msgrcv(2) System Calls Manual msgrcv(2)
NAME
msgrcv - Receives a message from a message queue
SYNOPSIS
#include <sys/msg.h>
int msgrcv(
int msqid,
void *msgp,
size_t msgsz,
long msgtyp,
int msgflg);
Application developers may want to specify #include statements for <sys/types.h> and <sys/ipc.h> before the one for <sys/msg.h> if programs
are being developed for multiple platforms. The additional #include statements are not required on Tru64 UNIX systems or by ISO or X/Open
standards, but may be required on other vendors' systems that conform to these standards.
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
msgrcv(): XSH5.0
Refer to the standards(5) reference page for more information about industry standards and associated tags.
PARAMETERS
Specifies the ID of the message queue from which to receive the message. Specifies a pointer to the msgbuf structure that is to receive
the message. See the NOTES section for more information. Specifies the maximum number of bytes allowed for the received data. The msgsz
parameter is only used to specify the size of the msgbuf.mtext field. It cannot include the msgbuf.mtype field or else the msgbuf buffer
is overwritten. Specifies the message type to read from the queue. Specifies the action to be taken by the kernel if there are no msgtyp
messages on the queue.
DESCRIPTION
The msgrcv() function receives a message from the queue associated with the msqid parameter. The function returns the number of bytes in
the received message.
The msgp parameter points to a user-defined msgbuf structure. This structure receives the message read from the queue.
The msgsz parameter specifies the maximum size allowed for the received data. If the message is longer than msgsz, the following occurs,
depending upon the setting of the MSG_NOERROR flag: If MSG_NOERROR is not set, the kernel returns an [E2BIG] error to the calling process
and leaves the message on the queue. If MSG_NOERROR is set, the kernel truncates the message to msgsz and discards the truncated portion
without notifying the calling process.
The msgtyp parameter specifies the message type that the process expects to receive. Possible values and their results are as follows: The
process receives the message at the head of the queue. The process receives the first message of the requested positive-integer type. The
process receives the first message of the lowest type on the queue. To qualify as the lowest type, the negative-integer type must be less
than or equal to the absolute value of msgtyp.
The msgflg parameter specifies the action that the kernel should take if the queue does not contain a message of the requested type.
Either of two kernel actions can be specified, as follows: If IPC_NOWAIT is set, the function returns immediately with a return value of -1
and sets errno to [ENOMSG]. If IPC_NOWAIT is not set, the kernel suspends the calling process.
The process remains suspended until one of the following occurs: A message of the requested type appears on the queue. In this
case, the kernel wakes the process to receive the message. The specified message queue ID is removed from the system. In this
case, the kernel sets errno to [EIDRM] and returns -1 to the calling process. The process catches a signal. In this case, the
process does not receive the message and, instead, resumes execution as directed by the sigaction() call.
NOTES
The parameter msgp points to a user-specified buffer that must contain first a field of type long int that specifies the type of the mes-
sage, and then a data portion that holds the data bytes of the message. For example: struct msgbuf { long int mtype; char
mtext[]; }
The mytpe field is set to the message type supplied by the sender.
The mtext field is set to the message text. Unless MSG_NOERROR is set, the message size will be less than or equal to the msgsz specified
on the call to msgrcv().
RETURN VALUES
Upon successful completion, the msgrcv() function returns a value equal to the number of bytes actually stored in mtext. Also, the kernel
updates the msqid_ds structure associated with the message queue ID as follows: Decrements msg_qnum by 1. Sets msg_lrpid equal to the
process ID of the calling process. Sets msg_rtime equal to the current time. [Tru64 UNIX] Decrements msg_cbytes by the message text
size.
When the msgrcv() function fails, it returns a value of -1 and sets errno to indicate the error.
ERRORS
The msgrcv() function sets errno to the specified values for the following conditions: The number of bytes to be received in mtext is
greater than msgsz and the MSG_NOERROR flag is not set. The calling process does not have permission for the operation. Indicates that
the requested address is in some way invalid, for example, out of bounds. The message queue identified by the msqid parameter has been
removed from the system. The operation was interrupted by a signal. The msqid parameter is not a valid message queue ID. The queue does
not contain a message of the requested type and the IPC_NOWAIT flag is set.
RELATED INFORMATION
Functions: msgctl(2), msgget(2), msgsnd(2), sigaction(2)
Data Structures: msqid_ds(4)
Standards: standards(5) delim off
msgrcv(2)