Who sent the message?
QNX SDP8.0Getting Started with the QNX OSDeveloperUser
Often a server will need to know who sent it a message. There are a number of reasons for this.
- accounting
- access control
- context association
- class of service
- compatibility
It would be cumbersome (and a security hole) to have the client provide
this information with each and every message sent.
Therefore, there's a structure filled in by the kernel whenever the
MsgReceive() function unblocks because it got a message.
This structure is of type
struct _msg_info,
and contains at least the following:
struct _msg_info
{
pid_t pid;
int32_t tid;
int32_t chid;
int32_t scoid;
int32_t coid;
int16_t priority;
int16_t flags;
ssize64_t msglen;
ssize64_t srcmsglen;
ssize64_t dstmsglen;
};
You pass it to the MsgReceive() function as the last argument. If you pass a NULL, then nothing happens. (The information can be retrieved later via the MsgInfo() call, so it's not gone forever!)
Let's look at the fields:
- pid, and tid
- Process ID, and thread ID of the client.
- priority
- The priority of the sending thread.
- chid, coid
- Channel ID that the message was sent to, and the connection ID used.
- scoid
- Server Connection ID. This is an internal identifier used by the kernel to route the message from the server back to the client. You don't need to know about it, except for the interesting fact that it will be a small integer that uniquely represents the client.
- flags
- Contains a variety of flag bits, including the following:
- We'll look at _NTO_MI_UNBLOCK_REQ in the section
Using the _NTO_MI_UNBLOCK_REQ,
below.
- We'll look at _NTO_MI_UNBLOCK_REQ in the section
- msglen
- Number of bytes received.
- srcmsglen
- The length of the source message, in bytes, as sent by the client. This may be greater than the value in msglen, as would be the case when receiving less data than what was sent.
- dstmsglen
- The length of the client's reply buffer, in bytes.
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