13 2015/07/11 16:23:59 deraadt Exp $
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.\"
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.\" Copyright (c) 2010 Nicholas Marriott <nicm@openbsd.org>
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.\"
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\"
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
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.\" IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
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.\" OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd $Mdocdate: July 11 2015 $
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.Dt IMSG_INIT 3
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.Os
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.Sh NAME
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.Nm imsg_init ,
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.Nm imsg_read ,
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.Nm imsg_get ,
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.Nm imsg_compose ,
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.Nm imsg_composev ,
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.Nm imsg_create ,
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.Nm imsg_add ,
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.Nm imsg_close ,
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.Nm imsg_free ,
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.Nm imsg_flush ,
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.Nm imsg_clear ,
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.Nm ibuf_open ,
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.Nm ibuf_dynamic ,
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.Nm ibuf_add ,
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.Nm ibuf_reserve ,
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.Nm ibuf_seek ,
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.Nm ibuf_size ,
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.Nm ibuf_left ,
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.Nm ibuf_close ,
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.Nm ibuf_write ,
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.Nm ibuf_free ,
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.Nm msgbuf_init ,
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.Nm msgbuf_clear ,
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.Nm msgbuf_write ,
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.Nm msgbuf_drain
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.Nd IPC messaging functions
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/queue.h
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.In sys/uio.h
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.In imsg.h
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.Ft void
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.Fn imsg_init "struct imsgbuf *ibuf" "int fd"
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.Ft ssize_t
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.Fn imsg_read "struct imsgbuf *ibuf"
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.Ft ssize_t
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.Fn imsg_get "struct imsgbuf *ibuf" "struct imsg *imsg"
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.Ft int
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.Fn imsg_compose "struct imsgbuf *ibuf" "u_int32_t type" "uint32_t peerid" \
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"pid_t pid" "int fd" "const void *data" "u_int16_t datalen"
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.Ft int
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.Fn imsg_composev "struct imsgbuf *ibuf" "u_int32_t type" "u_int32_t peerid" \
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"pid_t pid" "int fd" "const struct iovec *iov" "int iovcnt"
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.Ft "struct ibuf *"
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.Fn imsg_create "struct imsgbuf *ibuf" "u_int32_t type" "u_int32_t peerid" \
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"pid_t pid" "u_int16_t datalen"
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.Ft int
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.Fn imsg_add "struct ibuf *buf" "const void *data" "u_int16_t datalen"
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.Ft void
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.Fn imsg_close "struct imsgbuf *ibuf" "struct ibuf *msg"
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.Ft void
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.Fn imsg_free "struct imsg *imsg"
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.Ft int
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.Fn imsg_flush "struct imsgbuf *ibuf"
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.Ft void
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.Fn imsg_clear "struct imsgbuf *ibuf"
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.Ft "struct ibuf *"
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.Fn ibuf_open "size_t len"
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.Ft "struct ibuf *"
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.Fn ibuf_dynamic "size_t len" "size_t max"
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.Ft int
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.Fn ibuf_add "struct ibuf *buf" "const void *data" "size_t len"
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.Ft "void *"
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.Fn ibuf_reserve "struct ibuf *buf" "size_t len"
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.Ft "void *"
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.Fn ibuf_seek "struct ibuf *buf" "size_t pos" "size_t len"
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.Ft size_t
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.Fn ibuf_size "struct ibuf *buf"
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.Ft size_t
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.Fn ibuf_left "struct ibuf *buf"
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.Ft void
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.Fn ibuf_close "struct msgbuf *msgbuf" "struct ibuf *buf"
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.Ft int
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.Fn ibuf_write "struct msgbuf *msgbuf"
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.Ft void
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.Fn ibuf_free "struct ibuf *buf"
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.Ft void
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.Fn msgbuf_init "struct msgbuf *msgbuf"
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.Ft void
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.Fn msgbuf_clear "struct msgbuf *msgbuf"
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.Ft int
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.Fn msgbuf_write "struct msgbuf *msgbuf"
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.Ft void
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.Fn msgbuf_drain "struct msgbuf *msgbuf" "size_t n"
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.Sh DESCRIPTION
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The
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.Nm imsg
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functions provide a simple mechanism for communication between processes
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using sockets.
