msgop(2)

NAME

msgsnd, msgrcv — message operations

SYNOPSIS

#include <sys/msg.h> 

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

int msgrcv( 
    int     msqid, 
    void   *msgp, 
    size_t  msgsz, 
    long    msgtyp, 
    int     msgflg 
); 

DESCRIPTION

The msgsnd() system call sends a message to the queue associated with the message queue identifier specified by msqid.

msgp points to a user-defined buffer that must contain first a field of type long that specifies the type of the message, followed by 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:

long  mtype;     /* message type */ 
char  mtext[];   /* message text */ 

mtype is a positive integer that can be used by the receiving process for message selection (see msgrcv() below). mtext is any text of length msgsz bytes. msgsz 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 msg_qbytes (see message queue identifier in glossary(9)).
The total number of messages on all queues system-wide is equal to the system-imposed limit.

These actions are as follows:

If (msgflg & IPC_NOWAIT) is true, the message is not sent and the calling process returns immediately.
If (msgflg & IPC_NOWAIT) 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.
- msqid is removed from the system (see msgctl(2)). When this occurs, errno is set to [EIDRM] and a value of -1 is returned.
- The calling process receives a signal to be caught. In this case, the message is not sent and the calling process resumes execution in the manner prescribed in signal(5).

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid:

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

The msgrcv() system call 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. 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 & MSG_NOERROR) 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:

msgtyp = 0: First message on the queue is received.
msgtyp > 0: First message of type msgtyp is received.
msgtyp < 0: 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 & IPC_NOWAIT) is true, the calling process returns immediately with a value of -1 and errno set to [ENOMSG].
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.
- msqid is removed from the system. When this occurs, errno is set 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(5)).

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid.

msg_qnum is decremented by 1.
msg_lrpid is set to the process ID of the calling process.
msg_rtime is set to the current time.

RETURN VALUE

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.

ERRORS

If msgrcv() fails, errno is set to one of the following values.

[E2BIG]: mtext is greater than msgsz and (msgflg & MSG_NOERROR) is false.
[EACCES]: Operation permission is denied to the calling process.
[EFAULT]: msgp points to an illegal address. The reliable detection of this error is implementation dependent.
[EIDRM]: The message queue identifier msqid has been removed from the system.
[EINTR]: The function msgrcv() was interrupted by a signal.
[EINVAL]: msqid is not a valid message queue identifier.
[EINVAL]: msgsz is less than 0.
[ENOMSG]: The queue does not contain a message of the desired type and (msgflg & IPC_NOWAIT) is true.

If msgsnd() fails, errno is set to one of the following values.

[EACCES]: Operation permission is denied to the calling process.
[EAGAIN]: The message cannot be sent for one of the reasons cited above and (msgflg & IPC_NOWAIT) is true.
[EFAULT]: msgp points to an illegal address. The reliable detection of this error is implementation dependent.
[EIDRM]: The message queue identifier msqid has been removed from the system.
[EINTR]: msgsnd() was interrupted by a signal.
[EINVAL]: msqid is not a valid message queue identifier.
[EINVAL]: mtype is less than 1.
[EINVAL]: msgsz is less than zero or greater than the system-imposed limit.

STANDARDS CONFORMANCE

msgrcv(): SVID2, SVID3, XPG2, XPG3, XPG4

msgsnd(): SVID2, SVID3, XPG2, XPG3, XPG4

msgop(2)

Technical documentation

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