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Reid Spencer 2004-08-27 02:08:04 +00:00
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@ -18,14 +18,18 @@
<li><a href="#requirements">System Library Requirements</a>
<ol>
<li><a href="#headers">Hide System Header Files</a></li>
<li><a href="#c_headers">Allow Standard C Header Files</a></li>
<li><a href="#cpp_headers">Allow Standard C++ Header Files</a></li>
<li><a href="#nofunc">No Exposed Functions</a></li>
<li><a href="#nodata">No Exposed Data</a></li>
<li><a href="#xcptns">No Exceptions</a></li>
<li><a href="#errors">Standard Error Codes</a></li>
<li><a href="#overhead">Minimize Overhead</a></li>
<li><a href="#throw">Throw Only std::string</a></li>
<li><a href="#throw_spec">No throw() Specifications</a></li>
<li><a href="#nodupl">No Duplicate Impementations</a></li>
</ol></li>
<li><a href="#design">System Library Design</a>
<ol>
<li><a href="#nounused">No Unused Functionality</a></li>
<li><a href="#highlev">High-Level Interface</a></li>
<li><a href="#opaque">Use Opaque Classes</a></li>
<li><a href="#common">Common Implementations</a></li>
<li><a href="#multi_imps">Multiple Implementations</a></li>
@ -54,12 +58,17 @@
<tt>llvm/lib/System</tt>. The goal of this library is to completely shield
LLVM from the variations in operating system interfaces. By centralizing
LLVM's use of operating system interfaces, we make it possible for the LLVM
tool chain and runtime libraries to be more easily ported to new platforms.
The library also unclutters the rest of LLVM from #ifdef use and special
cases for specific operating systems.</p>
<p>The System Library was donated to LLVM by Reid Spencer who formulated the
original design as part of the eXtensible Programming System (XPS) which is
based, in part, on LLVM.</p>
tool chain and runtime libraries to be more easily ported to new platforms
since (theoretically) only <tt>llvm/lib/System</tt> needs to be ported. This
library also unclutters the rest of LLVM from #ifdef use and special
cases for specific operating systems. Such uses are replaced with simple calls
to the interfaces provided in <tt>llvm/include/llvm/System</tt>.</p> Note that
lib/System is not intended to be a complete operating system wrapper (such as
the Adaptive Communications Environment (ACE) or Apache Portable Runtime
(APR)), but only to provide the functionality necessary to support LLVM.
<p>The System Library was written by Reid Spencer who formulated the
design based on similar original work as part of the eXtensible Programming
System (XPS).</p>
</div>
<!-- *********************************************************************** -->
@ -69,43 +78,113 @@
<div class="doc_text">
<p>The System library's requirements are aimed at shielding LLVM from the
variations in operating system interfaces. The following sections define the
requirements needed to fulfill this objective.</p>
requirements needed to fulfill this objective. Of necessity, these requirements
must be strictly followed in order to ensure the library's goal is reached.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="headers">Hide System Header Files</a></div>
<div class="doc_text">
<p>To be written.</p>
<p>The library must sheild LLVM from <em>all</em> system libraries. To obtain
system level functionality, LLVM must <tt>#include "llvm/System/Thing.h"</tt>
and nothing else. This means that <tt>Thing.h</tt> cannot expose any system
header files. This protects LLVM from accidentally using system specific
functionality except through the lib/System interface. Specifically this
means that header files like "unistd.h", "windows.h", "stdio.h", and
"string.h" are verbotten outside the implementation of lib/System.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="c_headers">Allow Standard C Headers</a>
</div>
<div class="doc_text">
<p>The <em>standard</em> C headers (the ones beginning with "c") are allowed
to be exposed through the lib/System interface. These headers and the things
they declare are considered to be platform agnostic. LLVM source files may
include them or obtain their inclusion through lib/System interfaces.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="cpp_headers">Allow Standard C++ Headers</a>
</div>
<div class="doc_text">
<p>The <em>standard</em> C++ headers from the standard C++ library and
standard template library are allowed to be exposed through the lib/System
interface. These headers and the things they declare are considered to be
platform agnostic. LLVM source files may include them or obtain their
inclusion through lib/System interfaces.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
<div class="doc_text">
<p>To be written.</p>
<p>Any functions defined by system libraries (i.e. not defined by lib/System)
must not be exposed through the lib/System interface, even if the header file
for that function is not exposed. This prevents inadvertent use of system
specific functionality.</p>