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Each transmitted message is guaranteed to be presented to the receiving program
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whole.
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They are commonly used in privilege separated processes, where processes with
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different rights are required to cooperate.
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.Pp
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A program using these functions should be linked with
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.Em -lutil .
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.Pp
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The basic
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.Nm
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structure is the
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.Em imsgbuf ,
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which wraps a file descriptor and represents one side of a channel on which
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messages are sent and received:
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.Bd -literal -offset indent
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struct imsgbuf {
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TAILQ_HEAD(, imsg_fd) fds;
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struct ibuf_read r;
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struct msgbuf w;
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int fd;
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pid_t pid;
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};
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.Ed
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.Pp
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.Fn imsg_init
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is a routine which initializes
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.Fa ibuf
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as one side of a channel associated with
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.Fa fd .
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The file descriptor is used to send and receive messages,
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but is not closed by any of the imsg functions.
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An imsgbuf is initialized with the
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.Em w
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member as the output buffer queue,
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.Em fd
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with the file descriptor passed to
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.Fn imsg_init
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and the other members for internal use only.
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.Pp
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The
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.Fn imsg_clear
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function frees any data allocated as part of an imsgbuf.
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.Pp
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.Fn imsg_create ,
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.Fn imsg_add
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and
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.Fn imsg_close
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are generic construction routines for messages that are to be sent using an
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imsgbuf.
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.Pp
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.Fn imsg_create
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creates a new message with header specified by
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.Fa type ,
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.Fa peerid
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and
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.Fa pid .
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A
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.Fa pid
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of zero uses the process ID returned by
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.Xr getpid 2
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when
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.Fa ibuf
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was initialized.
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In addition to this common imsg header,
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.Fa datalen
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bytes of space may be reserved for attaching to this imsg.
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This space is populated using
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.Fn imsg_add .
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Additionally, the file descriptor
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.Fa fd
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may be passed over the socket to the other process.
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If
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.Fa fd
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is given, it is closed in the sending program after the message is sent.
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A value of \-1 indicates no file descriptor should be passed.
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.Fn imsg_create
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returns a pointer to a new message if it succeeds, NULL otherwise.
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.Pp
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.Fn imsg_add
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appends to
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.Fa imsg
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.Fa len
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bytes of ancillary data pointed to by
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.Fa buf .
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It returns
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.Fa len
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if it succeeds, \-1 otherwise.
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.Pp
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.Fn imsg_close
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completes creation of
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.Fa imsg
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by adding it to
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.Fa imsgbuf
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output buffer.
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.Pp
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.Fn imsg_compose
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is a routine which is used to quickly create and queue an imsg.
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It takes the same parameters as the
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.Fn imsg_create ,
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.Fn imsg_add
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and
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.Fn imsg_close
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routines,
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except that only one ancillary data buffer can be provided.
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This routine returns 1 if it succeeds, \-1 otherwise.
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.Pp
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.Fn imsg_composev
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is similar to
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.Fn imsg_compose .
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It takes the same parameters, except that the ancillary data buffer is specified
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by
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.Fa iovec .
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.Pp
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.Fn imsg_flush
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is a function which calls
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.Fn msgbuf_write
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in a loop until all imsgs in the output buffer are sent.
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It returns 0 if it succeeds, \-1 otherwise.
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.Pp
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The
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.Fn imsg_read
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routine reads pending data with
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.Xr recvmsg 2
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and queues it as individual messages on
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.Fa imsgbuf .
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It returns the number of bytes read on success, or \-1 on error.
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A return value of \-1 from
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.Fn imsg_read
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invalidates
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.Fa imsgbuf ,
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and renders it suitable only for passing to
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.Fn imsg_clear .
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.Pp
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.Fn imsg_get
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fills in an individual imsg pending on
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.Fa imsgbuf
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into the structure pointed to by
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.Fa imsg .
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It returns the total size of the message, 0 if no messages are ready, or \-1
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for an error.
