1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3<html> 4<head> 5 <title>System Library</title> 6 <link rel="stylesheet" href="llvm.css" type="text/css"> 7</head> 8<body> 9 10<h1>System Library</h1> 11<ul> 12 <li><a href="#abstract">Abstract</a></li> 13 <li><a href="#requirements">Keeping LLVM Portable</a> 14 <ol> 15 <li><a href="#headers">Don't Include System Headers</a></li> 16 <li><a href="#expose">Don't Expose System Headers</a></li> 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> 19 <li><a href="#highlev">High-Level Interface</a></li> 20 <li><a href="#nofunc">No Exposed Functions</a></li> 21 <li><a href="#nodata">No Exposed Data</a></li> 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> 26 <li><a href="#throw_spec">No throw() Specifications</a></li> 27 <li><a href="#organization">Code Organization</a></li> 28 <li><a href="#semantics">Consistent Semantics</a></li> 29 <li><a href="#bug">Tracking Bugzilla Bug: 351</a></li> 30 </ol></li> 31</ul> 32 33<div class="doc_author"> 34 <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p> 35</div> 36 37 38<!-- *********************************************************************** --> 39<h2><a name="abstract">Abstract</a></h2> 40<div> 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. 59 <p>The System Library was written by Reid Spencer who formulated the 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> 63</div> 64 65<!-- *********************************************************************** --> 66<h2> 67 <a name="requirements">Keeping LLVM Portable</a> 68</h2> 69<div> 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> 75 76<!-- ======================================================================= --> 77<h3><a name="headers">Don't Include System Headers</a></h3> 78<div> 79 <p>Except in <tt>lib/System</tt>, no LLVM source code should directly 80 <tt>#include</tt> a system header. Care has been taken to remove all such 81 <tt>#includes</tt> from LLVM while <tt>lib/System</tt> was being 82 developed. Specifically this means that header files like "unistd.h", 83 "windows.h", "stdio.h", and "string.h" are forbidden to be included by LLVM 84 source code outside the implementation of <tt>lib/System</tt>.</p> 85 <p>To obtain system-dependent functionality, existing interfaces to the system 86 found in <tt>include/llvm/System</tt> should be used. If an appropriate 87 interface is not available, it should be added to <tt>include/llvm/System</tt> 88 and implemented in <tt>lib/System</tt> for all supported platforms.</p> 89</div> 90 91<!-- ======================================================================= --> 92<h3><a name="expose">Don't Expose System Headers</a></h3> 93<div> 94 <p>The System Library must shield LLVM from <em>all</em> system headers. To 95 obtain system level functionality, LLVM source must 96 <tt>#include "llvm/System/Thing.h"</tt> and nothing else. This means that 97 <tt>Thing.h</tt> cannot expose any system header files. This protects LLVM 98 from accidentally using system specific functionality and only allows it 99 via the <tt>lib/System</tt> interface.</p> 100</div> 101 102<!-- ======================================================================= --> 103<h3><a name="c_headers">Use Standard C Headers</a></h3> 104<div> 105 <p>The <em>standard</em> C headers (the ones beginning with "c") are allowed 106 to be exposed through the <tt>lib/System</tt> interface. These headers and 107 the things they declare are considered to be platform agnostic. LLVM source 108 files may include them directly or obtain their inclusion through 109 <tt>lib/System</tt> interfaces.</p> 110</div> 111 112<!-- ======================================================================= --> 113<h3><a name="cpp_headers">Use Standard C++ Headers</a></h3> 114<div> 115 <p>The <em>standard</em> C++ headers from the standard C++ library and 116 standard template library may be exposed through the <tt>lib/System</tt> 117 interface. These headers and the things they declare are considered to be 118 platform agnostic. LLVM source files may include them or obtain their 119 inclusion through lib/System interfaces.</p> 120</div> 121 122<!-- ======================================================================= --> 123<h3><a name="highlev">High Level Interface</a></h3> 124<div> 125 <p>The entry points specified in the interface of lib/System must be aimed at 126 completing some reasonably high level task needed by LLVM. We do not want to 127 simply wrap each operating system call. It would be preferable to wrap several 128 operating system calls that are always used in conjunction with one another by 129 LLVM.</p> 130 <p>For example, consider what is needed to execute a program, wait for it to 131 complete, and return its result code. On Unix, this involves the following 132 operating system calls: <tt>getenv, fork, execve,</tt> and <tt>wait</tt>. The 133 correct thing for lib/System to provide is a function, say 134 <tt>ExecuteProgramAndWait</tt>, that implements the functionality completely. 