msgop(S) 6 January 1993 msgop(S) Name msgop - message operations Syntax cc . . . -lc #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgsnd (msqid, msgp, msgsz, msgflg) int msqid; struct msgbuf *msgp; int msgsz, msgflg; int msgrcv (msqid, msgp, msgsz, msgtyp, msgflg) int msqid; struct msgbuf *msgp; int msgsz; long msgtyp; int msgflg; Description The msgsnd system call is used to send a message to the queue associated with the message queue identifier specified by msqid. {WRITE} msgp 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 integer is positive and can be used by the receiving process for message selection (see msgrcv below). The array mtext is any text of length msgsz bytes. The msgsz argument can range from 0 to a system- imposed maximum. msgflg specifies the action to be taken if one or more of the following is true: + The number of bytes already on the queue is equal to msgqbytes. + The total number of messages on all queues system-wide is equal to the system-imposed limit. These actions are: + If (msgflg & IPCNOWAIT) is ``true'', the message is not sent and the calling process returns immediately. + If (msgflg & IPCNOWAIT) is ``false'', the calling process suspends execution until one of the following occurs: - The condition responsible for the suspension no longer exists, in which case the message is sent. - The msqid argument is removed from the system (see msgctl(S)). 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 the message is not sent and the calling process resumes execution in the manner prescribed in signal(S). msgsnd fails and no message is sent if one or more of the following is true: [EACCES] Operation permission is denied to the calling process (see Intro(S)). [EAGAIN] The message cannot be sent for one of the reasons cited above and (msgflg & IPCNOWAIT) is ``true''. [EFAULT] msgp points to an illegal address. [EINVAL] msqid is not a valid message queue identifier. mtype is less than 1. msgsz is less than zero or greater than the system-imposed limit. Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see Intro(S)): + msgqnum is incremented by 1. + msglspid is set equal to the process ID of the calling process. + msgstime is set equal to the current time. msgrcv reads a message from the queue associated with the message queue identifier specified by msqid and places it in the structure pointed to by msgp. {READ} This structure is composed of the following members: long mtype; /* message type */ char mtext[]; /* message text */ mtype is the received message's type as specified by the sending process. mtext is the text of the message. msgsz specifies the size in bytes of mtext. The received message is truncated to msgsz bytes if it is larger than msgsz and (msgflg & MSGNOERROR) is ``true''. The truncated part of the message is lost and no indication of the truncation is given to the calling process. msgtyp specifies the type of message requested as follows: + If msgtyp is equal to 0, the first message on the queue is received. + If msgtyp is greater than 0, the first message of type msgtyp is received. + 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. msgflg specifies the action to be taken if a message of the desired type is not on the queue. These are as follows: + If (msgflg & IPCNOWAIT) is ``true'', the calling process returns immediately with a return value of -1 and errno set to ENOMSG. + If (msgflg & IPCNOWAIT) is ``false'', the calling process suspends execution until one of the following occurs: - A message of the desired type is placed on the queue. - msqid 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 signal(S). msgrcv fails and no message is received if one or more of the following is true: [E2BIG] mtext is greater than msgsz and (msgflg & MSGNOERROR) is ``false''. [EACCES] Operation permission is denied to the calling process. [EFAULT] msgp points to an illegal address. [EINVAL] msqid is not a valid message queue identifier. msgsz is less than 0. [ENOMSG] The queue does not contain a message of the desired type and (msgtyp & IPCNOWAIT) is ``true''. Upon successful completion, the following actions are taken with respect to the data structure associated with msqid. msgqnum is decremented by 1. + msglrpid is set equal to the process ID of the calling process. + msgrtime is set equal to the current time. Diagnostics If msgsnd or msgrcv 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: + msgsnd returns a value of 0. + msgrcv 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 msgctl(S), msgget(S), signal(S) Standards conformance msgrcv and msgsnd are conformant with: AT&T SVID Issue 2; and X/Open Portability Guide, Issue 3, 1989.