2024-11-16

<!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang=""> <head> <meta charset="utf-8" /> <meta name="generator" content="pandoc" /> <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" /> <title>Blog Article on Message Passing in VExec</title> <style>

</style> </head> <body> Message exchange using AmigaOS-style message ports and message headers works only between threads. Messages are identified by and queued using direct pointers to the message headers, which are intrusively prepended to the intended message payload; additionally, message payloads frequently include pointers to ancillary data structures, basically rendering the <code>msg_length</code> field basically useless. By making messages a proper kernel-managed resource, we can treat messages similarly to virtual files, exposing a precisely managed window into the sender’s address space. Messages encapsulated this way retain much of the desirable developer experience that the original AmigaOS is well known for. Although message payloads must now be copied, this approach reduces the number of copies to a minimum of just one. NOTE. The text that follows is hypothetical and forward looking. The design contained herein is preliminary. It’s also written in C, even though the system call interfaces will be designed around the RISC-V assembly language. <h1 id="using-message-channels-to-receive-messages">Using Message Channels To Receive Messages</h1> To receive messages, a task must first create a message channel. The recommended process for creating a new message channel is to first allocate a signal (see <code>AllocSignal()</code>) for it to use when receiving a new message. Next, create the channel proper (see <code>NewChannel()</code>), setting optional attributes as desired by the application. The following code illustrates how to create a new channel with a custom-assigned signal bit:

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb1-1" aria-hidden="true"></a>int errcode; <a href="#cb1-2" aria-hidden="true"></a>uint8_t my_sig_bit = AllocSignal(&errcode); <a href="#cb1-3" aria-hidden="true"></a>if(errcode != 0) { <a href="#cb1-4" aria-hidden="true"></a> // No more signal bits <a href="#cb1-5" aria-hidden="true"></a>} <a href="#cb1-6" aria-hidden="true"></a> <a href="#cb1-7" aria-hidden="true"></a>ChannelAttrs chan_attrs = { <a href="#cb1-8" aria-hidden="true"></a> .sig_bit = my_sig_bit, <a href="#cb1-9" aria-hidden="true"></a>}; <a href="#cb1-10" aria-hidden="true"></a>uint64_t chanID = NewChannel(&chan_attrs, CHATTR_SIGBIT, &errcode); <a href="#cb1-11" aria-hidden="true"></a>if(errcode != 0) { <a href="#cb1-12" aria-hidden="true"></a> // Channel creation failed for some reason <a href="#cb1-13" aria-hidden="true"></a>}</code></pre>

}</code></pre> At this point, the server code can wait on the channel in the normal way.

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb3-1" aria-hidden="true"></a>uint64_t channel_sigmask = 1 << my_sig_bit; <a href="#cb3-2" aria-hidden="true"></a>// Assume this_mask and that_mask have already been initialized elsewhere. <a href="#cb3-3" aria-hidden="true"></a> <a href="#cb3-4" aria-hidden="true"></a>sigs = Wait(channel_sigmask | this_mask | that_mask); <a href="#cb3-5" aria-hidden="true"></a> <a href="#cb3-6" aria-hidden="true"></a>if(sigs & my_channel_event) { <a href="#cb3-7" aria-hidden="true"></a> handle_my_channel(chanID); <a href="#cb3-8" aria-hidden="true"></a>} <a href="#cb3-9" aria-hidden="true"></a> <a href="#cb3-10" aria-hidden="true"></a>if(sigs & this_mask) { <a href="#cb3-11" aria-hidden="true"></a> handle_this_thing(...etc...); <a href="#cb3-12" aria-hidden="true"></a>} <a href="#cb3-13" aria-hidden="true"></a> <a href="#cb3-14" aria-hidden="true"></a>if(sigs & that_mask) { <a href="#cb3-15" aria-hidden="true"></a> handle_that_thing(...etc...); <a href="#cb3-16" aria-hidden="true"></a>}</code></pre>

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb4-1" aria-hidden="true"></a>void handle_my_channel(uint64_t chanID) { <a href="#cb4-2" aria-hidden="true"></a> MsgID msg; <a href="#cb4-3" aria-hidden="true"></a> int errcode; <a href="#cb4-4" aria-hidden="true"></a> <a href="#cb4-5" aria-hidden="true"></a> for(;;) { <a href="#cb4-6" aria-hidden="true"></a> msg = GetChannelMsg(chanID, &errcode); <a href="#cb4-7" aria-hidden="true"></a> if(errcode == 0) { <a href="#cb4-8" aria-hidden="true"></a> handle_nonlocal_message(msg); <a href="#cb4-9" aria-hidden="true"></a> } else { <a href="#cb4-10" aria-hidden="true"></a> // Most likely, we just emptied the channel of messages. <a href="#cb4-11" aria-hidden="true"></a> break; <a href="#cb4-12" aria-hidden="true"></a> } <a href="#cb4-13" aria-hidden="true"></a> } <a href="#cb4-14" aria-hidden="true"></a>}</code></pre>

