Occasionally, you may not be able to send or receive a message in My eBay. While most messages go through, certain messages are blocked to protect the safety of eBay members. Here are the most common reasons a message will be returned unsent: Viruses If we detect a virus in an that's been sent, the message will be returned to the sender.

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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. If you pass a NULL, then nothing happens. The information can be retrieved later via the MsgInfo call, so it's not gone forever!

Note that nd is the receiving node's node descriptor for the transmitting node; srcnd is the transmitting node's node descriptor for the receiving node. 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.

This may be greater than the value in msglen, as would be the case when gratis sexchat less data than what was sent. What if you lose it? It's gone. Don't depend on the value of the receive ID to have any particular meaning — it may change in future versions of the operating system.

You can assume that it will be unique, in that you'll never have two outstanding clients identified by the same receive IDs in that case, the kernel couldn't tell them apart either when you do the MsgReply. Also, note that except in one special case the MsgDeliverEvent function which we'll look at lateronce you've done the MsgReplymezsage particular receive ID ceases to have meaning.

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This brings us to the MsgReply function. Replying to the client MsgReply accepts a receive ID, a status, a message pointer, and a message size.

We've just finished discussing the receive ID; it identifies who the reply message should be sent text friends online. The status variable indicates the aout status that should be passed to the client's MsgSend function. Finally, the message pointer and size indicate the location and size of the optional reply message that should be sent. The MsgReply function may appear to be very simple and it isbut its applications require some examination.

Not replying to the client There's absolutely no requirement that you reply to a client before accepting new messages from other clients via MsgReceive! This can be used in a of different scenarios.

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In a typical device driver, a client swansea sex chatroom make a request that won't be serviced for a message time. Other clients might wish to have requests serviced for example, there might be multiple analog channels, or there might be sending information that should be available immediately, etc. Architecturally, the ADC driver will simply queue the receive ID that it got from the MsgReceivestart up the second accumulation process, and go off and handle other requests.

When the 45 chat roulette nudes are up and the samples have been accumulated, the ADC driver can find the receive ID associated with the request and then reply to the client. Since the subservers are sebding for work, you'd simply make a note of their receive IDs and store those away. When actual work arrived, then and only then would you reply to the subserver, thus indicating that it should do some work.

Replying with no data, or an errno When you finally reply to the client, there's no requirement that you transfer any data. This is used in two scenarios. You may choose to reply with no data if the sole purpose of the reply is to unblock the client. Let's say the client just wants to be blocked until some particular event occurs, but it doesn't need to know which event.

As a slight modification of san antonio free phone chat lines, you may wish to return an error status to the client. In this case, you can't do that with MsgReplybut about must use MsgError : MsgError rcvid, EROFS ; In the aboutt example, the server detects that the client is attempting to write to a read-only filesystem, and, instead of returning any actual data, simply returns an messahe of EROFS back to the client.

Alternatively and we'll look at the calls shortlyyou may have already transferred the data via MsgWriteand there's no additional data to transfer. Why the its calls?

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They're subtly different. While both MsgError and MsgReply will unblock the client, MsgError will not transfer any additional data, will cause the client's MsgSend function to return -1, and will cause the client to have errno set to whatever was passed as the sendding argument to MsgError. On the other hand, MsgReply could transfer data as indicated by the third and fourth argumentsand gay chat modern cause the client's MsgSend function to return whatever was passed as the second argument to MsgReply.

MsgReply has no effect on the client's errno. Messwge, when you do return data, the second argument to MsgReply will be a positive integer indicating the of bytes being returned.

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If one process creates the other, then it's easy — the process creation call senving with the process ID of the newly created process. What if we have two perfect strangers? This would be the case if, for example, a Bi male phone chat Marathon party created a server and an application that you wrote wanted to talk to that server. This is typically used in multi-threaded servers that need to send themselves messages, and is, by its nature, a very limited case.

Take over a portion of the pathname space and become a resource manager. We'll talk about this when we look at resource managers in the Resource Managers chapter. There's another related problem. Since the process that created the file can die without removing the file, there's no way of knowing whether or not the process is still alive until you try to send a message to it.

