MsgReceive(), MsgReceive_r()

QNX SDP8.0C Library ReferenceAPIDeveloper

Wait for a message or pulse on a channel

Synopsis:

#include <sys/neutrino.h>

rcvid_t MsgReceive( int chid,
                    void * msg,
                    size_t bytes,
                    struct _msg_info * info );

rcvid_t MsgReceive_r( int chid,
                      void * msg,
                      size_t bytes,
                      struct _msg_info * info );

Arguments:

chid: The ID of a channel that you established by calling ChannelCreate(), or -1 to dissociate the thread from the last channel it received on (see Server boost in the Interprocess Communication chapter of the System Architecture guide).
msg: A pointer to a buffer where the function can store the received data.
bytes: The size of the buffer. This number must not exceed SSIZE_MAX, or the function will behave unpredictably.
info: NULL, or a pointer to a _msg_info structure where the function can store additional information about the message.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The MsgReceive() and MsgReceive_r() kernel calls wait for a message or pulse to arrive on the channel identified by chid, and store the received data in the buffer pointed to by msg.

These functions are identical, except in the way they indicate errors; see the Returns section for details.

The number of bytes transferred is the minimum of that specified by both the sender and the receiver. The received data isn't allowed to overflow the receive buffer area provided.

Note: The msg buffer must be big enough to contain a pulse. If it isn't, the functions indicate an error of EFAULT.

If a message is waiting on the channel when you call MsgReceive(), the calling thread doesn't block, and the message is immediately copied. If a message isn't waiting, the calling thread enters the RECEIVE-blocked state until a message arrives.

If multiple messages are sent to a channel without a thread waiting to receive them, the messages are queued in priority order.

Note: The thread's effective priority might change when it receives a message. For more information, see Priority inheritance and messages in the Interprocess Communication (IPC) chapter of the System Architecture guide.

If you pass a non-NULL pointer for info, the functions store additional information about the message and the thread that sent it in the _msg_info structure that info points to. You can get this information later by calling MsgInfo().

On success, MsgReceive() and MsgReceive_r() return:

>0: A message was received; the returned value is a a rcvid (receive identifier). You'll use the rcvid with other Msg*() kernel calls to interact with and reply to the sending thread. MsgReceive() changes the state of the sending thread to REPLY-blocked when the message is received. When you use MsgReply*() to reply to the received message, the sending thread is made ready again. The rcvid encodes the sending thread's ID and a local connection ID.
0: A pulse was received; msg contains a pulse message of type _pulse. When a pulse is received, the kernel space allocated to hold it is immediately released. The _msg_info structure isn't updated.

Note: Don't reply to a pulse.

Blocking states

STATE_RECEIVE: There's no message waiting.

Returns:

On success, both functions return a positive rcvid if they received a message, or EOK if they received a pulse. The only difference between MsgReceive() and MsgReceive_r() is the way they indicate errors:

MsgReceive(): If an error occurs, this function returns -1 and sets errno.
MsgReceive_r(): If an error occurs, this function may return the negative of any value from the Errors section. This function does NOT set errno, even on success.

Errors:

EFAULT: A fault occurred when the kernel tried to access the buffers provided. Because the OS accesses the sender's buffers only when MsgReceive() is called, a fault could occur in the sender if the sender's buffers are invalid. If a fault occurs when accessing the sender buffers (only) they'll receive an EFAULT and MsgReceive() won't unblock.
This error also occurs if the kernel tries to deliver a pulse to the server, but the size of the receive buffer is less than the size of a struct _pulse. The pulse is lost in this case.
EINTR: The call was interrupted by a signal.
ESRCH: The channel indicated by chid doesn't exist.
ETIMEDOUT: A kernel timeout unblocked the call. See TimerTimeout().

Classification:

QNX OS


Safety:
Cancellation point	Yes
Signal handler	Yes
Thread	Yes

Caveats:

The maximum size for a one-part message depends on the architecture:

For 32-bit architectures, it's SSIZE_MAX (i.e., 2³¹ − 1) bytes.
For 64-bit architectures, the API allows up to SSIZE_MAX (i.e., 2⁶³ − 1) bytes, but the virtual address space limits the size to 2³⁹ bytes.

Page updated: May 17, 2024