msgop(2)

NAME

msgop − message operations

SYNTAX

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

int msgsnd(msqid, msgp, msgsz, msgflg)
int msqid, msgsz, msgflg;
struct msgbuf ∗msgp;

int msgrcv(msqid, msgp, msgsz, msgtyp, msgflg)
int msqid, msgsz, msgflg;
struct msgbuf ∗msgp;
long msgtyp;

DESCRIPTION

The msgsnd system call is used to send a message to the queue associated with the message queue identifier specified by msqid. The 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 is a positive integer that can be used by the receiving process for message selection (see below). The mtext is any text of length msgsz bytes. The specified msgsz 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:

•The number of bytes already on the queue is equal to msg_qbytes. For further information, see intro(2).

•The total number of messages on all system queues is equal to the system-imposed limit.

Actions are as follows:

•If ( msgflg & IPC_NOWAIT ) is true, the message will not be sent, and the calling process will return immediately.

•If ( msgflg & IPC_NOWAIT ) is false, the calling process will suspend execution until one of the following occurs:

1.The condition responsible for the suspension no longer exists, and the message is sent.

2.The msqid argument is removed from the system. For further information, see msgctl(2). When this occurs, the global variable errno is set equal to EIDRM, and a value of −1 is returned.

3.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(2).

Upon successful completion, the msgsnd call takes the following actions 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.

For further information, see intro(2).

The msgrcv 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 consists of:

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

The mtype is the received message’s type as specified by the sending process. The mtext is the text of the message. The 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.

The 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.

The 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 will return immediately with a return value of −1, and the global variable errno is set to ENOMSG.

•If ( msgflg & IPC_NOWAIT ) is false, the calling process will suspend execution until one of the following occurs:

1.A message of the desired type is placed on the queue.

2.The msqid is removed from the system. When this occurs, the global variable errno is set equal to EIDRM, and a value of −1 is returned.

3.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(2).

Upon successful completion, the msgrcv call takes the following actions with respect to the data structure associated with msqid. For further information, see intro(2).

•The msg_qnum is decremented by 1.

•The msg_lrpid is set equal to the process ID of the calling process.

•The msg_rtime is set equal to the current time.

RETURN VALUES

If successful, the return values are as follows:

•The msgsnd call returns a value of 0.

•The msgrcv call returns a value equal to the number of bytes actually placed into mtext.

If msgsnd or msgrcv return due to the receipt of a signal, they return a −1, and the global variable errno is set to EINTR. If they return due to removal of msqid from the system, they return a −1, and the global variable errno is set to EIDRM.

Otherwise, if unsuccessful, both calls return a −1, and the global variable errno indicates the error code.

DIAGNOSTICS

The msgsnd call will fail if:

[EINVAL] The specified msqid is not a valid message queue identifier.

[EACCES] Operation permission is denied to the calling process. For further information, see intro(2).

[EINVAL] The mtype 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 specified msgsz is less than zero or greater than the system-imposed limit.

[EFAULT] The specified msgp points to an illegal address.

The msgrcv call will fail if:

[EINVAL] The specified msqid is not a valid message queue identifier.

[EACCES] Operation permission is denied to the calling process.

[EINVAL] The specified msgsz is less than 0.

[E2BIG] The mtext 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.