MSGOP(2)

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 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 that can be used by the receiving process for message selection (see the section about msgrcv).

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

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 on 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 initialised to MSGMNB bytes, but this limit can be modified using msgctl.) If insufficient space is available on the queue, then the default behaviour of msgsnd is to block until space becomes available. If IPC_NOWAIT is asserted 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 reads a message from the message queue specified by msqid into the msgbuf pointed to by the msgp argument, removing the read message from the queue.

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 first message in the queue is read.

    If msgtyp is greater than 0, then the first message on the queue of type msgtyp is read, unless MSG_EXCEPT was asserted in msgflg, in which case 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) of 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 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 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 permission 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 caught a signal.

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 does not have read permission 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 8192 bytes.

MSGMNB
    Default maximum size in bytes of a message queue: 16384 bytes. 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.

SEE ALSO
ipc(5), msgctl(2), msgget(2), msgrcv(2), msgsnd(2)