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Received messages are returned as a
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.Em struct imsg ,
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which must be freed by
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.Fn imsg_free
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when no longer required.
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.Em struct imsg
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has this form:
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.Bd -literal -offset indent
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struct imsg {
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struct imsg_hdr hdr;
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int fd;
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void *data;
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};
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struct imsg_hdr {
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u_int32_t type;
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u_int16_t len;
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u_int16_t flags;
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u_int32_t peerid;
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u_int32_t pid;
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};
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.Ed
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.Pp
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The header members are:
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.Bl -tag -width Ds -offset indent
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.It type
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A integer identifier, typically used to express the meaning of the message.
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.It len
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The total length of the imsg, including the header and any ancillary data
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transmitted with the message (pointed to by the
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.Em data
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member of the message itself).
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.It flags
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Flags used internally by the imsg functions: should not be used by application
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programs.
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.It peerid, pid
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32-bit values specified on message creation and free for any use by the
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caller, normally used to identify the message sender.
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.El
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.Pp
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In addition,
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.Em struct imsg
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has the following:
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.Bl -tag -width Ds -offset indent
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.It fd
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The file descriptor specified when the message was created and passed using the
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socket control message API, or \-1 if no file descriptor was sent.
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.It data
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A pointer to the ancillary data transmitted with the imsg.
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.El
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.Pp
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The IMSG_HEADER_SIZE define is the size of the imsg message header, which
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may be subtracted from the
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.Fa len
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member of
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.Em struct imsg_hdr
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to obtain the length of any additional data passed with the message.
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.Pp
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MAX_IMSGSIZE is defined as the maximum size of a single imsg, currently
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16384 bytes.
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.Sh BUFFERS
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The imsg API defines functions to manipulate buffers, used internally and during
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construction of imsgs with
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.Fn imsg_create .
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A
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.Em struct ibuf
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is a single buffer and a
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.Em struct msgbuf
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a queue of output buffers for transmission:
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.Bd -literal -offset indent
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struct ibuf {
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TAILQ_ENTRY(ibuf) entry;
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u_char *buf;
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size_t size;
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size_t max;
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size_t wpos;
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size_t rpos;
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int fd;
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};
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struct msgbuf {
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TAILQ_HEAD(, ibuf) bufs;
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u_int32_t queued;
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int fd;
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};
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.Ed
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.Pp
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The
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.Fn ibuf_open
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function allocates a fixed-length buffer.
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The buffer may not be resized and may contain a maximum of
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.Fa len
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bytes.
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On success
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.Fn ibuf_open
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returns a pointer to the buffer; on failure it returns NULL.
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.Pp
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.Fn ibuf_dynamic
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allocates a resizeable buffer of initial length
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.Fa len
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and maximum size
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.Fa max .
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Buffers allocated with
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.Fn ibuf_dynamic
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are automatically grown if necessary when data is added.
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.Pp
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.Fn ibuf_add
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is a routine which appends a block of data to
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.Fa buf .
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0 is returned on success and \-1 on failure.
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.Pp
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.Fn ibuf_reserve
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is used to reserve
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.Fa len
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bytes in
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.Fa buf .
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A pointer to the start of the reserved space is returned, or NULL on error.
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.Pp
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.Fn ibuf_seek
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is a function which returns a pointer to the part of the buffer at offset
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.Fa pos
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and of extent
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.Fa len .
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NULL is returned if the requested range is outside the part of the buffer
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in use.
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.Pp
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.Fn ibuf_size
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and
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.Fn ibuf_left
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are functions which return the total bytes used and available in
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.Fa buf
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respectively.
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.Pp
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.Fn ibuf_close
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appends
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.Fa buf
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to
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.Fa msgbuf
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ready to be sent.
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.Pp
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The
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.Fn ibuf_write
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routine transmits as many pending buffers as possible from
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.Fn msgbuf
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using
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.Xr writev 2 .
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It returns 1 if it succeeds, \-1 on error and 0 when no buffers were
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pending or an EOF condition on the socket is detected.
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Temporary resource shortages are returned with errno
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.Er EAGAIN
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and require the application to retry again in the future.
