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+ <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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+ <link rel="stylesheet" href="_static/llvm.css" type="text/css">
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-<div class="doc_title">System Library</div>
+<h1>System Library</h1>
<ul>
<li><a href="#abstract">Abstract</a></li>
<li><a href="#requirements">Keeping LLVM Portable</a>
<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>
</ul>
<div class="doc_author">
- <p>Written by <a href="rspencer@x10sys.com">Reid Spencer</a></p>
+ <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">
+<h2><a name="abstract">Abstract</a></h2>
+<div>
<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
</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="requirements">Keeping LLVM Portable</a>
-</div>
-<div class="doc_text">
+</h2>
+<div>
<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">
+<h3><a name="headers">Don't Include System Headers</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="expose">Don't Expose System Headers</a>
-</div>
-<div class="doc_text">
+<h3><a name="expose">Don't Expose System Headers</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="c_headers">Use Standard C Headers</a></div>
-<div class="doc_text">
+<h3><a name="c_headers">Use Standard C Headers</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="cpp_headers">Use Standard C++ Headers</a>
-</div>
-<div class="doc_text">
+<h3><a name="cpp_headers">Use Standard C++ Headers</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="highlev">High Level Interface</a></div>
-<div class="doc_text">
+<h3><a name="highlev">High Level Interface</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="nounused">No Unused Functionality</a></div>
-<div class="doc_text">
+<h3><a name="nounused">No Unused Functionality</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="nodupl">No Duplicate Implementations</a>
-</div>
-<div class="doc_text">
+<h3><a name="nodupl">No Duplicate Implementations</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
-<div class="doc_text">
+<h3><a name="virtuals">No Virtual Methods</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
-<div class="doc_text">
+<h3><a name="nofunc">No Exposed Functions</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
-<div class="doc_text">
+<h3><a name="nodata">No Exposed Data</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="softerrors">Minimize Soft Errors</a></div>
-<div class="doc_text">
+<h3><a name="softerrors">Minimize Soft Errors</a></h3>
+<div>
<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
"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
+ <p>lib/System must always attempt to minimize soft 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
</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">
+<h3><a name="throw_spec">No throw Specifications</a></h3>
+<div>
<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>
+ need them to be as efficient as possible. However, no routines in the
+ system library should actually throw exceptions.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="organization">Code Organization</a></div>
-<div class="doc_text">
+<h3><a name="organization">Code Organization</a></h3>
+<div>
<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.
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="semantics">Consistent Semantics</a></div>
-<div class="doc_text">
+<h3><a name="semantics">Consistent Semantics</a></h3>
+<div>
<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
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="bug">Bug 351</a></div>
-<div class="doc_text">
- <p>See <a href="http://llvm.cs.uiuc.edu/PR351">bug 351</a>
+<h3><a name="bug">Bug 351</a></h3>
+<div>
+ <p>See <a href="http://llvm.org/PR351">bug 351</a>
for further details on the progress of this work</p>
</div>
+</div>
+
<!-- *********************************************************************** -->
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- <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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