msgrcv Subroutine

Base Operating System and Extensions Technical Reference, Volume 1

msgrcv Subroutine

Purpose

Reads a message from a queue.

Library

Standard C Library (libc.a)

Syntax

#include <sys/msg.h>

int msgrcv (MessageQueueID, MessagePointer,MessageSize,MessageType, MessageFlag)
int MessageQueueID, MessageFlag;
void *MessagePointer;
size_t MessageSize;
long int MessageType;

Description

The msgrcv subroutine reads a message from the queue specified by the MessageQueueID parameter and stores it into the structure pointed to by the MessagePointer parameter. The current process must have read permission in order to perform this operation.

Note: The routine may coredump instead of returning EFAULT when an invalid pointer is passed in case of 64-bit application calling 32-bit kernel interface.

The following limits apply to the message queue:

Maximum message size is 65,535 bytes for releases prior to AIX 4.1.5 and is 4 Megabytes for release 4.1.5 and later releases.
Maximum number of messages per queue is 8192.
Maximum number of message queue IDs is 4096 for AIX releases before 4.3.2 and 131072 for AIX 4.3.2 and following.
Maximum number of bytes in a queue is 4 65,535 for releases prior to AIX 4.1.5 and is 4 Megabytes for release 4.1.5 and later releases.

Note: For a 64-bit process, the mtype field is 64 bits long. However, for compatibility with 32-bit processes, the most significant 32 bits must be 0 and will not be put on the message queue. For a 64-bit receiver process, the mtype will again be extended to 64 bits with the most significant bits 0.

Parameters

MessageQueueID	Specifies the message queue identifier.
MessagePointer	Points to a msgbuf structure containing the message. The msgbuf structure is defined in the sys/msg.h file and contains the following fields: mtyp_t mtype; /* Message type / char mtext[1]; / Beginning of message text */ The `mtype` field contains the type of the received message as specified by the sending process. The `mtext` field is the text of the message.
MessageSize	Specifies the size of the `mtext` field in bytes. The received message is truncated to the size specified by the MessageSize parameter if it is longer than the size specified by the MessageSize parameter and if the MSG_NOERROR value is set in the MessageFlag parameter. The truncated part of the message is lost and no indication of the truncation is given to the calling process.
MessageType	Specifies the type of message requested as follows: If equal to the value of 0, the first message on the queue is received. If greater than 0, the first message of the type specified by the MessageType parameter is received. If less than 0, the first message of the lowest type that is less than or equal to the absolute value of the MessageType parameter is received.
MessageFlag	Specifies either a value of 0 or is constructed by logically ORing one or more of the following values: MSG_NOERROR Truncates the message if it is longer than the MessageSize parameter. IPC_NOWAIT Specifies the action to take if a message of the desired type is not on the queue: If the IPC_NOWAIT value is set, the calling process returns a value of -1 and sets the errno global variable to the ENOMSG error code. If the IPC_NOWAIT value is not set, the calling process suspends execution until one of the following occurs: A message of the desired type is placed on the queue. The message queue identifier specified by the MessageQueueID parameter is removed from the system. When this occurs, the errno global variable is set to the EIDRM error code, 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 in the manner described in the sigaction subroutine.

Return Values

Upon successful completion, the msgrcv subroutine returns a value equal to the number of bytes actually stored into the mtext field and the following actions are taken with respect to fields of the data structure associated with the MessageQueueID parameter:

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

If the msgrcv subroutine is unsuccessful, a value of -1 is returned and the errno global variable is set to indicate the error.

Error Codes

The msgrcv subroutine is unsuccessful if any of the following conditions is true:

EINVAL	The MessageQueueID parameter is not a valid message queue identifier.
EACCES	The calling process is denied permission for the specified operation.
EINVAL	The MessageSize parameter is less than 0.
E2BIG	The `mtext` field is greater than the MessageSize parameter, and the MSG_NOERROR value is not set.
ENOMSG	The queue does not contain a message of the desired type and the IPC_NOWAIT value is set.
EFAULT	The MessagePointer parameter points outside of the allocated address space of the process.
EINTR	The msgrcv subroutine is interrupted by a signal.
EIDRM	The message queue identifier specified by the MessageQueueID parameter has been removed from the system.

Implementation Specifics

This subroutine is part of Base Operating System (BOS) Runtime.

Related Information

The msgctl subroutine, msgget subroutine, msgsnd subroutine, msgxrcv subroutine, sigaction subroutine.