diff options
Diffstat (limited to 'docs/SystemLibrary.html')
| -rw-r--r-- | docs/SystemLibrary.html | 316 |
1 files changed, 0 insertions, 316 deletions
diff --git a/docs/SystemLibrary.html b/docs/SystemLibrary.html deleted file mode 100644 index 1ef221fa27..0000000000 --- a/docs/SystemLibrary.html +++ /dev/null @@ -1,316 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> - <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> - <title>System Library</title> - <link rel="stylesheet" href="_static/llvm.css" type="text/css"> -</head> -<body> - -<h1>System Library</h1> -<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_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> - - -<!-- *********************************************************************** --> -<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 - 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> - -<!-- *********************************************************************** --> -<h2> - <a name="requirements">Keeping LLVM Portable</a> -</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> - -<!-- ======================================================================= --> -<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 - 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> - -<!-- ======================================================================= --> -<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 - <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> - -<!-- ======================================================================= --> -<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 - files may include them directly or obtain their inclusion through - <tt>lib/System</tt> interfaces.</p> -</div> - -<!-- ======================================================================= --> -<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 - platform agnostic. LLVM source files may include them or obtain their - inclusion through lib/System interfaces.</p> -</div> - -<!-- ======================================================================= --> -<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 - 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> - -<!-- ======================================================================= --> -<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 - 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> - -<!-- ======================================================================= --> -<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 - 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> - -<!-- ======================================================================= --> -<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 - differences, it just adds complexity. The <tt>#include</tt> mechanism works - just fine.</p> -</div> - -<!-- ======================================================================= --> -<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 - 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> - -<!-- ======================================================================= --> -<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 - of data that might not exist on all platforms.</p> -</div> - -<!-- ======================================================================= --> -<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 - 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. - 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> - -<!-- ======================================================================= --> -<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. However, no routines in the - system library should actually throw exceptions.</p> -</div> - -<!-- ======================================================================= --> -<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. - 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> - -<!-- ======================================================================= --> -<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 - 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> - -<!-- ======================================================================= --> -<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> - -<!-- *********************************************************************** --> - -<hr> -<address> - <a href="http://jigsaw.w3.org/css-validator/check/referer"><img - src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> - <a href="http://validator.w3.org/check/referer"><img - src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> - - <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br> - <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> - Last modified: $Date$ -</address> -</body> -</html> |