MSGOP(2) — SYSTEM CALLS

NAME

msgop, msgsnd, msgrcv − message operations

SYNOPSIS

#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

msgsnd() is used to send a message to the queue associated with the message queue identifier specified by msqid. [WRITE] (see msgctl(2)) msgp points to a structure containing the message. This structure is composed of the following members:

longmtype;/∗ message type ∗/
charmtext[1];/∗ 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 are true:

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

• 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 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:

• 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 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(3V).

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see intro(2)).

• msg_qnum is incremented by 1.

• msg_lspid is set equal to the process ID of the calling process.

• msg_stime 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:

longmtype;/∗ message type ∗/
charmtext[1];/∗ 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:

• 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 & IPC_NOWAIT) is “true”, the calling process will return immediately with a return value of −1 and errno set to ENOMSG.

• If (msgflg & IPC_NOWAIT) is “false”, the calling process will suspend 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(3V).

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see intro(2)).

• msg_qnum is decremented by 1.

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

• msg_rtime is set equal to the current time.

RETURN VALUES

msgsnd() returns:

0 on success.

−1 on failure and sets errno to indicate the error.

msgrcv() returns the number of bytes actually placed into mtext on success. On failure, it returns −1 and sets errno to indicate the error.

ERRORS

msgsnd() will fail and no message will be sent if one or more of the following are true:

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 “true”.

EFAULT msgp points to an illegal address.

EIDRM The message queue referred to by msqid was removed from the system.

EINTR The call was interrupted by the delivery of a signal.

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.

msgrcv() will fail and no message will be received if one or more of the following are true:

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.

EIDRM The message queue referred to by msqid was removed from the system.

EINTR The call was interrupted by the delivery of a signal.

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 & IPC_NOWAIT) is “true”.

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