NAME

msgrcv — XSI message receive operation

SYNOPSIS

[XSI] #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 XBD 3.206 Message Queue). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in 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 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 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.

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 2.7 XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.7 XSI Interprocess Communication, 2.8 Realtime, mq_close, mq_getattr, mq_notify, mq_open, mq_receive, mq_send, mq_setattr, mq_unlink, msgctl, msgget, msgsnd, sigaction

XBD 3.206 Message Queue, <sys/msg.h>

CHANGE HISTORY

First released in Issue 2. Derived from Issue 2 of the SVID.

Issue 5

The type of the return value is changed from int to ssize_t, and a warning is added to the DESCRIPTION about values of msgsz larger the {SSIZE_MAX}.

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to the APPLICATION USAGE section.

Issue 6

The normative text is updated to avoid use of the term "must" for application requirements.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0398 [345] and XSH/TC1-2008/0399 [421] are applied.