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5 <title>System Library</title>
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8<body>
9
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]10<h1>System Library</h1>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]11<ul>
12 <li><a href="#abstract">Abstract</a></li>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]13 <li><a href="#requirements">Keeping LLVM Portable</a>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]14 <ol>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]15 <li><a href="#headers">Don't Include System Headers</a></li>
16 <li><a href="#expose">Don't Expose System Headers</a></li>
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]17 <li><a href="#c_headers">Allow Standard C Header Files</a></li>
18 <li><a href="#cpp_headers">Allow Standard C++ Header Files</a></li>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]19 <li><a href="#highlev">High-Level Interface</a></li>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]20 <li><a href="#nofunc">No Exposed Functions</a></li>
21 <li><a href="#nodata">No Exposed Data</a></li>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]22 <li><a href="#nodupl">No Duplicate Implementations</a></li>
23 <li><a href="#nounused">No Unused Functionality</a></li>
24 <li><a href="#virtuals">No Virtual Methods</a></li>
25 <li><a href="#softerrors">Minimize Soft Errors</a></li>
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]26 <li><a href="#throw_spec">No throw() Specifications</a></li>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]27 <li><a href="#organization">Code Organization</a></li>
28 <li><a href="#semantics">Consistent Semantics</a></li>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]29 <li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]30 </ol></li>
31</ul>
32
33<div class="doc_author">
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]34 <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]35</div>
36
37
38<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]39<h2><a name="abstract">Abstract</a></h2>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]40<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]41 <p>This document provides some details on LLVM's System Library, located in
42 the source at <tt>lib/System</tt> and <tt>include/llvm/System</tt>. The
43 library's purpose is to shield LLVM from the differences between operating
44 systems for the few services LLVM needs from the operating system. Much of
45 LLVM is written using portability features of standard C++. However, in a few
46 areas, system dependent facilities are needed and the System Library is the
47 wrapper around those system calls.</p>
48 <p>By centralizing LLVM's use of operating system interfaces, we make it
49 possible for the LLVM tool chain and runtime libraries to be more easily
50 ported to new platforms since (theoretically) only <tt>lib/System</tt> needs
51 to be ported. This library also unclutters the rest of LLVM from #ifdef use
52 and special cases for specific operating systems. Such uses are replaced
53 with simple calls to the interfaces provided in <tt>include/llvm/System</tt>.
54 </p>
55 <p>Note that the System Library is not intended to be a complete operating
56 system wrapper (such as the Adaptive Communications Environment (ACE) or
57 Apache Portable Runtime (APR)), but only provides the functionality necessary
58 to support LLVM.
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]59 <p>The System Library was written by Reid Spencer who formulated the
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]60 design based on similar work originating from the eXtensible Programming
61 System (XPS). Several people helped with the effort; especially,
62 Jeff Cohen and Henrik Bach on the Win32 port.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]63</div>
64
65<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]66<h2>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]67 <a name="requirements">Keeping LLVM Portable</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]68</h2>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]69<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]70 <p>In order to keep LLVM portable, LLVM developers should adhere to a set of
71 portability rules associated with the System Library. Adherence to these rules
72 should help the System Library achieve its goal of shielding LLVM from the
73 variations in operating system interfaces and doing so efficiently. The
74 following sections define the rules needed to fulfill this objective.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]75</div>
76
77<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]78<h3><a name="headers">Don't Include System Headers</a></h3>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]79<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]80 <p>Except in <tt>lib/System</tt>, no LLVM source code should directly
81 <tt>#include</tt> a system header. Care has been taken to remove all such
82 <tt>#includes</tt> from LLVM while <tt>lib/System</tt> was being
83 developed. Specifically this means that header files like "unistd.h",
84 "windows.h", "stdio.h", and "string.h" are forbidden to be included by LLVM
85 source code outside the implementation of <tt>lib/System</tt>.</p>
86 <p>To obtain system-dependent functionality, existing interfaces to the system
87 found in <tt>include/llvm/System</tt> should be used. If an appropriate
88 interface is not available, it should be added to <tt>include/llvm/System</tt>
89 and implemented in <tt>lib/System</tt> for all supported platforms.</p>
90</div>
91
92<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]93<h3><a name="expose">Don't Expose System Headers</a></h3>
Reid Spencerd593afc2005-01-05 18:21:39 +0000[diff] [blame]94<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]95 <p>The System Library must shield LLVM from <em>all</em> system headers. To
96 obtain system level functionality, LLVM source must
97 <tt>#include "llvm/System/Thing.h"</tt> and nothing else. This means that
98 <tt>Thing.h</tt> cannot expose any system header files. This protects LLVM
99 from accidentally using system specific functionality and only allows it
100 via the <tt>lib/System</tt> interface.</p>
101</div>
102
103<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]104<h3><a name="c_headers">Use Standard C Headers</a></h3>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]105<div class="doc_text">
106 <p>The <em>standard</em> C headers (the ones beginning with "c") are allowed
