NAME

msgsnd — send a message to a message queue

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/msg.h>

int
msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

DESCRIPTION

The msgsnd() function sends a message from the message queue specified in msqid. The msgp argument is a pointer to a user-defined structure containing the message. This structure must contain a first field of type long that will indicate the user-defined type of the message. The remaining fields will contain the contents of the message. The following is an example of what this user-defined structure might look like:

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

The mtype field is an integer greater than 0 that can be used for selecting messages (see msgrcv(2)). The mtext field is an array of bytes of length msgsz, with size up to the system limit MSGMAX.

If the number of bytes already on the message queue plus msgsz is greater than the maximum number of bytes in the message queue (msg_qbytes, see msgctl(2)), or if the number of messages on all queues system-wide is already equal to the system limit, msgflg determines the action of msgsnd(). If msgflg has IPC_NOWAIT mask set in it, the call will return immediately. If msgflg does not have IPC_NOWAIT set in it, the call will block until:

The condition which caused the call to block no longer exists. The message was sent.
The message queue is removed, in which case -1 will be returned and errno set to EINVAL.
The caller catches a signal. The call returns with errno set to EINTR.

After a successful call, the data structure associated with the message queue is updated in the following way:

msg_qnum is incremented by 1.
msg_lspid is set to the pid of the calling process.
msg_stime is set to the current time.

RETURN VALUES

Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

msgsnd() will fail if:

[EACCES]

The calling process does not have write access to the message queue.

[EAGAIN]

There was no space for this message either on the queue or in the whole system, and IPC_NOWAIT was set in msgflg.

[EFAULT]

msgp points to an invalid address.

[EINTR]

The system call was interrupted by the delivery of a signal.

[EINVAL]

The msqid argument is not a valid message queue identifier, or the value of mtype is less than 1.

The message queue was removed while msgsnd() was waiting for a resource to become available in order to deliver the message.

The msgsz argument is greater than msg_qbytes or SSIZE_MAX.

STANDARDS

The msgsnd system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”) and IEEE Std 1003.1-2001 (“POSIX.1”).

HISTORY

Message queues appeared in the first release of AT&T System V UNIX.