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9
Misha Brukmanebbbf252004-07-21 18:02:43 +0000[diff] [blame]10<div class="doc_title">System Library</div>
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<!-- *********************************************************************** -->
39<div class="doc_section"><a name="abstract">Abstract</a></div>
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<!-- *********************************************************************** -->
66<div class="doc_section">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]67 <a name="requirements">Keeping LLVM Portable</a>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]68</div>
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<!-- ======================================================================= -->
Daniel Dunbar445c89a2009-07-17 16:41:57 +0000[diff] [blame]78<div class="doc_subsection"><a name="headers">Don't Include System Headers</a>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]79</div>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]80<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]81 <p>Except in <tt>lib/System</tt>, no LLVM source code should directly
82 <tt>#include</tt> a system header. Care has been taken to remove all such
83 <tt>#includes</tt> from LLVM while <tt>lib/System</tt> was being
84 developed. Specifically this means that header files like "unistd.h",
85 "windows.h", "stdio.h", and "string.h" are forbidden to be included by LLVM
86 source code outside the implementation of <tt>lib/System</tt>.</p>
87 <p>To obtain system-dependent functionality, existing interfaces to the system
88 found in <tt>include/llvm/System</tt> should be used. If an appropriate
89 interface is not available, it should be added to <tt>include/llvm/System</tt>
90 and implemented in <tt>lib/System</tt> for all supported platforms.</p>
91</div>
92
93<!-- ======================================================================= -->
94<div class="doc_subsection"><a name="expose">Don't Expose System Headers</a>
95</div>
Reid Spencerd593afc2005-01-05 18:21:39 +0000[diff] [blame]96<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]97 <p>The System Library must shield LLVM from <em>all</em> system headers. To
98 obtain system level functionality, LLVM source must
99 <tt>#include "llvm/System/Thing.h"</tt> and nothing else. This means that
100 <tt>Thing.h</tt> cannot expose any system header files. This protects LLVM
101 from accidentally using system specific functionality and only allows it
102 via the <tt>lib/System</tt> interface.</p>
103</div>
104
105<!-- ======================================================================= -->
Reid Spencerd593afc2005-01-05 18:21:39 +0000[diff] [blame]106<div class="doc_subsection"><a name="c_headers">Use Standard C Headers</a></div>
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]107<div class="doc_text">
108 <p>The <em>standard</em> C headers (the ones beginning with "c") are allowed
109 to be exposed through the <tt>lib/System</tt> interface. These headers and
110 the things they declare are considered to be platform agnostic. LLVM source
111 files may include them directly or obtain their inclusion through
112 <tt>lib/System</tt> interfaces.</p>
113</div>
114
115<!-- ======================================================================= -->
116<div class="doc_subsection"><a name="cpp_headers">Use Standard C++ Headers</a>
117</div>
118<div class="doc_text">
119 <p>The <em>standard</em> C++ headers from the standard C++ library and
120 standard template library may be exposed through the <tt>lib/System</tt>
121 interface. These headers and the things they declare are considered to be
122 platform agnostic. LLVM source files may include them or obtain their
123 inclusion through lib/System interfaces.</p>
124</div>
125
126<!-- ======================================================================= -->
127<div class="doc_subsection"><a name="highlev">High Level Interface</a></div>
128<div class="doc_text">
129 <p>The entry points specified in the interface of lib/System must be aimed at
130 completing some reasonably high level task needed by LLVM. We do not want to
131 simply wrap each operating system call. It would be preferable to wrap several
132 operating system calls that are always used in conjunction with one another by
133 LLVM.</p>
134 <p>For example, consider what is needed to execute a program, wait for it to
135 complete, and return its result code. On Unix, this involves the following
136 operating system calls: <tt>getenv, fork, execve,</tt> and <tt>wait</tt>. The
137 correct thing for lib/System to provide is a function, say
138 <tt>ExecuteProgramAndWait</tt>, that implements the functionality completely.