The message that you send to that program will at best be rejected, and at worst may cause damage. So that approach is out. And since this requires shared memory, it certainly won't work across a network! This generally gets used in either tiny test case programs or in very special cases, but always in the context of a multithreaded program. Effectively, all that happens is mesage one thread in religious chat rooms program is the client, and another thread is the server.

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Abouy server thread creates the channel and then places the channel ID into a global variable the node ID and process ID are the same for all thre in the process, so they don't need to be advertised. Itw client thread then aboout up the global channel ID and performs the ConnectAttach to it. The last approach, where the server becomes a resource manager, is definitely the cleanest and is the recommended general-purpose solution. The beauty of this scheme is that since the file descriptor that's returned from the open is the connection ID, no further work is required on the client's end to be able to use that particular connection.

For example, when the client calls read later, passing it the file descriptor, this translates with very little overhead into a MsgSend function.

irs What if a low-priority process and a high-priority process send a message to a server at the same time? Messages are always delivered in priority order. If both processes are at the same priority, then the messages will be abokt in time order since there's no such thing as absolutely simultaneous on a single-processor machine — even on an SMP box there will be some ordering as the CPUs arbitrate kernel access among themselves.

We'll come back to some of the other subtleties introduced by this question when we look at priority inversions later in this chapter. So far you've seen the basic message-passing primitives. As I mentioned earlier, these are all that you need.

However, there are a few extra functions that make life much easier. Let's consider an example using a client and server where we might need other functions. The client issues a MsgSend to transfer some data to the server. After the aboout issues the MsgSend it blocks; it's now waiting for the server to reply.

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An interesting thing happens on the server side. The server has called MsgReceive to receive the sending from the client. Depending on the de that you choose for your messages, the server may or may not know how big the client's message is. Why on earth would the server not know how big the message is? Consider the filesystem example that we've been using. Suppose the client does: write fd, buf, 16 ; This works as expected if the server does a MsgReceive and specifies a buffer size of, say, bytes.

Since our client sent only a tiny message 28 byteswe have no problems. However, what if the client sends something bigger than bytes, say 1 megabyte? We could, arbitrarily, say that the client isn't allowed to write more than n bytes. Then, in the client-side C library code for writewe could look at this requirement and split up the write request into several requests of n bytes each.

This is awkward. This in itself can be a fair amount of work. Also, it can have unexpected side effects for multi-threaded functions — what if the first part of the message from one thread gets sent, and about another thread in the client preempts the current thread and sends its own message. Where does that leave the original thread? All servers must now be prepared to handle the largest possible message size that may arrive. This means that all servers will have he texted me have a data area that's big, or the library will have to break its big requests into many smaller ones, thereby impacting speed.

Luckily, this problem has a fairly simple workaround that also gives us some advantages. Two functions, MsgRead and MsgWriteare especially useful here.

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The important fact to keep in mind is that the client is blocked. This means that the client isn't going to go and sending data structures while the server is about to examine them. In a multi-threaded client, the potential sendijg for another thread to mess around with the data area of mdssage client thread that's blocked on a server.

This is considered a bug bad de — its message thread assumes that it has exclusive access to a client's data area until the server thread unblocks the client. The server doesn't abouf, and the client doesn't unblock. MsgRead returns the of bytes it actually local chat rooms australia, or -1 if there was an error. So let's think about how we'd use this in our write example.

The C Library write function constructs a message with a header that it sends to the filesystem server, fs-qnx4. The server receives a small portion of the message via MsgReceivelooks at it, and decides where it's going to put the rest of the message. The fs-qnx4 server may decide that the best place to put the data is into messafe cache buffers it's already allocated.

Let's track an example: Avout fs-qnx4 message example, showing contiguous data view. So, the client has decided to send 4 KB to the filesystem. Notice how chat ave forums C Library stuck a tiny header in front of the data so that the filesystem could tell just what kind of request it actually was — we'll come back to this when we look at multi-part messages, and in even more detail when we look at resource managers.