107 to be exposed through the <tt>lib/System</tt> interface. These headers and
108 the things they declare are considered to be platform agnostic. LLVM source
109 files may include them directly or obtain their inclusion through
110 <tt>lib/System</tt> interfaces.</p>
111</div>
112
113<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]114<h3><a name="cpp_headers">Use Standard C++ Headers</a></h3>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]115<div class="doc_text">
116 <p>The <em>standard</em> C++ headers from the standard C++ library and
117 standard template library may be exposed through the <tt>lib/System</tt>
118 interface. These headers and the things they declare are considered to be
119 platform agnostic. LLVM source files may include them or obtain their
120 inclusion through lib/System interfaces.</p>
121</div>
122
123<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]124<h3><a name="highlev">High Level Interface</a></h3>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]125<div class="doc_text">
126 <p>The entry points specified in the interface of lib/System must be aimed at
127 completing some reasonably high level task needed by LLVM. We do not want to
128 simply wrap each operating system call. It would be preferable to wrap several
129 operating system calls that are always used in conjunction with one another by
130 LLVM.</p>
131 <p>For example, consider what is needed to execute a program, wait for it to
132 complete, and return its result code. On Unix, this involves the following
133 operating system calls: <tt>getenv, fork, execve,</tt> and <tt>wait</tt>. The
134 correct thing for lib/System to provide is a function, say
135 <tt>ExecuteProgramAndWait</tt>, that implements the functionality completely.
136 what we don't want is wrappers for the operating system calls involved.</p>
137 <p>There must <em>not</em> be a one-to-one relationship between operating
138 system calls and the System library's interface. Any such interface function
139 will be suspicious.</p>
140</div>
141
142<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]143<h3><a name="nounused">No Unused Functionality</a></h3>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]144<div class="doc_text">
145 <p>There must be no functionality specified in the interface of lib/System
146 that isn't actually used by LLVM. We're not writing a general purpose
147 operating system wrapper here, just enough to satisfy LLVM's needs. And, LLVM
148 doesn't need much. This design goal aims to keep the lib/System interface
149 small and understandable which should foster its actual use and adoption.</p>
150</div>
151
152<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]153<h3><a name="nodupl">No Duplicate Implementations</a></h3>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]154<div class="doc_text">
155 <p>The implementation of a function for a given platform must be written
156 exactly once. This implies that it must be possible to apply a function's
157 implementation to multiple operating systems if those operating systems can
158 share the same implementation. This rule applies to the set of operating
159 systems supported for a given class of operating system (e.g. Unix, Win32).
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]160 </p>
161</div>
162
163<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]164<h3><a name="virtuals">No Virtual Methods</a></h3>
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]165<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]166 <p>The System Library interfaces can be called quite frequently by LLVM. In
167 order to make those calls as efficient as possible, we discourage the use of
168 virtual methods. There is no need to use inheritance for implementation
169 differences, it just adds complexity. The <tt>#include</tt> mechanism works
170 just fine.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]171</div>
172
173<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]174<h3><a name="nofunc">No Exposed Functions</a></h3>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]175<div class="doc_text">
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]176 <p>Any functions defined by system libraries (i.e. not defined by lib/System)
177 must not be exposed through the lib/System interface, even if the header file
178 for that function is not exposed. This prevents inadvertent use of system
179 specific functionality.</p>
180 <p>For example, the <tt>stat</tt> system call is notorious for having
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]181 variations in the data it provides. <tt>lib/System</tt> must not declare
182 <tt>stat</tt> nor allow it to be declared. Instead it should provide its own
183 interface to discovering information about files and directories. Those
184 interfaces may be implemented in terms of <tt>stat</tt> but that is strictly
185 an implementation detail. The interface provided by the System Library must
186 be implemented on all platforms (even those without <tt>stat</tt>).</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]187</div>
188
189<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]190<h3><a name="nodata">No Exposed Data</a></h3>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]191<div class="doc_text">
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]192 <p>Any data defined by system libraries (i.e. not defined by lib/System) must
193 not be exposed through the lib/System interface, even if the header file for
194 that function is not exposed. As with functions, this prevents inadvertent use
195 of data that might not exist on all platforms.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]196</div>
197
198<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]199<h3><a name="softerrors">Minimize Soft Errors</a></h3>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]200<div class="doc_text">
201 <p>Operating system interfaces will generally provide error results for every
202 little thing that could go wrong. In almost all cases, you can divide these
203 error results into two groups: normal/good/soft and abnormal/bad/hard. That
204 is, some of the errors are simply information like "file not found",
205 "insufficient privileges", etc. while other errors are much harder like
206 "out of space", "bad disk sector", or "system call interrupted". We'll call
207 the first group "<i>soft</i>" errors and the second group "<i>hard</i>"
208 errors.<p>
Chris Lattner3ddd7172009-07-17 21:11:24 +0000[diff] [blame]209 <p>lib/System must always attempt to minimize soft errors.
