llvm/docs/SystemLibrary.html - toolchain/llvm-project - Gitiles

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 <head>
   <title>System Library</title>
   <link rel="stylesheet" href="llvm.css" type="text/css">
 </head>
 <body>

 <div class="doc_title">System Library</div>
 <ul>
   <li><a href="#abstract">Abstract</a></li>
   <li><a href="#requirements">Keeping LLVM Portable</a>
   <ol>
     <li><a href="#headers">Don't Include System Headers</a></li>
     <li><a href="#expose">Don't Expose System Headers</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="#highlev">High-Level Interface</a></li>
     <li><a href="#nofunc">No Exposed Functions</a></li>
     <li><a href="#nodata">No Exposed Data</a></li>
     <li><a href="#nodupl">No Duplicate Implementations</a></li>
     <li><a href="#nounused">No Unused Functionality</a></li>
     <li><a href="#virtuals">No Virtual Methods</a></li>
     <li><a href="#softerrors">Minimize Soft Errors</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="#organization">Code Organization</a></li>
     <li><a href="#semantics">Consistent Semantics</a></li>
     <li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
   </ol></li>
 </ul>

 <div class="doc_author">
   <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
 </div>


 <!-- *********************************************************************** -->
 <div class="doc_section"><a name="abstract">Abstract</a></div>
 <div class="doc_text">
   <p>This document provides some details on LLVM's System Library, located in
   the source at <tt>lib/System</tt> and <tt>include/llvm/System</tt>. The
   library's purpose is to shield LLVM from the differences between operating
   systems for the few services LLVM needs from the operating system. Much of
   LLVM is written using portability features of standard C++. However, in a few
   areas, system dependent facilities are needed and the System Library is the
   wrapper around those system calls.</p>
   <p>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 since (theoretically) only <tt>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>include/llvm/System</tt>.
   </p>
   <p>Note that the System Library is not intended to be a complete operating
   system wrapper (such as the Adaptive Communications Environment (ACE) or
   Apache Portable Runtime (APR)), but only provides the functionality necessary
   to support LLVM.
   <p>The System Library was written by Reid Spencer who formulated the
   design based on similar work originating from the eXtensible Programming
   System (XPS). Several people helped with the effort; especially,
   Jeff Cohen and Henrik Bach on the Win32 port.</p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section">
   <a name="requirements">Keeping LLVM Portable</a>
 </div>
 <div class="doc_text">
   <p>In order to keep LLVM portable, LLVM developers should adhere to a set of
   portability rules associated with the System Library. Adherence to these rules
   should help the System Library achieve its goal of shielding LLVM from the
   variations in operating system interfaces and doing so efficiently.  The
   following sections define the rules needed to fulfill this objective.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="headers">Don't Inlcude System Headers</a>
 </div>
 <div class="doc_text">
   <p>Except in <tt>lib/System</tt>, no LLVM source code should directly
   <tt>#include</tt> a system header. Care has been taken to remove all such
   <tt>#includes</tt> from LLVM while <tt>lib/System</tt> was being
   developed.  Specifically this means that header files like "unistd.h",
   "windows.h", "stdio.h", and "string.h" are forbidden to be included by LLVM
   source code outside the implementation of <tt>lib/System</tt>.</p>
   <p>To obtain system-dependent functionality, existing interfaces to the system
   found in <tt>include/llvm/System</tt> should be used. If an appropriate
   interface is not available, it should be added to <tt>include/llvm/System</tt>
   and implemented in <tt>lib/System</tt> for all supported platforms.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="expose">Don't Expose System Headers</a>
 </div>
 <div class="doc_text">
   <p>The System Library must shield LLVM from <em>all</em> system headers. To
   obtain system level functionality, LLVM source 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 and only allows it
   via the <tt>lib/System</tt> interface.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="c_headers">Use 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 <tt>lib/System</tt> interface. These headers and
   the things they declare are considered to be platform agnostic. LLVM source
   files may include them directly or obtain their inclusion through
   <tt>lib/System</tt> interfaces.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="cpp_headers">Use 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 may be exposed through the <tt>lib/System</tt>
   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="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="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="nodupl">No Duplicate Implementations</a>
 </div>
 <div class="doc_text">
   <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. This rule applies to the set of operating
   systems supported for a given class of operating system (e.g. Unix, Win32).
   </p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
 <div class="doc_text">
   <p>The System Library interfaces can be called quite frequently by LLVM. In
   order to make those calls as efficient as possible, we discourage the use of
   virtual methods. There is no need to use inheritance for implementation
   differences, it just adds complexity. The <tt>#include</tt> mechanism works
   just fine.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
 <div class="doc_text">
   <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. <tt>lib/System</tt> 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. The interface provided by the System Library must
   be implemented on all platforms (even those without <tt>stat</tt>).</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
 <div class="doc_text">
   <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="softerrors">Minimize Soft Errors</a></div>
 <div class="doc_text">
   <p>Operating system interfaces will generally provide error 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". We'll call
   the first group "<i>soft</i>" errors and the second group "<i>hard</i>"
   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 <em>can't</em> 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>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="throw">Throw Only std::string</a></div>
 <div class="doc_text">
   <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 implementation 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="throw_spec">No throw Specifications</a>
 </div>
 <div class="doc_text">
   <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
   compiler does not insert additional exception handling code into the interface
   functions. This is a performance consideration: lib/System functions are at
   the bottom of 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="organization">Code Organization</a></div>
 <div class="doc_text">
   <p>Implementations of the System Library interface are separated by their
   general class of operating system. Currently only Unix and Win32 classes are
   defined but more could be added for other operating system classifications.
   To distinguish which implementation to compile, the code in lib/System uses
   the LLVM_ON_UNIX and LLVM_ON_WIN32 #defines provided via configure through the
   llvm/Config/config.h file. Each source file in lib/System, after implementing
   the generic (operating system independent) functionality needs to include the
   correct implementation using a set of <tt>#if defined(LLVM_ON_XYZ)</tt>
   directives. For example, if we had lib/System/File.cpp, we'd expect to see in
   that file:</p>
   <pre><tt>
   #if defined(LLVM_ON_UNIX)
   #include "Unix/File.cpp"
   #endif
   #if defined(LLVM_ON_WIN32)
   #include "Win32/File.cpp"
   #endif
   </tt></pre>
   <p>The implementation in lib/System/Unix/File.cpp should handle all Unix
   variants. The implementation in lib/System/Win32/File.cpp should handle all
   Win32 variants.  What this does is quickly differentiate the basic class of
   operating system that will provide the implementation. The specific details
   for a given platform must still be determined through the use of
   <tt>#ifdef</tt>.</p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="semantics">Consistent Semantics</a></div>
 <div class="doc_text">
   <p>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.  In all cases, the interface and the implementation
   must be semantically consistent. </p>
 </div>