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb5-1" aria-hidden="true"></a>errcode = DisposeChannel(chanID); <a href="#cb5-2" aria-hidden="true"></a>if(errcode != 0) { <a href="#cb5-3" aria-hidden="true"></a> // Most likely, the channel isn't empty. <a href="#cb5-4" aria-hidden="true"></a>}</code></pre>

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb6-1" aria-hidden="true"></a>void handle_nonlocal_message(uint64_t msg) { <a href="#cb6-2" aria-hidden="true"></a> struct MyMessageHeader header; <a href="#cb6-3" aria-hidden="true"></a> struct Body1 b1; <a href="#cb6-4" aria-hidden="true"></a> struct Body2 b2; <a href="#cb6-5" aria-hidden="true"></a> int errcode; <a href="#cb6-6" aria-hidden="true"></a> <a href="#cb6-7" aria-hidden="true"></a> int64_t actual = ReadChanMsg(msg, &header, sizeof(header)); <a href="#cb6-8" aria-hidden="true"></a> if(actual != sizeof(header)) { <a href="#cb6-9" aria-hidden="true"></a> // Message that was sent is too small even for a proper header. <a href="#cb6-10" aria-hidden="true"></a> goto msg_doone; <a href="#cb6-11" aria-hidden="true"></a> } <a href="#cb6-12" aria-hidden="true"></a> <a href="#cb6-13" aria-hidden="true"></a> switch(header.request_code) { <a href="#cb6-14" aria-hidden="true"></a> case DO_REQUEST_1: <a href="#cb6-15" aria-hidden="true"></a> actual = ReadChanMsg(msg, &b1, sizeof(b1)); <a href="#cb6-16" aria-hidden="true"></a> if(actual == sizeof(b1)) { <a href="#cb6-17" aria-hidden="true"></a> // Process request 1 here. <a href="#cb6-18" aria-hidden="true"></a> } <a href="#cb6-19" aria-hidden="true"></a> goto msg_done; <a href="#cb6-20" aria-hidden="true"></a> <a href="#cb6-21" aria-hidden="true"></a> case DO_REQUEST_2: <a href="#cb6-22" aria-hidden="true"></a> actual = ReadChanMsg(msg, &b2, sizeof(b2)); <a href="#cb6-23" aria-hidden="true"></a> if(actual == sizeof(b2)) { <a href="#cb6-24" aria-hidden="true"></a> // Process request 1 here. <a href="#cb6-25" aria-hidden="true"></a> } <a href="#cb6-26" aria-hidden="true"></a> goto msg_done; <a href="#cb6-27" aria-hidden="true"></a> <a href="#cb6-28" aria-hidden="true"></a> default: <a href="#cb6-29" aria-hidden="true"></a> // Unknown message type; just reply to it. <a href="#cb6-30" aria-hidden="true"></a> } <a href="#cb6-31" aria-hidden="true"></a>msg_done: <a href="#cb6-32" aria-hidden="true"></a> errcode = ReplyChanMsg(msg); <a href="#cb6-33" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb6-34" aria-hidden="true"></a> // Message reply failed; even so, it is no longer eligible for use <a href="#cb6-35" aria-hidden="true"></a> // by this task anymore. <a href="#cb6-36" aria-hidden="true"></a> } <a href="#cb6-37" aria-hidden="true"></a>}</code></pre>