210 This is a design requirement because the
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]211 minimization of soft errors can affect the granularity and the nature of the
212 interface. In general, if you find that you're wanting to throw soft errors,
213 you must review the granularity of the interface because it is likely you're
214 trying to implement something that is too low level. The rule of thumb is to
215 provide interface functions that <em>can't</em> fail, except when faced with
216 hard errors.</p>
217 <p>For a trivial example, suppose we wanted to add an "OpenFileForWriting"
218 function. For many operating systems, if the file doesn't exist, attempting
219 to open the file will produce an error. However, lib/System should not
220 simply throw that error if it occurs because its a soft error. The problem
221 is that the interface function, OpenFileForWriting is too low level. It should
222 be OpenOrCreateFileForWriting. In the case of the soft "doesn't exist" error,
223 this function would just create it and then open it for writing.</p>
224 <p>This design principle needs to be maintained in lib/System because it
225 avoids the propagation of soft error handling throughout the rest of LLVM.
226 Hard errors will generally just cause a termination for an LLVM tool so don't
227 be bashful about throwing them.</p>
228 <p>Rules of thumb:</p>
229 <ol>
230 <li>Don't throw soft errors, only hard errors.</li>
231 <li>If you're tempted to throw a soft error, re-think the interface.</li>
232 <li>Handle internally the most common normal/good/soft error conditions
233 so the rest of LLVM doesn't have to.</li>
234 </ol>
235</div>
236
237<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]238<h3><a name="throw_spec">No throw Specifications</a></h3>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]239<div class="doc_text">
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]240 <p>None of the lib/System interface functions may be declared with C++
241 <tt>throw()</tt> specifications on them. This requirement makes sure that the
John Criswell98d06362004-11-21 14:58:12 +0000[diff] [blame]242 compiler does not insert additional exception handling code into the interface
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]243 functions. This is a performance consideration: lib/System functions are at
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]244 the bottom of many call chains and as such can be frequently called. We
Chris Lattner3ddd7172009-07-17 21:11:24 +0000[diff] [blame]245 need them to be as efficient as possible. However, no routines in the
246 system library should actually throw exceptions.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]247</div>
248
249<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]250<h3><a name="organization">Code Organization</a></h3>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]251<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]252 <p>Implementations of the System Library interface are separated by their
253 general class of operating system. Currently only Unix and Win32 classes are
254 defined but more could be added for other operating system classifications.
255 To distinguish which implementation to compile, the code in lib/System uses
256 the LLVM_ON_UNIX and LLVM_ON_WIN32 #defines provided via configure through the
257 llvm/Config/config.h file. Each source file in lib/System, after implementing
258 the generic (operating system independent) functionality needs to include the
259 correct implementation using a set of <tt>#if defined(LLVM_ON_XYZ)</tt>
260 directives. For example, if we had lib/System/File.cpp, we'd expect to see in
261 that file:</p>
262 <pre><tt>
263 #if defined(LLVM_ON_UNIX)
264 #include "Unix/File.cpp"
265 #endif
266 #if defined(LLVM_ON_WIN32)
267 #include "Win32/File.cpp"
268 #endif
269 </tt></pre>
270 <p>The implementation in lib/System/Unix/File.cpp should handle all Unix
271 variants. The implementation in lib/System/Win32/File.cpp should handle all
272 Win32 variants. What this does is quickly differentiate the basic class of
273 operating system that will provide the implementation. The specific details
274 for a given platform must still be determined through the use of
275 <tt>#ifdef</tt>.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]276</div>
277
278<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]279<h3><a name="semantics">Consistent Semantics</a></h3>
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]280<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]281 <p>The implementation of a lib/System interface can vary drastically between
282 platforms. That's okay as long as the end result of the interface function
283 is the same. For example, a function to create a directory is pretty straight
284 forward on all operating system. System V IPC on the other hand isn't even
285 supported on all platforms. Instead of "supporting" System V IPC, lib/System
286 should provide an interface to the basic concept of inter-process
287 communications. The implementations might use System V IPC if that was
288 available or named pipes, or whatever gets the job done effectively for a
289 given operating system. In all cases, the interface and the implementation
290 must be semantically consistent. </p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]291</div>
292
293<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame^]294<h3><a name="bug">Bug 351</a></h3>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]295<div class="doc_text">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]296 <p>See <a href="http://llvm.org/PR351">bug 351</a>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]297 for further details on the progress of this work</p>
298</div>
299
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]300<!-- *********************************************************************** -->
301
302<hr>
303<address>
304 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]308
309 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]310 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]311 Last modified: $Date$
312</address>
313</body>
314</html>