139 what we don't want is wrappers for the operating system calls involved.</p>
140 <p>There must <em>not</em> be a one-to-one relationship between operating
141 system calls and the System library's interface. Any such interface function
142 will be suspicious.</p>
143</div>
144
145<!-- ======================================================================= -->
146<div class="doc_subsection"><a name="nounused">No Unused Functionality</a></div>
147<div class="doc_text">
148 <p>There must be no functionality specified in the interface of lib/System
149 that isn't actually used by LLVM. We're not writing a general purpose
150 operating system wrapper here, just enough to satisfy LLVM's needs. And, LLVM
151 doesn't need much. This design goal aims to keep the lib/System interface
152 small and understandable which should foster its actual use and adoption.</p>
153</div>
154
155<!-- ======================================================================= -->
156<div class="doc_subsection"><a name="nodupl">No Duplicate Implementations</a>
157</div>
158<div class="doc_text">
159 <p>The implementation of a function for a given platform must be written
160 exactly once. This implies that it must be possible to apply a function's
161 implementation to multiple operating systems if those operating systems can
162 share the same implementation. This rule applies to the set of operating
163 systems supported for a given class of operating system (e.g. Unix, Win32).
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]164 </p>
165</div>
166
167<!-- ======================================================================= -->
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]168<div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]169<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]170 <p>The System Library interfaces can be called quite frequently by LLVM. In
171 order to make those calls as efficient as possible, we discourage the use of
172 virtual methods. There is no need to use inheritance for implementation
173 differences, it just adds complexity. The <tt>#include</tt> mechanism works
174 just fine.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]175</div>
176
177<!-- ======================================================================= -->
178<div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
179<div class="doc_text">
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]180 <p>Any functions defined by system libraries (i.e. not defined by lib/System)
181 must not be exposed through the lib/System interface, even if the header file
182 for that function is not exposed. This prevents inadvertent use of system
183 specific functionality.</p>
184 <p>For example, the <tt>stat</tt> system call is notorious for having
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]185 variations in the data it provides. <tt>lib/System</tt> must not declare
186 <tt>stat</tt> nor allow it to be declared. Instead it should provide its own
187 interface to discovering information about files and directories. Those
188 interfaces may be implemented in terms of <tt>stat</tt> but that is strictly
189 an implementation detail. The interface provided by the System Library must
190 be implemented on all platforms (even those without <tt>stat</tt>).</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]191</div>
192
193<!-- ======================================================================= -->
194<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
195<div class="doc_text">
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]196 <p>Any data defined by system libraries (i.e. not defined by lib/System) must
197 not be exposed through the lib/System interface, even if the header file for
198 that function is not exposed. As with functions, this prevents inadvertent use
199 of data that might not exist on all platforms.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]200</div>
201
202<!-- ======================================================================= -->
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]203<div class="doc_subsection"><a name="softerrors">Minimize Soft Errors</a></div>
204<div class="doc_text">
205 <p>Operating system interfaces will generally provide error results for every
206 little thing that could go wrong. In almost all cases, you can divide these
207 error results into two groups: normal/good/soft and abnormal/bad/hard. That
208 is, some of the errors are simply information like "file not found",
209 "insufficient privileges", etc. while other errors are much harder like
210 "out of space", "bad disk sector", or "system call interrupted". We'll call
211 the first group "<i>soft</i>" errors and the second group "<i>hard</i>"
212 errors.<p>
Chris Lattner3ddd7172009-07-17 21:11:24 +0000[diff] [blame]213 <p>lib/System must always attempt to minimize soft errors.
214 This is a design requirement because the
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]215 minimization of soft errors can affect the granularity and the nature of the
216 interface. In general, if you find that you're wanting to throw soft errors,
217 you must review the granularity of the interface because it is likely you're
218 trying to implement something that is too low level. The rule of thumb is to
219 provide interface functions that <em>can't</em> fail, except when faced with
220 hard errors.</p>
221 <p>For a trivial example, suppose we wanted to add an "OpenFileForWriting"
222 function. For many operating systems, if the file doesn't exist, attempting
223 to open the file will produce an error. However, lib/System should not
224 simply throw that error if it occurs because its a soft error. The problem
225 is that the interface function, OpenFileForWriting is too low level. It should
226 be OpenOrCreateFileForWriting. In the case of the soft "doesn't exist" error,
227 this function would just create it and then open it for writing.</p>
228 <p>This design principle needs to be maintained in lib/System because it
229 avoids the propagation of soft error handling throughout the rest of LLVM.
