MSGOP

Section: Linux Programmer's Manual (2)
Updated: 2000-07-10
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NAME

msgop - message operations

SYNOPSIS

# include <sys/types.h>


# include <sys/ipc.h>


# include <sys/msg.h>

int msgsnd ( int msqid, struct msgbuf *msgp, size_t msgsz, int msgflg )

ssize_t msgrcv ( int msqid, struct msgbuf *msgp, size_t msgsz, long msgtyp, int msgflg )

DESCRIPTION

To send or receive a message, the calling process allocates a structure that looks like the following

       struct msgbuf {
               long    mtype;   /* message type, must be > 0 */

               char    mtext[1];        /* message data */

       };

but with an array
mtext of size msgsz, a non-negative integer value. The structure member mtype must have a strictly positive integer value that can be used by the receiving process for message selection (see the section about msgrcv).

The calling process must have write access permissions to send and read access permissions to receive a message on the queue.

The msgsnd system call enqueues a copy of the message pointed to by the msgp argument on the message queue whose identifier is specified by the value of the msqid argument.

The argument msgflg specifies the system call behaviour if enqueuing the new message will require more than msg_qbytes in the queue. Asserting IPC_NOWAIT the message will not be sent and the system call fails returning with errno set to EAGAIN. Otherwise the process is suspended until the condition for the suspension no longer exists (in which case the message is sent and the system call succeeds), or the queue is removed (in which case the system call fails with errno set to EIDRM), or the process receives a signal that has to be caught (in which case the system call fails with errno set to EINTR).

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 reads a message from the message queue specified by msqid into the msgbuf pointed to by the msgp argument removing from the queue, on success, the read message.

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 if the msgflg argument asserts MSG_NOERROR, the message text will be truncated (and the truncated part will be lost), otherwise the message isn't removed from the queue and the system call fails returning with errno set to E2BIG.

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

: If msgtyp is 0, then the message on the queue's front is read.
: If msgtyp is greater than 0, then the first message on the queue of type msgtyp is read if MSG_EXCEPT isn't asserted by the msgflg argument, otherwise the first message on the queue of type not equal to msgtyp will be read.
: If msgtyp is less than 0, then the first message on the queue with the lowest type less than or equal to the absolute value of msgtyp will be read.

The msgflg argument asserts none, one or more (or-ing them) among the following flags:

: IPC_NOWAIT For immediate return if no message of the requested type is on the queue. The system call fails with errno set to ENOMSG.
: MSG_EXCEPT Used with msgtyp greater than 0 to read the first message on 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 asserted in msgflg, the calling process is blocked until one of the following conditions occurs:

: A message of the desired type is placed on the queue.
: The message queue is removed from the system. In such a case the system call fails with errno set to EIDRM.
: The calling process receives a signal that has to be caught. In such a 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 a failure both functions return -1 with errno indicating the error, otherwise msgsnd returns 0 and msgrvc returns the number of bytes actually copied into the mtext array.

ERRORS

When msgsnd fails, at return errno will be set to one among the following values:

EAGAIN: The message can't be sent due to the msg_qbytes limit for the queue and IPC_NOWAIT was asserted in mgsflg.
EACCES: The calling process has no write access permissions on the message queue.
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 received a signal that had to be caught.
EINVAL: Invalid msqid value, or nonpositive mtype value, or invalid msgsz value (less than 0 or greater than the system value MSGMAX).
ENOMEM: The system has not enough memory to make a copy of the supplied msgbuf.

When msgrcv fails, at return errno will be set to one among the following values:

E2BIG: The message text length is greater than msgsz and MSG_NOERROR isn't asserted in msgflg.
EACCES: The calling process has no read access permissions on the message queue.
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 received a signal that had to be caught.
EINVAL: Illegal msgqid value, or msgsz less than 0.
ENOMSG: IPC_NOWAIT was asserted in msgflg and no message of the requested type existed on the message queue.

NOTES

The followings are system limits affecting a msgsnd system call:

MSGMAX: Maximum size for a message text: the implementation set this value to 4080 bytes.
MSGMNB: Default maximum size in bytes of a message queue: policy dependent. The super-user can increase the size of a message queue beyond MSGMNB by a msgctl 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).

CONFORMING TO

SVr4, SVID.

NOTE

The pointer argument is declared as struct msgbuf * with libc4, libc5, glibc 2.0, glibc 2.1. It is declared as void * (const void * for msgsnd()) with glibc 2.2, following the SUSv2.

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Time: 02:14:36 GMT, January 21, 2025