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb7-1" aria-hidden="true"></a>// Example implementation of write(). <a href="#cb7-2" aria-hidden="true"></a>// I am going to skip error handling for brevity's sake. <a href="#cb7-3" aria-hidden="true"></a>size_t write(int fd, void *buffer, size_t length) { <a href="#cb7-4" aria-hidden="true"></a> struct WriteRequest wr = { <a href="#cb7-5" aria-hidden="true"></a> .fd = fd, <a href="#cb7-6" aria-hidden="true"></a> .req_length = length, <a href="#cb7-7" aria-hidden="true"></a> .actual_length = 0, // will be filled in later <a href="#cb7-8" aria-hidden="true"></a> .errcode = 0, // will be filled in later <a href="#cb7-9" aria-hidden="true"></a> }; <a href="#cb7-10" aria-hidden="true"></a> <a href="#cb7-11" aria-hidden="true"></a> // Because no data copies happen until the last possible moment, <a href="#cb7-12" aria-hidden="true"></a> // we can directly reference the supplied memory buffer in our IOVec. <a href="#cb7-13" aria-hidden="true"></a> // The file server will "see" this message as a single WriteRequest- <a href="#cb7-14" aria-hidden="true"></a> // prefixed buffer of length sizeof(wr)+length bytes. <a href="#cb7-15" aria-hidden="true"></a> struct io_vec iov[2] = { <a href="#cb7-16" aria-hidden="true"></a> [0] {.io_addr = &wr, .io_len = sizeof(wr) }, <a href="#cb7-17" aria-hidden="true"></a> [1] {.io_addr = buffer, .io_len = length }, <a href="#cb7-18" aria-hidden="true"></a> }; <a href="#cb7-19" aria-hidden="true"></a> <a href="#cb7-20" aria-hidden="true"></a> msg = NewMsg(our_replay_chan_id, &iov, 2, &errcode); <a href="#cb7-21" aria-hidden="true"></a> SendChanMsg(our_fileserver_id, msg); <a href="#cb7-22" aria-hidden="true"></a> // arm I/O response timeout timer here... <a href="#cb7-23" aria-hidden="true"></a> uint64_t sigs = Wait(our_fileserver_mask | timer_mask); <a href="#cb7-24" aria-hidden="true"></a> if(sigs & our_fileserver_mask) { <a href="#cb7-25" aria-hidden="true"></a> GetChanMsg(my_reply_id, &errcode); // we know it's our msg. <a href="#cb7-26" aria-hidden="true"></a> DisposeMsg(msg); <a href="#cb7-27" aria-hidden="true"></a> errno = wr.errcode; <a href="#cb7-28" aria-hidden="true"></a> return wr.actual_length; <a href="#cb7-29" aria-hidden="true"></a> } <a href="#cb7-30" aria-hidden="true"></a> <a href="#cb7-31" aria-hidden="true"></a> if(sigs & timer_mask) { <a href="#cb7-32" aria-hidden="true"></a> // I/O timeout happened; clean up and "cancel" the message, etc. <a href="#cb7-33" aria-hidden="true"></a> // Return EAGAIN or EINTR something. <a href="#cb7-34" aria-hidden="true"></a> } <a href="#cb7-35" aria-hidden="true"></a>}</code></pre>

