Name

msgop, msgrcv, msgsnd — message operations

Synopsis

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd( int   msqid,
  const void *  msgp,
  size_t   msgsz,
  int   msgflg);
ssize_t msgrcv( int   msqid,
  void *  msgp,
  size_t   msgsz,
  long   msgtyp,
  int   msgflg);

DESCRIPTION

The msgsnd() and msgrcv() system calls are used, respectively, to send messages to, and receive messages from, a message queue. The calling process must have write permission on the message queue in order to send a message, and read permission to receive a message.

The msgp argument is a pointer to caller-defined structure of the following general form:

struct msgbuf {
  long   mtype;
/* message type, must be > 0 */
  char   mtext[1];
/* message data */
};

The mtext field is an array (or other structure) whose size is specified by msgsz, a non-negative integer value. Messages of zero length (i.e., no mtext field) are permitted. The mtype field must have a strictly positive integer value. This value can be used by the receiving process for message selection (see the description of msgrcv() below).

The msgsnd() system call appends a copy of the message pointed to by msgp to the message queue whose identifier is specified by msqid.

If sufficient space is available in the queue, msgsnd() succeeds immediately. (The queue capacity is defined by the msg_bytes field in the associated data structure for the message queue. During queue creation this field is initialized to MSGMNB bytes, but this limit can be modified using msgctl(2).) If insufficient space is available in the queue, then the default behavior of msgsnd() is to block until space becomes available. If IPC_NOWAIT is specified in msgflg, then the call instead fails with the error EAGAIN.

A blocked msgsnd() call may also fail if the queue is removed (in which case the system call fails with errno set to EIDRM), or a signal is caught (in which case the system call fails with errno set to EINTR). (msgsnd and msgrcv are never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flag when establishing a signal handler.)

Upon successful completion the message queue data structure is updated as follows:

  • msg_lspid is set to the process ID of the calling process.

  • msg_qnum is incremented by 1.

  • msg_stime is set to the current time.

The system call msgrcv() removes a message from the queue specified by msqid and places it in the buffer pointed to msgp.

The argument msgsz specifies the maximum size in bytes for the member mtext of the structure pointed to by the msgp argument. If the message text has length greater than msgsz, then the behavior depends on whether MSG_NOERROR is specified in msgflg. If MSG_NOERROR is specified, then the message text will be truncated (and the truncated part will be lost); if MSG_NOERROR is not specified, then the message isn't removed from the queue and the system call fails returning −1 with errno set to E2BIG.

The argument msgtyp specifies the type of message requested as follows:

  • If msgtyp is 0, then the first message in the queue is read.

  • If msgtyp is greater than 0, then the first message in the queue of type msgtyp is read, unless MSG_EXCEPT was specified in msgflg, in which case the first message in the queue of type not equal to msgtyp will be read.

  • If msgtyp is less than 0, then the first message in the queue with the lowest type less than or equal to the absolute value of msgtyp will be read.

The msgflg argument is a bit mask constructed by ORing together zero or more of the following flags:

IPC_NOWAIT

Return immediately if no message of the requested type is in the queue. The system call fails with errno set to ENOMSG.

MSG_EXCEPT

Used with msgtyp greater than 0 to read the first message in the queue with message type that differs from msgtyp.

MSG_NOERROR

To truncate the message text if longer than msgsz bytes.

If no message of the requested type is available and IPC_NOWAIT isn't specified in msgflg, the calling process is blocked until one of the following conditions occurs:

  • A message of the desired type is placed in the queue.

  • The message queue is removed from the system. In this case the system call fails with errno set to EIDRM.

  • The calling process catches a signal. In this case the system call fails with errno set to EINTR.

Upon successful completion the message queue data structure is updated as follows:

  • msg_lrpid is set to the process ID of the calling process.

  • msg_qnum is decremented by 1.

  • msg_rtime is set to the current time.

RETURN VALUE

On failure both functions return −1 with errno indicating the error, otherwise msgsnd() returns 0 and msgrcv() returns the number of bytes actually copied into the mtext array.

ERRORS

When msgsnd() fails, errno will be set to one among the following values:

EACCES

The calling process does not have write permission on the message queue, and does not have the CAP_IPC_OWNER capability.

EAGAIN

The message can't be sent due to the msg_qbytes limit for the queue and IPC_NOWAIT was specified in msgflg.

EFAULT

The address pointed to by msgp isn't accessible.

EIDRM

The message queue was removed.

EINTR

Sleeping on a full message queue condition, the process caught a signal.

EINVAL

Invalid msqid value, or non-positive mtype value, or invalid msgsz value (less than 0 or greater than the system value MSGMAX).

ENOMEM

The system does not have enough memory to make a copy of the message pointed to by msgp.

When msgrcv() fails, errno will be set to one among the following values:

E2BIG

The message text length is greater than msgsz and MSG_NOERROR isn't specified in msgflg.

EACCES

The calling process does not have read permission on the message queue, and does not have the CAP_IPC_OWNER capability.

EAGAIN

No message was available in the queue and IPC_NOWAIT was specified in msgflg.

EFAULT

The address pointed to by msgp isn't accessible.

EIDRM

While the process was sleeping to receive a message, the message queue was removed.

EINTR

While the process was sleeping to receive a message, the process caught a signal.

EINVAL

msgqid was invalid, or msgsz was less than 0.

ENOMSG

IPC_NOWAIT was specified in msgflg and no message of the requested type existed on the message queue.

CONFORMING TO

SVr4, POSIX.1-2001.

NOTES

The msgp argument is declared as struct msgbuf * with libc4, libc5, glibc 2.0, glibc 2.1. It is declared as void * with glibc 2.2 and later, as required by SUSv2 and SUSv3.

The following limits on message queue resources affect the msgsnd() call:

MSGMAX

Maximum size for a message text: 8192 bytes (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmax).

MSGMNB

Default maximum size in bytes of a message queue: 16384 bytes (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmnb). The superuser can increase the size of a message queue beyond MSGMNB by a msgctl(2) system call.

The implementation has no intrinsic limits for the system wide maximum number of message headers (MSGTQL) and for the system wide maximum size in bytes of the message pool (MSGPOOL).

SEE ALSO

msgctl(2), msgget(2), capabilities(7), mq_overview(7), svipc(7)

COLOPHON

This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


  Copyright 1993 Giorgio Ciucci <giorgiocrcc.it>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esrthyrsus.com>
Modified Mon Jul 10 21:09:59 2000 by aeb
Modified 1 Jun 2002, Michael Kerrisk <mtk.manpagesgmail.com>
Language clean-ups.
Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX
Added note on restart behavior of msgsnd() and msgrcv()
Formatting clean-ups (argument and field names marked as .I
instead of .B)
Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com>
    Added notes on capability requirements
Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com>
Language and formatting clean-ups
Added notes on /proc files