The msgsnd() function is used to send a message to the queue associated with the message queue identifier specified by msqid.

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 mtype; /* message type */ char mtext[1]; /* message text */ }

The mtype member is a non-zero positive type long int that can be used by the receiving process for message selection.

The mtext member is any text of length msgsz bytes. The msgsz argument can range from 0 to a system-imposed maximum.

The msgflg argument specifies the action to be taken if one or more of the following are true:

• The number of bytes already on the queue is equal to msg_qbytes; see intro(2).
• The total number of messages on all queues system-wide is equal to the system-imposed limit.

These actions are as follows:

• If (msgflg&IPC_NOWAIT) is non-zero, the message will not be sent and the calling process will return immediately.
• If (msgflg&IPC_NOWAIT) is 0, the calling process will suspend execution until one of the following occurs:
  • The condition responsible for the suspension no longer exists, in which case the message is sent.
  • 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 the message is not sent 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 incremented by 1.
• msg_lspid is set equal to the process ID of the calling process.
• msg_stime is set equal to the current time.

Upon successful completion, 0 is returned. Otherwise, -1 is returned, no message is sent, and errno is set to indicate the error.

The msgsnd() function will fail if:

EACCES

Operation permission is denied to the calling process. See intro(2).

EAGAIN

The message cannot be sent for one of the reasons cited above and (msgflg&IPC_NOWAIT) is non-zero.

EIDRM

The message queue identifier msgid is removed from the system.

EINTR

The msgsnd() function was interrupted by a signal.

EINVAL

The value of msqid is not a valid message queue identifier, or the value of mtype is less than 1; or the value of msgsz is less than 0 or greater than the system-imposed limit.

The msgsnd() 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), msgrcv(2), sigaction(2)