X-Git-Url: http://info.iut-bm.univ-fcomte.fr/pub/gitweb/simgrid.git/blobdiff_plain/7fdd9d11419684606beb08a8a7bc673955eb08af..b47649fd1bd845d7c3d4c07e4400382570d080db:/doc/gtut-tour-8-exceptions.doc

diff --git a/doc/gtut-tour-8-exceptions.doc b/doc/gtut-tour-8-exceptions.doc
index 118ef2616d..260b5c3e5b 100644
--- a/doc/gtut-tour-8-exceptions.doc
+++ b/doc/gtut-tour-8-exceptions.doc
@@ -1,5 +1,5 @@
 /**
-@page GRAS_tut_tour_exceptions Lesson 8: Handling errors through exceptions (TODO)
+@page GRAS_tut_tour_exceptions Lesson 8: Handling errors through exceptions
 
 \section GRAS_tut_tour_exceptions_toc Table of Contents
  - \ref GRAS_tut_tour_exceptions_intro
@@ -10,17 +10,136 @@
 
 \section GRAS_tut_tour_exceptions_intro Introduction
 
+Exceptions are a great mecanism to deal with error exception, everyone knows
+that.
+
+Without exceptions, you have to rely on returning a value indicating whether
+the function call were right or not, and check the return values of every
+single function you call. If there is one point in the calling sequence
+where your forgot this check, the chain is broken and caller won't notice
+the issue. In practice, dealing with error without exceptions loads user
+code with *tons* of those stupid checks and you loose your functional code
+in the middle of that miasm.
+
+With them, you simply write your code. If you want to deal with errors (ie,
+you actually know how to react to errors at this point of your code), you
+write a catching block. If you don't, you don't. And exceptions flow through
+from trowing point to catching point without bothering you.
+
+At this point, you may be a bit surprised by the previous paragraphs.
+SimGrid and GRAS are written in C, and everybody knows that there is no
+exception in C but only in C++, Java and such. This is true, exceptions are
+not part of the C language, but this is such a great tool that we
+implemented an exception mecanism as part of the SimGrid library (with
+setjmp and longjmp, for the curious). 
+
+Being "home-grown" make our exception mecanic both stronger and weaker at
+the same time. First it is weaker because, well, we are more limitated
+within the library as we are than if we could change the compiler itself to
+add some extra checks here and specific treatment there. But it is also a
+advantage for us, since the exception mecanism is perfectly fitted to the
+distributed settings of GRAS processes. They can easily propagate on the
+net, as we will see in the next lesson (\ref GRAS_tut_tour_rpc) and contain
+information about the host on which they were thrown (#xbt_ex_t) along with
+the thrown point in the source code.
+
+The syntax of XBT exceptions should not sound unfamilliar to most of you.
+You throw them using the #THROW0...#THROW7 macros. They take 2 extra
+arguments in addition to the format and its self arguments: an error
+category (of type #xbt_errcat_t) and an error "value" (an integer;
+pratically, this is often left to 0 in my own code). So, you may have
+something like the following:
+\verbatim THROW3(system_error, 0, "Cannot connect to %s:%d because of %s", hostname, port, reason);\endverbatim
+
+Then, you simply add a #TRY/#CATCH block around your code:
+\verbatim TRY{ 
+  /* your code */ 
+} CATCH(e) { 
+  /* error handling code */
+} \endverbatim
+
+Another strange thing is that you should actually free the memory allocated
+to the exception with xbt_ex_fres() if you manage to deal with them. There
+is a bit more than this on the picture (#CLEANUP blocks, for example), and
+you should check the section \ref XBT_ex for more details.
+
+You should be <b>very carfull</b> when using the exceptions. They work great
+when used correctly, but there is a few golden rules you should never break.
+Moreover, the error messages and symptom can get <b>really crude</b> when
+misusing the exceptions.
+
+ - <b>Do not move out of a TRY block with a return, a break or any other
+   kind of jump. NEVER. EVER.</b>. This is the most tempting error, and this
+   drives the system nuts. You will probably segfault in the next exception
+   raising, far away from where you incidentally typed <tt>return</tt>
+   (this is because there is some cleanups to do at the end of a TRY block,
+   you cannot just leave it).
+ - <b>Play safe with variables modified in the TRY block</b>. You may want
+   to mark them as <tt>volatile</tt>, which is a modifier (just like
+   <tt>const</tt>) indicating that the value of the denoted variable may get
+   changed by external parts of the program during the run. This is the case
+   when your data gets modified by an exception raising function, I guess.
+
+So, as you can see, you don't want to include large sections of your program
+in TRY blocks. If you do so, it's quite sure that one day, you'll do a break
+or a return within this block. And believe me, finding such typos is a real
+pain.
+
 \section GRAS_tut_tour_exceptions_use Putting exceptions into action
 
+Okay. I hope those little warnings didn't discourage you from using the
+exceptions, because they really are a nice mecanism. We will now change a
+bit our program to take advantage of them. The only issue is that when a
+program run properly, it usually don't raise any exception. We could protect
+the calls we already have with exception handling, but it wouldn't be really
+exciting since we know this code does not throw any exception under the
+condition we use (actually, most of the GRAS functions may throw exception
+on problem).
+
+Instead, we will code a little game between the client and the server. We
+won't tell the client the exact port on which the server listen, and it will
+have to find from itself. For this, it will try to open socket and send the
+kill message to each ports of the search range. If it manage to close the
+socket after sending the message without being interrupted by an exception,
+it can assume that it killed the server and stop searching.
+\dontinclude 8-exceptions.c
+\skip port=3000
+\until end_of_loop
+
+To make the game a bit more fun (and to show you what an exception actually
+look like when it's not catched), we add a potential command line argument
+to the server, asking it to cheat and to not open its port within the search
+range but elsewhere:
+\dontinclude 8-exceptions.c
+\skip strcmp
+\until gras_socket_my_port
+\until }
+
+Then, when the client detects that it didn't manage to find&destroy the
+server, it throws a suicide exception (sorry for the bad jokes):
+\skip if(!found)
+\until THROW
 
 \section GRAS_tut_tour_exceptions_recap Recapping everything together
 
-The program now reads:
-\include 8-exceptions.c
+Here is the output produced by this new program. Note that when the program
+bails out because of an uncatched exception, it displays its backtrace just
+like a JVM would do (ok, it'a a bit cruder than the one of the JVM, but
+anyway). For each function frame of the calling stack, it displays the
+function name and its location in the source files (if it manage to retrieve
+it). Don't be jalous, you can display such stacks wherever you want with 
+xbt_backtrace_display() ;)
+
+Unfortunately, this feature is only offered under Linux for now since I have
+no idea of how to retrieve the call stack of the current process under the
+other operating systems. But help is always welcome in this area too ;)
 
-Which produces the expected output:
 \include 8-exceptions.output
 
+The complete program reads:
+\include 8-exceptions.c
+
+
 Go to \ref GRAS_tut_tour_rpc
 
 */