1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 6 <title>System Library</title> 7 <link rel="stylesheet" href="llvm.css" type="text/css"> 8 </head> 9 <body> 10 11 <h1>System Library</h1> 12 <ul> 13 <li><a href="#abstract">Abstract</a></li> 14 <li><a href="#requirements">Keeping LLVM Portable</a> 15 <ol> 16 <li><a href="#headers">Don't Include System Headers</a></li> 17 <li><a href="#expose">Don't Expose System Headers</a></li> 18 <li><a href="#c_headers">Allow Standard C Header Files</a></li> 19 <li><a href="#cpp_headers">Allow Standard C++ Header Files</a></li> 20 <li><a href="#highlev">High-Level Interface</a></li> 21 <li><a href="#nofunc">No Exposed Functions</a></li> 22 <li><a href="#nodata">No Exposed Data</a></li> 23 <li><a href="#nodupl">No Duplicate Implementations</a></li> 24 <li><a href="#nounused">No Unused Functionality</a></li> 25 <li><a href="#virtuals">No Virtual Methods</a></li> 26 <li><a href="#softerrors">Minimize Soft Errors</a></li> 27 <li><a href="#throw_spec">No throw() Specifications</a></li> 28 <li><a href="#organization">Code Organization</a></li> 29 <li><a href="#semantics">Consistent Semantics</a></li> 30 <li><a href="#bug">Tracking Bugzilla Bug: 351</a></li> 31 </ol></li> 32 </ul> 33 34 <div class="doc_author"> 35 <p>Written by <a href="mailto:rspencer (a] x10sys.com">Reid Spencer</a></p> 36 </div> 37 38 39 <!-- *********************************************************************** --> 40 <h2><a name="abstract">Abstract</a></h2> 41 <div> 42 <p>This document provides some details on LLVM's System Library, located in 43 the source at <tt>lib/System</tt> and <tt>include/llvm/System</tt>. The 44 library's purpose is to shield LLVM from the differences between operating 45 systems for the few services LLVM needs from the operating system. Much of 46 LLVM is written using portability features of standard C++. However, in a few 47 areas, system dependent facilities are needed and the System Library is the 48 wrapper around those system calls.</p> 49 <p>By centralizing LLVM's use of operating system interfaces, we make it 50 possible for the LLVM tool chain and runtime libraries to be more easily 51 ported to new platforms since (theoretically) only <tt>lib/System</tt> needs 52 to be ported. This library also unclutters the rest of LLVM from #ifdef use 53 and special cases for specific operating systems. Such uses are replaced 54 with simple calls to the interfaces provided in <tt>include/llvm/System</tt>. 55 </p> 56 <p>Note that the System Library is not intended to be a complete operating 57 system wrapper (such as the Adaptive Communications Environment (ACE) or 58 Apache Portable Runtime (APR)), but only provides the functionality necessary 59 to support LLVM. 60 <p>The System Library was written by Reid Spencer who formulated the 61 design based on similar work originating from the eXtensible Programming 62 System (XPS). Several people helped with the effort; especially, 63 Jeff Cohen and Henrik Bach on the Win32 port.</p> 64 </div> 65 66 <!-- *********************************************************************** --> 67 <h2> 68 <a name="requirements">Keeping LLVM Portable</a> 69 </h2> 70 <div> 71 <p>In order to keep LLVM portable, LLVM developers should adhere to a set of 72 portability rules associated with the System Library. Adherence to these rules 73 should help the System Library achieve its goal of shielding LLVM from the 74 variations in operating system interfaces and doing so efficiently. The 75 following sections define the rules needed to fulfill this objective.</p> 76 77 <!-- ======================================================================= --> 78 <h3><a name="headers">Don't Include System Headers</a></h3> 79 <div> 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 <!-- ======================================================================= --> 93 <h3><a name="expose">Don't Expose System Headers</a></h3> 94 <div> 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 <!-- ======================================================================= --> 104 <h3><a name="c_headers">Use Standard C Headers</a></h3> 105 <div> 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 <!-- ======================================================================= --> 114 <h3><a name="cpp_headers">Use Standard C++ Headers</a></h3> 115 <div> 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 <!-- ======================================================================= --> 124 <h3><a name="highlev">High Level Interface</a></h3> 125 <div> 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 <!-- ======================================================================= --> 143 <h3><a name="nounused">No Unused Functionality</a></h3> 144 <div> 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 <!-- ======================================================================= --> 153 <h3><a name="nodupl">No Duplicate Implementations</a></h3> 154 <div> 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). 160 </p> 161 </div> 162 163 <!-- ======================================================================= --> 164 <h3><a name="virtuals">No Virtual Methods</a></h3> 165 <div> 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> 171 </div> 172 173 <!-- ======================================================================= --> 174 <h3><a name="nofunc">No Exposed Functions</a></h3> 175 <div> 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 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> 187 </div> 188 189 <!-- ======================================================================= --> 190 <h3><a name="nodata">No Exposed Data</a></h3> 191 <div> 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> 196 </div> 197 198 <!-- ======================================================================= --> 199 <h3><a name="softerrors">Minimize Soft Errors</a></h3> 200 <div> 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> 209 <p>lib/System must always attempt to minimize soft errors. 210 This is a design requirement because the 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 <!-- ======================================================================= --> 238 <h3><a name="throw_spec">No throw Specifications</a></h3> 239 <div> 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 242 compiler does not insert additional exception handling code into the interface 243 functions. This is a performance consideration: lib/System functions are at 244 the bottom of many call chains and as such can be frequently called. We 245 need them to be as efficient as possible. However, no routines in the 246 system library should actually throw exceptions.</p> 247 </div> 248 249 <!-- ======================================================================= --> 250 <h3><a name="organization">Code Organization</a></h3> 251 <div> 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> 276 </div> 277 278 <!-- ======================================================================= --> 279 <h3><a name="semantics">Consistent Semantics</a></h3> 280 <div> 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> 291 </div> 292 293 <!-- ======================================================================= --> 294 <h3><a name="bug">Bug 351</a></h3> 295 <div> 296 <p>See <a href="http://llvm.org/PR351">bug 351</a> 297 for further details on the progress of this work</p> 298 </div> 299 300 </div> 301 302 <!-- *********************************************************************** --> 303 304 <hr> 305 <address> 306 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 307 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 308 <a href="http://validator.w3.org/check/referer"><img 309 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 310 311 <a href="mailto:rspencer (a] x10sys.com">Reid Spencer</a><br> 312 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 313 Last modified: $Date$ 314 </address> 315 </body> 316 </html> 317