<p>For example, the <tt>stat</tt> system call is notorious for having
variations in the data it provides. lib/System must not declare <tt>stat</tt>
nor allow it to be declared. Instead it should provide its own interface to
discovering information about files and directories. Those interfaces may be
implemented in terms of <tt>stat</tt> but that is strictly an implementation
detail.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
<div class="doc_text">
<p>To be written.</p>
<p>Any data defined by system libraries (i.e. not defined by lib/System) must
not be exposed through the lib/System interface, even if the header file for
that function is not exposed. As with functions, this prevents inadvertent use
of data that might not exist on all platforms.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="xcptns">No Exceptions</a></div>
<div class="doc_subsection"><a name="throw">Throw Only std::string</a></div>
<div class="doc_text">
<p>To be written.</p>
<p>If an error occurs that lib/System cannot handle, the only action taken by
lib/System is to throw an instance of std:string. The contents of the string
must explain both what happened and the context in which it happened. The
format of the string should be a (possibly empty) list of contexts each
terminated with a : and a space, followed by the error message, optionally
followed by a reason, and optionally followed by a suggestion.</p>
<p>For example, failure to open a file named "foo" could result in a message
like:</p>
<ul><li>foo: Unable to open file because it doesn't exist."</li></ul>
<p>The "foo:" part is the context. The "Unable to open file" part is the error
message. The "because it doesn't exist." part is the reason. This message has
no suggestion. Where possible, the imlementation of lib/System should use
operating system specific facilities for converting the error code returned by
a system call into an error message. This will help to make the error message
more familiar to users of that type of operating system.</p>
<p>Note that this requirement precludes the throwing of any other exceptions.
For example, various C++ standard library functions can cause exceptions to be
thrown (e.g. out of memory situation). In all cases, if there is a possibility
that non-string exceptions could be thrown, the lib/System library must ensure
that the exceptions are translated to std::string form.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="errors">Standard Error Codes</a></div>
<div class="doc_subsection"><a name="throw_spec">No throw Specifications</a>
</div>
<div class="doc_text">
<p>To be written.</p>
<p>None of the lib/System interface functions may be declared with C++
<tt>throw()</tt> specifications on them. This requirement makes sure that the
compler does not insert addtional exception handling code into the interface
functions. This is a performance consideration: lib/System functions are at
the bottom of the many call chains and as such can be frequently called. We
need them to be as efficient as possible.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="overhead">Minimize Overhead</a></div>
<div class="doc_subsection"><a name="nodupl">No Duplicate Implementations</a>
</div>
<div class="doc_text">
<p>To be written.</p>
<p>The implementation of a function for a given platform must be written
exactly once. This implies that it must be possible to apply a function's
implementation to multiple operating systems if those operating systems can
share the same implementation.</p>
</div>
<!-- *********************************************************************** -->
@ -118,6 +197,128 @@
remainder of LLVM is completely operating system agnostic.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="nounused">No Unused Functionality</a></div>
<div class="doc_text">
<p>There must be no functionality specified in the interface of lib/System
that isn't actually used by LLVM. We're not writing a general purpose
operating system wrapper here, just enough to satisfy LLVM's needs. And, LLVM
doesn't need much. This design goal aims to keep the lib/System interface
small and understandable which should foster its actual use and adoption.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="highlev">High Level Interface</a></div>
<div class="doc_text">
<p>The entry points specified in the interface of lib/System must be aimed at
completing some reasonably high level task needed by LLVM. We do not want to
simply wrap each operating system call. It would be preferable to wrap several
operating system calls that are always used in conjunction with one another by
LLVM.</p>
<p>For example, consider what is needed to execute a program, wait for it to
complete, and return its result code. On Unix, this involves the following
operating system calls: <tt>getenv, fork, execve,</tt> and <tt>wait</tt>. The
correct thing for lib/System to provide is a function, say
<tt>ExecuteProgramAndWait</tt>, that implements the functionality completely.
