msgop(S)

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

msgrcv- receives a message

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 */

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.

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

These actions are:

• 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.

  
  -

  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 sigaction(S) or 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 & IPC_NOWAIT) 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)):

• 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:

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

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 returns immediately with a return 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 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 sigaction(S) or 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 & MSG_NOERROR) 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 & IPC_NOWAIT) is ``true''.

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 equal to the process ID of the calling process.

• msg_rtime is set equal to the current time.

Diagnostics

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

Standards conformance

X/Open Portability Guide, Issue 3, 1989 .