msgop(2)
NAME
msgsnd, msgrcv − message operations, msgsnd and msgrcv
SYNTAX
#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
There are two message operations system calls, msgsnd and msgrcv.
msgsnd
The msgsnd 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:
longmtype;/∗ message type ∗/
charmtext[];/∗ 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 msgrcv 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 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(.).
The total number of messages on all queues system-wide is equal to the system-imposed limit.
These specified 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.
The msqid parameter is removed from the system. For further information, see msgctl(.). 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(.).
The msgsnd system call will fail and no message will be sent if one or more of the following are true:
[EINVAL] The msqid parameter is not a valid message queue identifier.
[EACCES] Operation permission is denied to the calling process. For more information, see errno(.).
[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 errno(.).
The msg_qnum is incremented by 1.
The msg_lspid is set equal to the process ID of the calling process.
The msg_stime is set equal to the current time.
msgrcv
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 ∗/
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:
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 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:
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.
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 signal(.).
The msgrcv system call will fail and no message will be received if one or more of the following are true:
[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.
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 the msgsnd or msgrcv 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:
The msgsnd system call returns a value of 0.
The msgrcv 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.