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

The msgp argument points to a user-defined buffer that must contain first a field of type long int that will specify the type of the message, and then a data portion that will hold the data bytes of the message. The structure below is an example of what this user-defined buffer might look like:

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

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

The mtext member is the text of the message.

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

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

• If msgtyp is 0, the first message on the queue is received.
• If msgtyp is greater than 0, the first message of type msgtyp is 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 is received.

The msgflg argument specifies which of the following actions is to be taken if a message of the desired type is not on the queue:

• If (msgflg&IPC_NOWAIT) is non-zero, the calling process will return immediately with a return value of -1 and errno set to ENOMSG.
• If (msgflg&IPC_NOWAIT) is 0, the calling process will 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 (see msgctl(2)); when this occurs, errno is set equal to EIDRM and -1 is returned.
  • The calling process receives a signal that is to be caught; in this case a message is not received and the calling process resumes execution in the manner prescribed in sigaction(2).

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see intro(2)):

• msg_qnum is decremented by 1.
• msg_lrpid is set equal to the process ID of the calling process.
• msg_rtime is set equal to the current time.

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

The msgrcv() function will 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 intro(2).

EIDRM

The message queue identifier msqid is removed from the system.

EINTR

The msgrcv() function was interrupted by a signal.

EINVAL

The msqid argument 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 msgrcv() function may fail if:

EFAULT

The msgp argument points to an illegal address.

The value passed as the msgp argument should be converted to type void *.

intro(2), msgctl(2), msgget(2), msgsnd(2), sigaction(2)