 <!-- ======================================================================= -->
 <div class="doc_subsection"><a name="bug">Bug 351</a></div>
 <div class="doc_text">
   <p>See <a href="http://llvm.org/PR351">bug 351</a>
   for further details on the progress of this work</p>
 </div>

 <!-- *********************************************************************** -->

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   <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
   Last modified: $Date$
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 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<title>System Library</title>
	<link rel="stylesheet" href="llvm.css" type="text/css">
	</head>
	<body>

	<div class="doc_title">System Library</div>
	<ul>
	<li><a href="#abstract">Abstract</a></li>
	<li><a href="#requirements">Keeping LLVM Portable</a>
	<ol>
	<li><a href="#headers">Don't Include System Headers</a></li>
	<li><a href="#expose">Don't Expose System Headers</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="#highlev">High-Level Interface</a></li>
	<li><a href="#nofunc">No Exposed Functions</a></li>
	<li><a href="#nodata">No Exposed Data</a></li>
	<li><a href="#nodupl">No Duplicate Implementations</a></li>
	<li><a href="#nounused">No Unused Functionality</a></li>
	<li><a href="#virtuals">No Virtual Methods</a></li>
	<li><a href="#softerrors">Minimize Soft Errors</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="#organization">Code Organization</a></li>
	<li><a href="#semantics">Consistent Semantics</a></li>
	<li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
	</ol></li>
	</ul>

	<div class="doc_author">
	<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
	</div>