135 what we don't want is wrappers for the operating system calls involved.</p> 136 <p>There must <em>not</em> be a one-to-one relationship between operating 137 system calls and the System library's interface. Any such interface function 138 will be suspicious.</p> 139</div> 140 141<!-- ======================================================================= --> 142<h3><a name="nounused">No Unused Functionality</a></h3> 143<div> 144 <p>There must be no functionality specified in the interface of lib/System 145 that isn't actually used by LLVM. We're not writing a general purpose 146 operating system wrapper here, just enough to satisfy LLVM's needs. And, LLVM 147 doesn't need much. This design goal aims to keep the lib/System interface 148 small and understandable which should foster its actual use and adoption.</p> 149</div> 150 151<!-- ======================================================================= --> 152<h3><a name="nodupl">No Duplicate Implementations</a></h3> 153<div> 154 <p>The implementation of a function for a given platform must be written 155 exactly once. This implies that it must be possible to apply a function's 156 implementation to multiple operating systems if those operating systems can 157 share the same implementation. This rule applies to the set of operating 158 systems supported for a given class of operating system (e.g. Unix, Win32). 159 </p> 160</div> 161 162<!-- ======================================================================= --> 163<h3><a name="virtuals">No Virtual Methods</a></h3> 164<div> 165 <p>The System Library interfaces can be called quite frequently by LLVM. In 166 order to make those calls as efficient as possible, we discourage the use of 167 virtual methods. There is no need to use inheritance for implementation 168 differences, it just adds complexity. The <tt>#include</tt> mechanism works 169 just fine.</p> 170</div> 171 172<!-- ======================================================================= --> 173<h3><a name="nofunc">No Exposed Functions</a></h3> 174<div> 175 <p>Any functions defined by system libraries (i.e. not defined by lib/System) 176 must not be exposed through the lib/System interface, even if the header file 177 for that function is not exposed. This prevents inadvertent use of system 178 specific functionality.</p> 179 <p>For example, the <tt>stat</tt> system call is notorious for having 180 variations in the data it provides. <tt>lib/System</tt> must not declare 181 <tt>stat</tt> nor allow it to be declared. Instead it should provide its own 182 interface to discovering information about files and directories. Those 183 interfaces may be implemented in terms of <tt>stat</tt> but that is strictly 184 an implementation detail. The interface provided by the System Library must 185 be implemented on all platforms (even those without <tt>stat</tt>).</p> 186</div> 187 188<!-- ======================================================================= --> 189<h3><a name="nodata">No Exposed Data</a></h3> 190<div> 191 <p>Any data defined by system libraries (i.e. not defined by lib/System) must 192 not be exposed through the lib/System interface, even if the header file for 193 that function is not exposed. As with functions, this prevents inadvertent use 194 of data that might not exist on all platforms.</p> 195</div> 196 197<!-- ======================================================================= --> 198<h3><a name="softerrors">Minimize Soft Errors</a></h3> 199<div> 200 <p>Operating system interfaces will generally provide error results for every 201 little thing that could go wrong. In almost all cases, you can divide these 202 error results into two groups: normal/good/soft and abnormal/bad/hard. That 203 is, some of the errors are simply information like "file not found", 204 "insufficient privileges", etc. while other errors are much harder like 205 "out of space", "bad disk sector", or "system call interrupted". We'll call 206 the first group "<i>soft</i>" errors and the second group "<i>hard</i>" 207 errors.<p> 208 <p>lib/System must always attempt to minimize soft errors. 209 This is a design requirement because the 210 minimization of soft errors can affect the granularity and the nature of the 211 interface. In general, if you find that you're wanting to throw soft errors, 212 you must review the granularity of the interface because it is likely you're 213 trying to implement something that is too low level. The rule of thumb is to 214 provide interface functions that <em>can't</em> fail, except when faced with 215 hard errors.