Reads a message from a queue.
Standard C Library (libc.a)
int msgrcv (MessageQueueID, MessagePointer,MessageSize,MessageType, MessageFlag)
int MessageQueueID, MessageFlag;
void * MessagePointer;
long int MessageType;
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.
Limits on message size and number of messages in the queue can be found in AIX 5L Version 5.1 General Programming Concepts: Writing and Debugging Programs.
Note: For a 64-bit process, the mtype field is 64 bits long. However, for compatibility with 32-bit processes, the mtype field must be a 32-bit signed value that is sign-extended to 64 bits. The most significant 32 bits are not put on the message queue. For a 64-bit process, the mtype field is again sign-extended to 64 bits.
|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; /* 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:
|MessageFlag||Specifies either a value of 0 or is constructed by logically ORing one or
more of the following 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:
If the msgrcv subroutine is unsuccessful, a value of -1 is returned and the errno global variable is set to indicate the error.
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.|
This subroutine is part of Base Operating System (BOS) Runtime.
The msgctl (msgctl Subroutine) subroutine, msgget (msgget Subroutine) subroutine, msgsnd (msgsnd Subroutine) subroutine, msgxrcv (msgxrcv Subroutine) subroutine, sigaction subroutine.