MSGRCV(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
msgrcv — XSI message receive operation

SYNOPSIS

#include <sys/msg.h>

ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
    int msgflg);

DESCRIPTION
The msgrcv() function operates on XSI message queues (see the Base Definitions volume of POSIX.1-2017, Section 3.226, Message Queue). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime.

The msgrcv() function shall read a message from the queue associated with the message queue identifier specified by msqid and place it in the user-defined buffer pointed to by msgp.

The application shall ensure that the argument msgp points to a user-defined buffer that contains first a field of type long specifying the type of the message, and then a data portion that holds the data bytes of the message. The structure below is an example of what this user-defined buffer might look like:

    struct mymsg {
        long    mtype;     /* Message type. */
        char    mtext[1];  /* Message text. */
    }

The structure member mtype is the received message's type as specified by the sending process.

The structure member mtext is the text of the message.

The argument msgsz specifies the size in bytes of mtext. The received message shall be truncated to msgsz bytes if it is larger than msgsz and (msgflg & MSG_NOERROR) is non-zero. The truncated part of the message shall be lost and no indication of the truncation shall be given to the calling process.

If the value of msgsz is greater than {SSIZE_MAX}, the result is implementation-defined.

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

*   If msgtyp is 0, the first message on the queue shall be received.

*   If msgtyp is greater than 0, the first message of type msgtyp shall be received.

*   If msgtyp is less than 0, the first message of the lowest type that is less than or equal to the absolute value of msgtyp shall be received.

The argument msgflg specifies the action to be taken if a message of the desired type is not on the queue. These are as follows:

*   If (msgflg & IPC_NOWAIT) is non-zero, the calling thread shall return immediately with a return value of -1 and errno set to [ENOMSG].

*   If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend execution until one of the following occurs:

    --  A message of the desired type is placed on the queue.

    --  The message queue identifier msqid is removed from the system; when this occurs, errno shall be set to [EIDRM] and -1 shall be returned.

    --  The calling thread receives a signal that is to be caught; in this case a message is not received and the calling thread resumes execution in the manner prescribed in sigaction().

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid:

*   msg_qnum shall be decremented by 1.

*   msg_lrpid shall be set to the process ID of the calling process.

*   msg_rtime shall be set to the current time, as described in Section 2.7.1, IPC General Description.

RETURN VALUE
Upon successful completion, msgrcv() shall return a value equal to the number of bytes actually placed into the buffer mtext. Otherwise, no message shall be received, msgrcv() shall return -1, and errno shall be set to indicate the error.

ERRORS
The msgrcv() function shall fail if:

E2BIG
    The value of mtext is greater than msgsz and (msgflg & MSG_NOERROR) is 0.

EACCES
    Operation permission is denied to the calling process; see Section 2.7, XSI Interprocess Communication.

EIDRM
    The message queue identifier msqid is removed from the system.

EINTR
    The msgrcv() function was interrupted by a signal.

EINVAL
    msqid is not a valid message queue identifier.

ENOMSG
    The queue does not contain a message of the desired type and (msgflg & IPC_NOWAIT) is non-zero.

The following sections are informative.

EXAMPLES

  Receiving a Message

The following example receives the first message on the queue (based on the value of the msgtyp argument, 0). The queue is identified by the msqid argument (assuming that the value has previously been set). This call specifies that an error should be reported if no message is available, but not if the message is too large. The message size is calculated directly using the sizeof operator.

    #include <sys/msg.h>
    ...
    int result;
    int msqid;
    struct message {
        long type;
        char text[20];
    } msg;
    long msgtyp = 0;
    ...
    result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
             msgtyp, MSG_NOERROR | IPC_NOWAIT);

APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for interprocess communication (IPC). Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE
None.

FUTURE DIRECTIONS
None.

SEE ALSO
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, mq_close(), mq_getattr(), mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgctl(), msgget(), msgsnd(), sigaction()

The Base Definitions volume of POSIX.1-2017, Section 3.226, Message Queue, <sys_msg.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

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