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msgsnd — send a message to a message queue
Standard C Library (libc, -lc)
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
The msgsnd() function sends a message to the message queue specified in msqid. The msgp argument points to a structure containing the message. This structure should consist of the following members: long mtype; /* message type */ char mtext; /* body of message */ mtype is an integer greater than 0 that can be used for selecting messages (see msgrcv(2)), mtext is an array of msgsz bytes. The argument msgsz can range from 0 to a system-imposed maximum, MSGMAX. If the number of bytes already on the message queue plus msgsz is bigger than the maximum number of bytes on the message queue (msg_qbytes, see msgctl(2)), or 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 does no longer exist. The message will be sent. • The message queue is removed, in which case -1 will be returned, and errno is 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_cbytes is incremented by the size of the message. • 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.
The msgsnd() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
The msgsnd() function will fail if: [EINVAL] The msqid argument is not a valid message queue identifier. 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. The mtype argument is not greater than 0. [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] The msgp argument points to an invalid address. [EINTR] The system call was interrupted by the delivery of a signal.
Message queues appeared in the first release of AT&T Unix System V.
NetBSD and FreeBSD do not define the EIDRM error value, which should be used in the case of a removed message queue.