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb8-1" aria-hidden="true"></a>enum Ops { <a href="#cb8-2" aria-hidden="true"></a> op_add, op_subtract, op_shutdown <a href="#cb8-3" aria-hidden="true"></a>}; <a href="#cb8-4" aria-hidden="true"></a> <a href="#cb8-5" aria-hidden="true"></a>struct MsgReq { <a href="#cb8-6" aria-hidden="true"></a> enum Ops function; <a href="#cb8-7" aria-hidden="true"></a> int64_t accumulator; <a href="#cb8-8" aria-hidden="true"></a> int64_t operand; <a href="#cb8-9" aria-hidden="true"></a>}; <a href="#cb8-10" aria-hidden="true"></a> <a href="#cb8-11" aria-hidden="true"></a>void <a href="#cb8-12" aria-hidden="true"></a>main(void) { <a href="#cb8-13" aria-hidden="true"></a> int errcode; <a href="#cb8-14" aria-hidden="true"></a> <a href="#cb8-15" aria-hidden="true"></a> // Allocate a signal, so that we can be notified when <a href="#cb8-16" aria-hidden="true"></a> // a message arrives on our channel. <a href="#cb8-17" aria-hidden="true"></a> uint8_t server_sigbit = AllocSignal(&errcode); <a href="#cb8-18" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb8-19" aria-hidden="true"></a> printf("Unable to allocate signal.\n"); <a href="#cb8-20" aria-hidden="true"></a> goto end; <a href="#cb8-21" aria-hidden="true"></a> } <a href="#cb8-22" aria-hidden="true"></a> uint64_t server_channel_mask = 1 << server_sigbit; <a href="#cb8-23" aria-hidden="true"></a> <a href="#cb8-24" aria-hidden="true"></a> // Create a public channel named TallyServer. Configure <a href="#cb8-25" aria-hidden="true"></a> // it so that, when a message arrives, we (this task) is <a href="#cb8-26" aria-hidden="true"></a> // notified via the signal we allocated above. <a href="#cb8-27" aria-hidden="true"></a> ChannelAttrs chattrs = { 0, }; <a href="#cb8-28" aria-hidden="true"></a> chattrs.sigbit = server_sigbit; <a href="#cb8-29" aria-hidden="true"></a> chattrs.name = "TallyServer"; <a href="#cb8-30" aria-hidden="true"></a> chattrs.is_public = true; <a href="#cb8-31" aria-hidden="true"></a> uint64_t chanID = NewChannel( <a href="#cb8-32" aria-hidden="true"></a> &chattrs, <a href="#cb8-33" aria-hidden="true"></a> // Set only these attrs; ignore or assume defaults for the rest. <a href="#cb8-34" aria-hidden="true"></a> CHATTRF_SIGBIT | CHATTRF_NAME | CHATTRF_ISPUBLIC, <a href="#cb8-35" aria-hidden="true"></a> &errcode <a href="#cb8-36" aria-hidden="true"></a> ); <a href="#cb8-37" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb8-38" aria-hidden="true"></a> printf("Unable to create public channel"); <a href="#cb8-39" aria-hidden="true"></a> goto no_channel; <a href="#cb8-40" aria-hidden="true"></a> } <a href="#cb8-41" aria-hidden="true"></a> <a href="#cb8-42" aria-hidden="true"></a> printf("Welcome to Tally Server. Now entering event loop.\n"); <a href="#cb8-43" aria-hidden="true"></a> <a href="#cb8-44" aria-hidden="true"></a> for(bool still_running = true; still_true; ) { <a href="#cb8-45" aria-hidden="true"></a> // We only need to wait on a single event source, so we ignore <a href="#cb8-46" aria-hidden="true"></a> // return code (it'll either be 0 or server_channel_mask). <a href="#cb8-47" aria-hidden="true"></a> Wait(server_channel_mask); <a href="#cb8-48" aria-hidden="true"></a> <a href="#cb8-49" aria-hidden="true"></a> // If we're here, we know that at least one message is awaiting <a href="#cb8-50" aria-hidden="true"></a> // us on the server channel. <a href="#cb8-51" aria-hidden="true"></a> for(;;) { <a href="#cb8-52" aria-hidden="true"></a> msg = GetChanMsg(chanID, &errcode); <a href="#cb8-53" aria-hidden="true"></a> if(errcode == GCME_CHAN_EMPTY) <a href="#cb8-54" aria-hidden="true"></a> break; <a href="#cb8-55" aria-hidden="true"></a> <a href="#cb8-56" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb8-57" aria-hidden="true"></a> printf("Unable to GetChanMsg."); <a href="#cb8-58" aria-hidden="true"></a> goto no_chan_msg; <a href="#cb8-59" aria-hidden="true"></a> } <a href="#cb8-60" aria-hidden="true"></a> <a href="#cb8-61" aria-hidden="true"></a> struct MsgReq mr; <a href="#cb8-62" aria-hidden="true"></a> uint64_t actual = ReadChanMsg(msg, &mr, sizeof(mr)); <a href="#cb8-63" aria-hidden="true"></a> switch(mr.function) { <a href="#cb8-64" aria-hidden="true"></a> case op_shutdown: <a href="#cb8-65" aria-hidden="true"></a> still_running = false; <a href="#cb8-66" aria-hidden="true"></a> prinf("Someone is asking us to shutdown.\n"); <a href="#cb8-67" aria-hidden="true"></a> break; <a href="#cb8-68" aria-hidden="true"></a> case op_add: <a href="#cb8-69" aria-hidden="true"></a> mr.accumulator += mr.operand; <a href="#cb8-70" aria-hidden="true"></a> break; <a href="#cb8-71" aria-hidden="true"></a> case op_subtract: <a href="#cb8-72" aria-hidden="true"></a> mr.accumulator -= mr.operand; <a href="#cb8-73" aria-hidden="true"></a> break; <a href="#cb8-74" aria-hidden="true"></a> } <a href="#cb8-75" aria-hidden="true"></a> // After calculating the results, return it to the sender. <a href="#cb8-76" aria-hidden="true"></a> // Note that we use WriteChanMsgAt() here because the message <a href="#cb8-77" aria-hidden="true"></a> // cursor is already set at the end of the message, thanks to <a href="#cb8-78" aria-hidden="true"></a> // the prior read. <a href="#cb8-79" aria-hidden="true"></a> actual = WriteChanMsgAt(msg, &mr, sizeof(mr), 0); <a href="#cb8-80" aria-hidden="true"></a> ReplyChanMsg(msg); <a href="#cb8-81" aria-hidden="true"></a> } <a href="#cb8-82" aria-hidden="true"></a> } <a href="#cb8-83" aria-hidden="true"></a> <a href="#cb8-84" aria-hidden="true"></a>no_chan_msg: <a href="#cb8-85" aria-hidden="true"></a> DisposeChannel(chanID); <a href="#cb8-86" aria-hidden="true"></a>no_channel: <a href="#cb8-87" aria-hidden="true"></a> FreeSignal(server_sigbit); <a href="#cb8-88" aria-hidden="true"></a>end: <a href="#cb8-89" aria-hidden="true"></a>}</code></pre>