	<!-- *********************************************************************** -->
	<div class="doc_section"><a name="abstract">Abstract</a></div>
	<div class="doc_text">
	<p>This document provides some details on LLVM's System Library, located in
	the source at <tt>lib/System</tt> and <tt>include/llvm/System</tt>. The
	library's purpose is to shield LLVM from the differences between operating
	systems for the few services LLVM needs from the operating system. Much of
	LLVM is written using portability features of standard C++. However, in a few
	areas, system dependent facilities are needed and the System Library is the
	wrapper around those system calls.</p>
	<p>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 since (theoretically) only <tt>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>include/llvm/System</tt>.
	</p>
	<p>Note that the System Library is not intended to be a complete operating
	system wrapper (such as the Adaptive Communications Environment (ACE) or
	Apache Portable Runtime (APR)), but only provides the functionality necessary
	to support LLVM.
	<p>The System Library was written by Reid Spencer who formulated the
	design based on similar work originating from the eXtensible Programming
	System (XPS). Several people helped with the effort; especially,
	Jeff Cohen and Henrik Bach on the Win32 port.</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section">
	<a name="requirements">Keeping LLVM Portable</a>
	</div>
	<div class="doc_text">
	<p>In order to keep LLVM portable, LLVM developers should adhere to a set of
	portability rules associated with the System Library. Adherence to these rules
	should help the System Library achieve its goal of shielding LLVM from the
	variations in operating system interfaces and doing so efficiently. The
	following sections define the rules needed to fulfill this objective.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="headers">Don't Inlcude System Headers</a>
	</div>
	<div class="doc_text">
	<p>Except in <tt>lib/System</tt>, no LLVM source code should directly
	<tt>#include</tt> a system header. Care has been taken to remove all such
	<tt>#includes</tt> from LLVM while <tt>lib/System</tt> was being
	developed. Specifically this means that header files like "unistd.h",
	"windows.h", "stdio.h", and "string.h" are forbidden to be included by LLVM
	source code outside the implementation of <tt>lib/System</tt>.</p>
	<p>To obtain system-dependent functionality, existing interfaces to the system
	found in <tt>include/llvm/System</tt> should be used. If an appropriate
	interface is not available, it should be added to <tt>include/llvm/System</tt>
	and implemented in <tt>lib/System</tt> for all supported platforms.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="expose">Don't Expose System Headers</a>
	</div>
	<div class="doc_text">
	<p>The System Library must shield LLVM from <em>all</em> system headers. To
	obtain system level functionality, LLVM source 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 and only allows it
	via the <tt>lib/System</tt> interface.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="c_headers">Use 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 <tt>lib/System</tt> interface. These headers and
	the things they declare are considered to be platform agnostic. LLVM source
	files may include them directly or obtain their inclusion through
	<tt>lib/System</tt> interfaces.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="cpp_headers">Use 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 may be exposed through the <tt>lib/System</tt>
	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="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="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="nodupl">No Duplicate Implementations</a>
	</div>
	<div class="doc_text">
	<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. This rule applies to the set of operating
	systems supported for a given class of operating system (e.g. Unix, Win32).
	</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
	<div class="doc_text">
	<p>The System Library interfaces can be called quite frequently by LLVM. In
	order to make those calls as efficient as possible, we discourage the use of
	virtual methods. There is no need to use inheritance for implementation
	differences, it just adds complexity. The <tt>#include</tt> mechanism works
	just fine.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
	<div class="doc_text">
	<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. <tt>lib/System</tt> 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. The interface provided by the System Library must
	be implemented on all platforms (even those without <tt>stat</tt>).</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
	<div class="doc_text">
	<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="softerrors">Minimize Soft Errors</a></div>
	<div class="doc_text">
	<p>Operating system interfaces will generally provide error 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". We'll call
	the first group "<i>soft</i>" errors and the second group "<i>hard</i>"
	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 <em>can't</em> 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>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="throw">Throw Only std::string</a></div>
	<div class="doc_text">
	<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 implementation 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="throw_spec">No throw Specifications</a>
	</div>
	<div class="doc_text">
	<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
	compiler does not insert additional exception handling code into the interface
	functions. This is a performance consideration: lib/System functions are at
	the bottom of 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="organization">Code Organization</a></div>
	<div class="doc_text">
	<p>Implementations of the System Library interface are separated by their
	general class of operating system. Currently only Unix and Win32 classes are
	defined but more could be added for other operating system classifications.
	To distinguish which implementation to compile, the code in lib/System uses
	the LLVM_ON_UNIX and LLVM_ON_WIN32 #defines provided via configure through the
	llvm/Config/config.h file. Each source file in lib/System, after implementing
	the generic (operating system independent) functionality needs to include the
	correct implementation using a set of <tt>#if defined(LLVM_ON_XYZ)</tt>
	directives. For example, if we had lib/System/File.cpp, we'd expect to see in
	that file:</p>
	<pre><tt>
	#if defined(LLVM_ON_UNIX)
	#include "Unix/File.cpp"
	#endif
	#if defined(LLVM_ON_WIN32)
	#include "Win32/File.cpp"
	#endif
	</tt></pre>
	<p>The implementation in lib/System/Unix/File.cpp should handle all Unix
	variants. The implementation in lib/System/Win32/File.cpp should handle all
	Win32 variants. What this does is quickly differentiate the basic class of
	operating system that will provide the implementation. The specific details
	for a given platform must still be determined through the use of
	<tt>#ifdef</tt>.</p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="semantics">Consistent Semantics</a></div>
	<div class="doc_text">
	<p>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. In all cases, the interface and the implementation
	must be semantically consistent. </p>
	</div>

	<!-- ======================================================================= -->
	<div class="doc_subsection"><a name="bug">Bug 351</a></div>
	<div class="doc_text">
	<p>See <a href="http://llvm.org/PR351">bug 351</a>
	for further details on the progress of this work</p>
	</div>

	<!-- *********************************************************************** -->

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