230 Hard errors will generally just cause a termination for an LLVM tool so don't
231 be bashful about throwing them.</p>
232 <p>Rules of thumb:</p>
233 <ol>
234 <li>Don't throw soft errors, only hard errors.</li>
235 <li>If you're tempted to throw a soft error, re-think the interface.</li>
236 <li>Handle internally the most common normal/good/soft error conditions
237 so the rest of LLVM doesn't have to.</li>
238 </ol>
239</div>
240
241<!-- ======================================================================= -->
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]242<div class="doc_subsection"><a name="throw_spec">No throw Specifications</a>
243</div>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]244<div class="doc_text">
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]245 <p>None of the lib/System interface functions may be declared with C++
246 <tt>throw()</tt> specifications on them. This requirement makes sure that the
John Criswell98d06362004-11-21 14:58:12 +0000[diff] [blame]247 compiler does not insert additional exception handling code into the interface
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]248 functions. This is a performance consideration: lib/System functions are at
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]249 the bottom of many call chains and as such can be frequently called. We
Chris Lattner3ddd7172009-07-17 21:11:24 +0000[diff] [blame]250 need them to be as efficient as possible. However, no routines in the
251 system library should actually throw exceptions.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]252</div>
253
254<!-- ======================================================================= -->
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]255<div class="doc_subsection"><a name="organization">Code Organization</a></div>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]256<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]257 <p>Implementations of the System Library interface are separated by their
258 general class of operating system. Currently only Unix and Win32 classes are
259 defined but more could be added for other operating system classifications.
260 To distinguish which implementation to compile, the code in lib/System uses
261 the LLVM_ON_UNIX and LLVM_ON_WIN32 #defines provided via configure through the
262 llvm/Config/config.h file. Each source file in lib/System, after implementing
263 the generic (operating system independent) functionality needs to include the
264 correct implementation using a set of <tt>#if defined(LLVM_ON_XYZ)</tt>
265 directives. For example, if we had lib/System/File.cpp, we'd expect to see in
266 that file:</p>
267 <pre><tt>
268 #if defined(LLVM_ON_UNIX)
269 #include "Unix/File.cpp"
270 #endif
271 #if defined(LLVM_ON_WIN32)
272 #include "Win32/File.cpp"
273 #endif
274 </tt></pre>
275 <p>The implementation in lib/System/Unix/File.cpp should handle all Unix
276 variants. The implementation in lib/System/Win32/File.cpp should handle all
277 Win32 variants. What this does is quickly differentiate the basic class of
278 operating system that will provide the implementation. The specific details
279 for a given platform must still be determined through the use of
280 <tt>#ifdef</tt>.</p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]281</div>
282
283<!-- ======================================================================= -->
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]284<div class="doc_subsection"><a name="semantics">Consistent Semantics</a></div>
Reid Spencer7acb8662004-08-27 02:08:04 +0000[diff] [blame]285<div class="doc_text">
Reid Spencer0c004852005-01-05 18:17:10 +0000[diff] [blame]286 <p>The implementation of a lib/System interface can vary drastically between
287 platforms. That's okay as long as the end result of the interface function
288 is the same. For example, a function to create a directory is pretty straight
289 forward on all operating system. System V IPC on the other hand isn't even
290 supported on all platforms. Instead of "supporting" System V IPC, lib/System
291 should provide an interface to the basic concept of inter-process
292 communications. The implementations might use System V IPC if that was
293 available or named pipes, or whatever gets the job done effectively for a
294 given operating system. In all cases, the interface and the implementation
295 must be semantically consistent. </p>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]296</div>
297
298<!-- ======================================================================= -->
Chris Lattner05540a72004-07-17 18:50:19 +0000[diff] [blame]299<div class="doc_subsection"><a name="bug">Bug 351</a></div>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]300<div class="doc_text">
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]301 <p>See <a href="http://llvm.org/PR351">bug 351</a>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]302 for further details on the progress of this work</p>
303</div>
304
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]305<!-- *********************************************************************** -->
306
307<hr>
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Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]313
314 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]315 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Reid Spencer4f6015a2004-07-17 10:04:49 +0000[diff] [blame]316 Last modified: $Date$
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