<pre class="sourceCode c"><code class="sourceCode c"><a href="#cb9-1" aria-hidden="true"></a>enum Ops { <a href="#cb9-2" aria-hidden="true"></a> op_add, op_subtract, op_shutdown <a href="#cb9-3" aria-hidden="true"></a>}; <a href="#cb9-4" aria-hidden="true"></a> <a href="#cb9-5" aria-hidden="true"></a>struct MsgReq { <a href="#cb9-6" aria-hidden="true"></a> enum Ops function; <a href="#cb9-7" aria-hidden="true"></a> int64_t accumulator; <a href="#cb9-8" aria-hidden="true"></a> int64_t operand; <a href="#cb9-9" aria-hidden="true"></a>}; <a href="#cb9-10" aria-hidden="true"></a> <a href="#cb9-11" aria-hidden="true"></a>void <a href="#cb9-12" aria-hidden="true"></a>main(int argc, char* argv[]) { <a href="#cb9-13" aria-hidden="true"></a> // Create our reply channel. After we send a message, <a href="#cb9-14" aria-hidden="true"></a> // it will eventually appear on this channel. <a href="#cb9-15" aria-hidden="true"></a> uint8_t reply_sigbit = AllocSignal(&errcode); <a href="#cb9-16" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb9-17" aria-hidden="true"></a> printf("Cannot allocate a signal.\n"); <a href="#cb9-18" aria-hidden="true"></a> goto end; <a href="#cb9-19" aria-hidden="true"></a> } <a href="#cb9-20" aria-hidden="true"></a> uint64_t reply_mask = 1 << reply_sigbit; <a href="#cb9-21" aria-hidden="true"></a> <a href="#cb9-22" aria-hidden="true"></a> struct ChannelAttrs chatters = { 0, }; <a href="#cb9-23" aria-hidden="true"></a> chattrs.sig_bit = reply_sigbit; <a href="#cb9-24" aria-hidden="true"></a> uint64_t replyID = NewChannel(&chattrs, CHATTRF_SIGBIT, &errcode); <a href="#cb9-25" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb9-26" aria-hidden="true"></a> printf("Unable to create reply channel.\n"); <a href="#cb9-27" aria-hidden="true"></a> goto no_reply_channel; <a href="#cb9-28" aria-hidden="true"></a> } <a href="#cb9-29" aria-hidden="true"></a> <a href="#cb9-30" aria-hidden="true"></a> // Locate the server; this is the program who will perform our <a href="#cb9-31" aria-hidden="true"></a> // arithmetic for us, since C just isn't up to the task. ;) <a href="#cb9-32" aria-hidden="true"></a> uint64_t serverID = FindChannel("TallyServer", &errcode); <a href="#cb9-33" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb9-34" aria-hidden="true"></a> printf("Cannot find Tally Server channel\n"); <a href="#cb9-35" aria-hidden="true"></a> goto bad_operation; <a href="#cb9-36" aria-hidden="true"></a> } <a href="#cb9-37" aria-hidden="true"></a> <a href="#cb9-38" aria-hidden="true"></a> // Build the message we want to send. <a href="#cb9-39" aria-hidden="true"></a> struct MsgReq mr; <a href="#cb9-40" aria-hidden="true"></a> <a href="#cb9-41" aria-hidden="true"></a> if(argc < 4) { <a href="#cb9-42" aria-hidden="true"></a> printf("Syntax: %s accumulator/N function/K operand/N\n", argv[0]); <a href="#cb9-43" aria-hidden="true"></a> goto bad_operation; <a href="#cb9-44" aria-hidden="true"></a> } <a href="#cb9-45" aria-hidden="true"></a> <a href="#cb9-46" aria-hidden="true"></a> switch(argv[2][0]) { <a href="#cb9-47" aria-hidden="true"></a> case '+': mr.function = op_add; break; <a href="#cb9-48" aria-hidden="true"></a> case '-': mr.function = op_subtract; break; <a href="#cb9-49" aria-hidden="true"></a> case 'Q': mr.function = op_shutdown; break; <a href="#cb9-50" aria-hidden="true"></a> default: <a href="#cb9-51" aria-hidden="true"></a> printf("I don't know what operation you asked for.