#include <sys/msg.h>
int msgrcv(int msqid, void *msgp, size_t msgsz,
long int msgtyp, int msgflg);
The msgrcv() function reads a message from the message queue specified by the msqid parameter and places it in the user-defined buffer pointed to by the *msgp parameter.
The *msgp parameter points to a user-defined buffer that must contain the following:
The following structure is an example of what this user-defined buffer might look like for a message that has 5 bytes of data:
struct mymsg {
long int mtype; /* message type */
char mtext[5]; /* message text */
}
The value of mtype is the type of the received message, as specified by the sender of the message.
The msgtyp parameter specifies the type of message to receive from the message queue as follows:
The msgsz parameter specifies the size in bytes of the data part of the message. The received message is truncated to msgsz bytes if it is larger than msgsz and the MSG_NOERROR flag is set in the msgflg parameter. The truncated part of the message is lost and no indication of the truncation is given to the calling thread. Otherwise, msgrcv() returns with return value -1 and errno set to E2BIG.
If a message of the desired type is not available on the message queue, the msgflg parameter specifies the action to be taken. The actions are as follows:
The system maintains status information about a message queue which can be retrieved with the msgctl() function. When a message is successfully received from the message queue, the system sets the members of the msqid_ds structure as follows:
Authorization Required for msgrcv()
| Object Referred to | Authority Required | errno |
|---|---|---|
| Message queue from which message is received | Read | EACCES |
| value | msgrcv() was successful. The value returned is the number of bytes of data placed in the data part of the buffer pointed to by the msgp parameter. |
| -1 | msgrcv() was not successful. The errno variable is set to indicate the error. |
If msgrcv() is not successful, errno usually indicates one of the following errors. Under some conditions, errno could indicate an error other than those listed here.
Argument list too long.
The size in bytes of the message is greater than msgsz and the MSG_NOERROR flag is not set in the msgflg parameter.
Permission denied.
An attempt was made to access an object in a way forbidden by its object access permissions.
The thread does not have access to the specified file, directory, component, or path.
The calling thread does not have read permission to the message queue.
A damaged object was encountered.
A referenced object is damaged. The object cannot be used.
The message queue has been damaged by a previous message queue operation.
The address used for an argument is not correct.
In attempting to use an argument in a call, the system detected an address that is not valid.
While attempting to access a parameter passed to this function, the system detected an address that is not valid.
ID has been removed.
The message queue identifier msqid was removed from the system.
Interrupted function call.
The function msgrcv() was interrupted by a signal.
The value specified for the argument is not correct.
A function was passed incorrect argument values, or an operation was attempted on an object and the operation specified is not supported for that type of object.
An argument value is not valid, out of range, or NULL.
One of the following has occurred:
Message does not exist.
The message queue does not contain a message of the desired type and the IPC_NOWAIT flag is set in the msgflg parameter.
Unknown system state.
The operation failed because of an unknown system state. See any messages in the job log and correct any errors that are indicated, then retry the operation.
None.
struct mymsg {
long int mtype; /* 12 bytes padding inserted after */
char *pointer; /* the mtype field by the compiler.*/
char c; /* 15 bytes padding inserted after */
char *pointer2; /* the c field by the compiler. */
} msg; /* After the mtype field, there are*/
/* 33 bytes of user data, but 60 */
/* bytes of data including padding.*/
msgsz = sizeof(msg) - sizeof(long int); /* 60 bytes. */
The following example receives a message from a message queue.
Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.
#include <sys/msg.h>
main() {
int msqid = 2;
int rc;
size_t msgsz;
long int msgtyp;
struct mymsg {
long int mtype;
char mtext[256];
};
msgsz = sizeof(struct mymsg) - sizeof(long int);
msgtyp = 1;
rc = msgrcv(msqid, &mymsg, msgsz, msgtyp, IPC_NOWAIT);
}