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.Pp
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.Fn ibuf_free
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frees
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.Fa buf
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and any associated storage.
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.Pp
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The
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.Fn msgbuf_init
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function initializes
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.Fa msgbuf
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so that buffers may be appended to it.
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The
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.Em fd
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member should also be set directly before
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.Fn msgbuf_write
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is used.
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.Pp
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.Fn msgbuf_clear
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empties a msgbuf, removing and discarding any queued buffers.
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.Pp
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The
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.Fn msgbuf_write
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routine calls
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.Xr sendmsg 2
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to transmit buffers queued in
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.Fa msgbuf .
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It returns 1 if it succeeds, \-1 on error, and 0 when the queue was empty
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or an EOF condition on the socket is detected.
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Temporary resource shortages are returned with errno
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.Er EAGAIN
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and require the application to retry again in the future.
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.Pp
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.Fn msgbuf_drain
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discards data from buffers queued in
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.Fa msgbuf
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until
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.Fa n
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bytes have been removed or
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.Fa msgbuf
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is empty.
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.Sh EXAMPLES
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In a typical program, a channel between two processes is created with
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.Xr socketpair 2 ,
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and an
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.Em imsgbuf
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created around one file descriptor in each process:
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.Bd -literal -offset indent
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struct imsgbuf parent_ibuf, child_ibuf;
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int imsg_fds[2];
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if (socketpair(AF_UNIX, SOCK_STREAM, PF_UNSPEC, imsg_fds) == -1)
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err(1, "socketpair");
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switch (fork()) {
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case -1:
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err(1, "fork");
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case 0:
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/* child */
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close(imsg_fds[0]);
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imsg_init(&child_ibuf, imsg_fds[1]);
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exit(child_main(&child_ibuf));
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}
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/* parent */
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close(imsg_fds[1]);
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imsg_init(&parent_ibuf, imsg_fds[0]);
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exit(parent_main(&parent_ibuf));
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.Ed
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.Pp
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Messages may then be composed and queued on the
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.Em imsgbuf ,
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for example using the
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.Fn imsg_compose
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function:
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.Bd -literal -offset indent
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enum imsg_type {
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IMSG_A_MESSAGE,
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IMSG_MESSAGE2
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};
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int
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child_main(struct imsgbuf *ibuf)
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{
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int idata;
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...
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idata = 42;
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imsg_compose(ibuf, IMSG_A_MESSAGE,
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0, 0, -1, &idata, sizeof idata);
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...
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}
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.Ed
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.Pp
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A mechanism such as
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.Xr poll 2
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or the
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.Xr event 3
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library is used to monitor the socket file descriptor.
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When the socket is ready for writing, queued messages are transmitted with
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.Fn msgbuf_write :
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.Bd -literal -offset indent
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if (msgbuf_write(&ibuf-\*(Gtw) \*(Lt= 0 && errno != EAGAIN) {
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/* handle write failure */
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}
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.Ed
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.Pp
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And when ready for reading, messages are first received using
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.Fn imsg_read
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and then extracted with
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.Fn imsg_get :
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.Bd -literal -offset indent
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void
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dispatch_imsg(struct imsgbuf *ibuf)
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{
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struct imsg imsg;
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ssize_t n, datalen;
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int idata;
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if ((n = imsg_read(ibuf)) == -1 || n == 0) {
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/* handle socket error */
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}
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for (;;) {
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if ((n = imsg_get(ibuf, &imsg)) == -1) {
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/* handle read error */
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}
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if (n == 0) /* no more messages */
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return;
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datalen = imsg.hdr.len - IMSG_HEADER_SIZE;
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switch (imsg.hdr.type) {
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case IMSG_A_MESSAGE:
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if (datalen \*(Lt sizeof idata) {
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/* handle corrupt message */
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}
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memcpy(&idata, imsg.data, sizeof idata);
|
|
/* handle message received */
|
|
break;
|
|
...
|
|
}
|
|
|
|
imsg_free(&imsg);
|
|
}
|
|
}
|
|
.Ed
|
|
.Sh SEE ALSO
|
|
.Xr socketpair 2 ,
|
|
.Xr unix 4
|