what we don't want is wrappers for the operating system calls involved.</p>
<p>There must <em>not</em> be a one-to-one relationship between operating
system calls and the System library's interface. Any such interface function
will be suspicious.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="highlev">Minimize Soft Errors</a></div>
<div class="doc_text">
<p>Operating system interfaces will generally provide errors results for every
little thing that could go wrong. In almost all cases, you can divide these
error results into two groups: normal/good/soft and abnormal/bad/hard. That
is, some of the errors are simply information like "file not found",
"insufficient privileges", etc. while other errors are much harder like
"out of space", "bad disk sector", or "system call interrupted". Well call the
first group "soft" errors and the second group "hard" errors.<p>
<p>lib/System must always attempt to minimize soft errors and always just
throw a std::string on hard errors. This is a design requirement because the
minimization of soft errors can affect the granularity and the nature of the
interface. In general, if you find that you're wanting to throw soft errors,
you must review the granularity of the interface because it is likely you're
trying to implement something that is too low level. The rule of thumb is to
provide interface functions that "can't" fail, except when faced with hard
errors.</p>
<p>For a trivial example, suppose we wanted to add an "OpenFileForWriting"
function. For many operating systems, if the file doesn't exist, attempting
to open the file will produce an error. However, lib/System should not
simply throw that error if it occurs because its a soft error. The problem
is that the interface function, OpenFileForWriting is too low level. It should
be OpenOrCreateFileForWriting. In the case of the soft "doesn't exist" error,
this function would just create it and then open it for writing.</p>
<p>This design principle needs to be maintained in lib/System because it
avoids the propagation of soft error handling throughout the rest of LLVM.
Hard errors will generally just cause a termination for an LLVM tool so don't
be bashful about throwing them.</p>
<p>Rules of thumb:</p>
<ol>
<li>Don't throw soft errors, only hard errors.</li>
<li>If you're tempted to throw a soft error, re-think the interface.</li>
<li>Handle internally the most common normal/good/soft error conditions
so the rest of LLVM doesn't have to.</li>
</ol>
<pre><tt>
Notes:
10. The implementation of a lib/System interface can vary drastically between
platforms. That's okay as long as the end result of the interface function is
the same. For example, a function to create a directory is pretty straight
forward on all operating system. System V IPC on the other hand isn't even
supported on all platforms. Instead of "supporting" System V IPC, lib/System
should provide an interface to the basic concept of inter-process
communications. The implementations might use System V IPC if that was
available or named pipes, or whatever gets the job done effectively for a
given operating system.
11. Implementations are separated first by the general class of operating system
as provided by the configure script's $build variable. This variable is used
to create a link from $BUILD_OBJ_ROOT/lib/System/platform to a directory in
$BUILD_SRC_ROOT/lib/System directory with the same name as the $build
variable. This provides a retargetable include mechanism. By using the link's
name (platform) we can actually include the operating specific
implementation. For example, support $build is "Darwin" for MacOS X. If we
place:
#include "platform/File.cpp"
into a a file in lib/System, it will actually include
lib/System/Darwin/File.cpp. What this does is quickly differentiate the basic
class of operating system that will provide the implementation.
12. Implementation files in lib/System need may only do two things: (1) define
functions and data that is *TRULY* generic (completely platform agnostic) and
(2) #include the platform specific implementation with:
#include "platform/Impl.cpp"
where Impl is the name of the implementation files.
13. Platform specific implementation files (platform/Impl.cpp) may only #include
other Impl.cpp files found in directories under lib/System. The order of
inclusion is very important (from most generic to most specific) so that we
don't inadvertently place an implementation in the wrong place. For example,
consider a fictitious implementation file named DoIt.cpp. Here's how the
#includes should work for a Linux platform
lib/System/DoIt.cpp
#include "platform/DoIt.cpp" // platform specific impl. of Doit
DoIt
lib/System/Linux/DoIt.cpp // impl that works on all Linux
#include "../Unix/DoIt.cpp" // generic Unix impl. of DoIt
#include "../Unix/SUS/DoIt.cpp // SUS specific impl. of DoIt
#include "../Unix/SUS/v3/DoIt.cpp // SUSv3 specific impl. of DoIt
Note that the #includes in lib/System/Linux/DoIt.cpp are all optional but
should be used where the implementation of some functionality can be shared
across some set of Unix variants. We don't want to duplicate code across
variants if their implementation could be shared.
</tt></pre>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="opaque">Use Opaque Classes</a></div>
<div class="doc_text">
@ -141,14 +342,6 @@
<p>To be written.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="low_level">Use Low Level Interfaces</a>
</div>
<div class="doc_text">
<p>To be written.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="memalloc">No Memory Allocation</a></div>
<div class="doc_text">