\n"); <a href="#cb9-52" aria-hidden="true"></a> goto bad_operation; <a href="#cb9-53" aria-hidden="true"></a> } <a href="#cb9-54" aria-hidden="true"></a> mr.accumulator = atoi(argv[1]); <a href="#cb9-55" aria-hidden="true"></a> mr.operand = atoi(argv[3]); <a href="#cb9-56" aria-hidden="true"></a> <a href="#cb9-57" aria-hidden="true"></a> // Send the message. When building the message, be sure to set the <a href="#cb9-58" aria-hidden="true"></a> // reply channel ID, so it knows where to go when it comes home! <a href="#cb9-59" aria-hidden="true"></a> uint64_t msg = NewMsgQuick(replyID, &mr, sizeof(mr), &errcode); <a href="#cb9-60" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb9-61" aria-hidden="true"></a> printf("Cannot create message from simple buffer\n"); <a href="#cb9-62" aria-hidden="true"></a> goto bad_operation; <a href="#cb9-63" aria-hidden="true"></a> } <a href="#cb9-64" aria-hidden="true"></a> <a href="#cb9-65" aria-hidden="true"></a> SendChanMsg(serverID, msg); <a href="#cb9-66" aria-hidden="true"></a> <a href="#cb9-67" aria-hidden="true"></a> // At this point, the message is away and queued onto the server's channel. <a href="#cb9-68" aria-hidden="true"></a> // NO DATA HAS BEEN COPIED YET. Thus, if you need to send more messages, <a href="#cb9-69" aria-hidden="true"></a> // you must create more buffers as they're needed. The memory this <a href="#cb9-70" aria-hidden="true"></a> // message refers to must remain stable until the receiver has copied the <a href="#cb9-71" aria-hidden="true"></a> // data for itself. <a href="#cb9-72" aria-hidden="true"></a> // <a href="#cb9-73" aria-hidden="true"></a> // We can now do whatever we want. For this simple example, we choose to <a href="#cb9-74" aria-hidden="true"></a> // just wait for a response by waiting on the reply port. <a href="#cb9-75" aria-hidden="true"></a> <a href="#cb9-76" aria-hidden="true"></a> Wait(reply_mask); <a href="#cb9-77" aria-hidden="true"></a> <a href="#cb9-78" aria-hidden="true"></a> // We complete the handshake by retrieving the sent message. <a href="#cb9-79" aria-hidden="true"></a> uint64_t reply_msg_id = GetChanMsg(replyID, &errcode); <a href="#cb9-80" aria-hidden="true"></a> if(errcode != 0) { <a href="#cb9-81" aria-hidden="true"></a> printf("Uh oh; something went wrong when collecting our reply.\n"); <a href="#cb9-82" aria-hidden="true"></a> goto no_reply; <a href="#cb9-83" aria-hidden="true"></a> } <a href="#cb9-84" aria-hidden="true"></a> <a href="#cb9-85" aria-hidden="true"></a> // Print out the results from our server. <a href="#cb9-86" aria-hidden="true"></a> <a href="#cb9-87" aria-hidden="true"></a> printf("The result is: %lld\n", mr.accumulator); <a href="#cb9-88" aria-hidden="true"></a> <a href="#cb9-89" aria-hidden="true"></a> // At this point, we could re-use the buffer/msg pair. <a href="#cb9-90" aria-hidden="true"></a> <a href="#cb9-91" aria-hidden="true"></a>no_reply: <a href="#cb9-92" aria-hidden="true"></a> DisposeMsg(msg); <a href="#cb9-93" aria-hidden="true"></a>bad_operation: <a href="#cb9-94" aria-hidden="true"></a> DisposeChannel(replyID); <a href="#cb9-95" aria-hidden="true"></a>no_reply_channel: <a href="#cb9-96" aria-hidden="true"></a> FreeSignal(reply_sigbit); <a href="#cb9-97" aria-hidden="true"></a>end: <a href="#cb9-98" aria-hidden="true"></a>}</code></pre>