</p> 216 <p>For a trivial example, suppose we wanted to add an "OpenFileForWriting" 217 function. For many operating systems, if the file doesn't exist, attempting 218 to open the file will produce an error. However, lib/System should not 219 simply throw that error if it occurs because its a soft error. The problem 220 is that the interface function, OpenFileForWriting is too low level. It should 221 be OpenOrCreateFileForWriting. In the case of the soft "doesn't exist" error, 222 this function would just create it and then open it for writing.</p> 223 <p>This design principle needs to be maintained in lib/System because it 224 avoids the propagation of soft error handling throughout the rest of LLVM. 225 Hard errors will generally just cause a termination for an LLVM tool so don't 226 be bashful about throwing them.</p> 227 <p>Rules of thumb:</p> 228 <ol> 229 <li>Don't throw soft errors, only hard errors.</li> 230 <li>If you're tempted to throw a soft error, re-think the interface.</li> 231 <li>Handle internally the most common normal/good/soft error conditions 232 so the rest of LLVM doesn't have to.</li> 233 </ol> 234</div> 235 236<!-- ======================================================================= --> 237<h3><a name="throw_spec">No throw Specifications</a></h3> 238<div> 239 <p>None of the lib/System interface functions may be declared with C++ 240 <tt>throw()</tt> specifications on them. This requirement makes sure that the 241 compiler does not insert additional exception handling code into the interface 242 functions. This is a performance consideration: lib/System functions are at 243 the bottom of many call chains and as such can be frequently called. We 244 need them to be as efficient as possible. However, no routines in the 245 system library should actually throw exceptions.</p> 246</div> 247 248<!-- ======================================================================= --> 249<h3><a name="organization">Code Organization</a></h3> 250<div> 251 <p>Implementations of the System Library interface are separated by their 252 general class of operating system. Currently only Unix and Win32 classes are 253 defined but more could be added for other operating system classifications. 254 To distinguish which implementation to compile, the code in lib/System uses 255 the LLVM_ON_UNIX and LLVM_ON_WIN32 #defines provided via configure through the 256 llvm/Config/config.h file. Each source file in lib/System, after implementing 257 the generic (operating system independent) functionality needs to include the 258 correct implementation using a set of <tt>#if defined(LLVM_ON_XYZ)</tt> 259 directives. For example, if we had lib/System/File.cpp, we'd expect to see in 260 that file:</p> 261 <pre><tt> 262 #if defined(LLVM_ON_UNIX) 263 #include "Unix/File.cpp" 264 #endif 265 #if defined(LLVM_ON_WIN32) 266 #include "Win32/File.cpp" 267 #endif 268 </tt></pre> 269 <p>The implementation in lib/System/Unix/File.cpp should handle all Unix 270 variants. The implementation in lib/System/Win32/File.cpp should handle all 271 Win32 variants. What this does is quickly differentiate the basic class of 272 operating system that will provide the implementation. The specific details 273 for a given platform must still be determined through the use of 274 <tt>#ifdef</tt>.</p> 275</div> 276 277<!-- ======================================================================= --> 278<h3><a name="semantics">Consistent Semantics</a></h3> 279<div> 280 <p>The implementation of a lib/System interface can vary drastically between 281 platforms. That's okay as long as the end result of the interface function 282 is the same. For example, a function to create a directory is pretty straight 283 forward on all operating system. System V IPC on the other hand isn't even 284 supported on all platforms. Instead of "supporting" System V IPC, lib/System 285 should provide an interface to the basic concept of inter-process 286 communications. The implementations might use System V IPC if that was 287 available or named pipes, or whatever gets the job done effectively for a 288 given operating system. In all cases, the interface and the implementation 289 must be semantically consistent. </p> 290</div> 291 292<!-- ======================================================================= --> 293<h3><a name="bug">Bug 351</a></h3> 294<div> 295 <p>See <a href="http://llvm.org/PR351">bug 351</a> 296 for further details on the progress of this work</p> 297</div> 298 299</div> 300 301<!-- *********************************************************************** --> 302 303<hr> 304<address> 305 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 306 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 307 <a href="http://validator.w3.org/check/referer"><img 308 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 309 310 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br> 311 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 312 Last modified: $Date: 2011-04-22 20:30:22 -0400 (Fri, 22 Apr 2011) $ 313</address> 314</body> 315</html> 316