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
Check Out this Related Man Page
msgop(2) System Calls Manual msgop(2) Name msgsnd, msgrcv - message operations Syntax #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgsnd (msqid, msgp, msgsz, msgflg) int msqid; void *msgp; size_t msgsz; int msgflg; int msgrcv (msqid, msgp, msgsz, msgtyp, msgflg) int msqid; void *msgp; size_t msgsz; long msgtyp; int msgflg; Description There are two message operations system calls, and msgsnd The system call is used to send a message to the queue associated with the message queue identifier specified by msqid. The msgp parameter points to a structure containing the message. This structure is composed of the following members: long mtype; /* message type */ char mtext[]; /* message text */ The mtype parameter is a positive integer that can be used by the receiving process for message selection. For more information, see the section of this reference page. The mtext parameter is any text of length msgsz bytes. The msgsz parameter can range from 0 to a system- imposed maximum. The msgflg parameter specifies the action to be taken if the number of bytes already on the queue is equal to msg_qbytes. (For further information, see ) The parameter also specifies what happens when the total number of messages on all queues system-wide is equal to the system-imposed limit. If either of these conditions is true, and if (msgflg & IPC_NOWAIT) is true, the message is not sent and the calling process returns imme- diately. However, if either of the conditions is true and (msgflg & IPC_NOWAIT) is false, the calling process suspends 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 msqid parameter is removed from the system. For further information, see When this occurs, errno is set equal to EIDRM, and a value of -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 The system call fails and no message is sent under the following conditions: [EINVAL] The msqid parameter is not a valid message queue identifier. [EACCES] Operation permission is denied to the calling process. For more information, see [EINVAL] The mtype parameter is less than 1. [EAGAIN] The message cannot be sent for one of the reasons cited above and (msgflg & IPC_NOWAIT) is true. [EINVAL] The msgsz parameter is less than zero or greater than the system-imposed limit. [EFAULT] The msgp parameter points to an illegal address. Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (for more information, see ): o The msg_qnum is incremented by 1. o The msg_lspid is set equal to the process ID of the calling process. o The msg_stime is set equal to the current time. msgrcv The system call reads a message from the queue associated with the message queue identifier specified by msqid and places it in the struc- ture pointed to by msgp. This structure is composed of the following members: long mtype; /* message type */ char mtext[]; /* message text */ The mtype parameter is the received message's type, as specified by the sending process. The mtext parameter is the text of the message. The msgsz parameter 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 true. The truncated part of the message is lost and no indication of the truncation is given to the calling process. The msgtyp parameter specifies the type of message requested, as follows: o If msgtyp is equal to 0, the first message on the queue is received. o If msgtyp is greater than 0, the first message of type msgtyp is received. o 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 parameter specifies the action to be taken if a message of the desired type is not on the queue. These specified actions are as follows: o If (msgflg & IPC_NOWAIT) is true, the calling process returns immediately with a value of -1 and errno set to ENOMSG. o If (msgflg & IPC_NOWAIT) is false, the calling process suspends execution until one of the following occurs: - A message of the desired type is placed on the queue. - The msqid parameter is removed from the system. When this occurs, errno is set equal to EIDRM, and a value of -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 The system call fails and no message is received under the following conditions: [EINVAL] The msqid parameter is not a valid message queue identifier. [EACCES] Operation permission is denied to the calling process. [EINVAL] The msgsz parameter is less than 0. [E2BIG] The mtext parameter is greater than msgsz and (msgflg & MSG_NOERROR) is false. [ENOMSG] The queue does not contain a message of the desired type and (msgtyp & IPC_NOWAIT) is true. [EFAULT] The msgp parameter points to an illegal address. Upon successful completion, the following actions are taken with respect to the data structure associated with msqid: o The msg_qnum is decremented by 1. o The msg_lrpid is set equal to the process ID of the calling process. o The msg_rtime is set equal to the current time. Return Values If the or system calls return due to the receipt of a signal, a value of -1 is returned to the calling process, and errno is set to EINTR. If they return due to removal of msqid from the system, a value of -1 is returned, and errno is set to EIDRM. Upon successful completion, the return value is as follows: o The system call returns a value of 0. o The system call returns a value equal to the number of bytes actually placed into mtext. Otherwise, a value of -1 is returned, and errno is set to indicate the error. See Also errno(2), intro(2), msgctl(2), msgget(2), signal(3) msgop(2)