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msgsnd(2) [debian man page]

MSGSND(2)						      BSD System Calls Manual							 MSGSND(2)

NAME
msgsnd -- send a message to a message queue LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg); DESCRIPTION
The msgsnd() function sends a message to the message queue specified in msqid. The msgp argument points to a structure containing the mes- sage. This structure should consist of the following members: long mtype; /* message type */ char mtext[1]; /* 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: o The condition which caused the call to block does no longer exist. The message will be sent. o The message queue is removed, in which case -1 will be returned, and errno is set to EINVAL. o 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: o msg_cbytes is incremented by the size of the message. o msg_qnum is incremented by 1. o msg_lspid is set to the pid of the calling process. o msg_stime is set to the current time. RETURN VALUES
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. ERRORS
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 mes- sage. 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. HISTORY
Message queues appeared in the first release of AT&T Unix System V. BUGS
NetBSD and FreeBSD do not define the EIDRM error value, which should be used in the case of a removed message queue. BSD
July 9, 2009 BSD

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msgsnd(2)							   System Calls 							 msgsnd(2)

NAME
msgsnd - message send operation SYNOPSIS
#include <sys/msg.h> int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg); DESCRIPTION
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 mes- sage, 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: o The number of bytes already on the queue is equal to msg_qbytes. See intro(2). o The total number of messages on the queue would exceed the maximum allowed by the system. See NOTES. These actions are as follows: o If (msgflg&IPC_NOWAIT) is non-zero, the message will not be sent and the calling process will return immediately. o If (msgflg&IPC_NOWAIT) is 0, the calling process will suspend execution until one of the following occurs: o The condition responsible for the suspension no longer exists, in which case the message is sent. o 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. o 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)): o msg_qnum is incremented by 1. o msg_lspid is set equal to the process ID of the calling process. o msg_stime is set equal to the current time. RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, -1 is returned, no message is sent, and errno is set to indicate the error. ERRORS
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. 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. USAGE
The value passed as the msgp argument should be converted to type void *. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ SEE ALSO
rctladm(1M), intro(2), msgctl(2), msgget(2), msgrcv(2), setrctl(2), sigaction(2), attributes(5), standards(5) NOTES
The maximum number of messages allowed on a message queue is the minimum enforced value of the process.max-msg-messages resource control of the creating process at the time msgget(2) was used to allocate the queue. See rctladm(1M) and setrctl(2) for information about using resource controls. SunOS 5.10 11 Feb 2003 msgsnd(2)

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