1=============== 2Support Library 3=============== 4 5Abstract 6======== 7 8This document provides some details on LLVM's Support Library, located in the 9source at ``lib/Support`` and ``include/llvm/Support``. The library's purpose 10is to shield LLVM from the differences between operating systems for the few 11services LLVM needs from the operating system. Much of LLVM is written using 12portability features of standard C++. However, in a few areas, system dependent 13facilities are needed and the Support Library is the wrapper around those 14system calls. 15 16By centralizing LLVM's use of operating system interfaces, we make it possible 17for the LLVM tool chain and runtime libraries to be more easily ported to new 18platforms since (theoretically) only ``lib/Support`` needs to be ported. This 19library also unclutters the rest of LLVM from #ifdef use and special cases for 20specific operating systems. Such uses are replaced with simple calls to the 21interfaces provided in ``include/llvm/Support``. 22 23Note that the Support Library is not intended to be a complete operating system 24wrapper (such as the Adaptive Communications Environment (ACE) or Apache 25Portable Runtime (APR)), but only provides the functionality necessary to 26support LLVM. 27 28The Support Library was originally referred to as the System Library, written 29by Reid Spencer who formulated the design based on similar work originating 30from the eXtensible Programming System (XPS). Several people helped with the 31effort; especially, Jeff Cohen and Henrik Bach on the Win32 port. 32 33Keeping LLVM Portable 34===================== 35 36In order to keep LLVM portable, LLVM developers should adhere to a set of 37portability rules associated with the Support Library. Adherence to these rules 38should help the Support Library achieve its goal of shielding LLVM from the 39variations in operating system interfaces and doing so efficiently. The 40following sections define the rules needed to fulfill this objective. 41 42Don't Include System Headers 43---------------------------- 44 45Except in ``lib/Support``, no LLVM source code should directly ``#include`` a 46system header. Care has been taken to remove all such ``#includes`` from LLVM 47while ``lib/Support`` was being developed. Specifically this means that header 48files like "``unistd.h``", "``windows.h``", "``stdio.h``", and "``string.h``" 49are forbidden to be included by LLVM source code outside the implementation of 50``lib/Support``. 51 52To obtain system-dependent functionality, existing interfaces to the system 53found in ``include/llvm/Support`` should be used. If an appropriate interface is 54not available, it should be added to ``include/llvm/Support`` and implemented in 55``lib/Support`` for all supported platforms. 56 57Don't Expose System Headers 58--------------------------- 59 60The Support Library must shield LLVM from **all** system headers. To obtain 61system level functionality, LLVM source must 62``#include "llvm/Support/Thing.h"`` and nothing else. This means that 63``Thing.h`` cannot expose any system header files. This protects LLVM from 64accidentally using system specific functionality and only allows it via 65the ``lib/Support`` interface. 66 67Use Standard C Headers 68---------------------- 69 70The **standard** C headers (the ones beginning with "c") are allowed to be 71exposed through the ``lib/Support`` interface. These headers and the things they 72declare are considered to be platform agnostic. LLVM source files may include 73them directly or obtain their inclusion through ``lib/Support`` interfaces. 74 75Use Standard C++ Headers 76------------------------ 77 78The **standard** C++ headers from the standard C++ library and standard 79template library may be exposed through the ``lib/Support`` interface. These 80headers and the things they declare are considered to be platform agnostic. 81LLVM source files may include them or obtain their inclusion through 82``lib/Support`` interfaces. 83 84High Level Interface 85-------------------- 86 87The entry points specified in the interface of ``lib/Support`` must be aimed at 88completing some reasonably high level task needed by LLVM. We do not want to 89simply wrap each operating system call. It would be preferable to wrap several 90operating system calls that are always used in conjunction with one another by 91LLVM. 92 93For example, consider what is needed to execute a program, wait for it to 94complete, and return its result code. On Unix, this involves the following 95operating system calls: ``getenv``, ``fork``, ``execve``, and ``wait``. The 96correct thing for ``lib/Support`` to provide is a function, say 97``ExecuteProgramAndWait``, that implements the functionality completely. what 98we don't want is wrappers for the operating system calls involved. 99 100There must **not** be a one-to-one relationship between operating system 101calls and the Support library's interface. Any such interface function will be 102suspicious. 103 104No Unused Functionality 105----------------------- 106 107There must be no functionality specified in the interface of ``lib/Support`` 108that isn't actually used by LLVM. We're not writing a general purpose operating 109system wrapper here, just enough to satisfy LLVM's needs. And, LLVM doesn't 110need much. This design goal aims to keep the ``lib/Support`` interface small and 111understandable which should foster its actual use and adoption. 112 113No Duplicate Implementations 114---------------------------- 115 116The implementation of a function for a given platform must be written exactly 117once. This implies that it must be possible to apply a function's 118implementation to multiple operating systems if those operating systems can 119share the same implementation. This rule applies to the set of operating 120systems supported for a given class of operating system (e.g. Unix, Win32). 121 122No Virtual Methods 123------------------ 124 125The Support Library interfaces can be called quite frequently by LLVM. In order 126to make those calls as efficient as possible, we discourage the use of virtual 127methods. There is no need to use inheritance for implementation differences, it 128just adds complexity. The ``#include`` mechanism works just fine. 129 130No Exposed Functions 131-------------------- 132 133Any functions defined by system libraries (i.e. not defined by ``lib/Support``) 134must not be exposed through the ``lib/Support`` interface, even if the header 135file for that function is not exposed. This prevents inadvertent use of system 136specific functionality. 137 138For example, the ``stat`` system call is notorious for having variations in the 139data it provides. ``lib/Support`` must not declare ``stat`` nor allow it to be 140declared. Instead it should provide its own interface to discovering 141information about files and directories. Those interfaces may be implemented in 142terms of ``stat`` but that is strictly an implementation detail. The interface 143provided by the Support Library must be implemented on all platforms (even 144those without ``stat``). 145 146No Exposed Data 147--------------- 148 149Any data defined by system libraries (i.e. not defined by ``lib/Support``) must 150not be exposed through the ``lib/Support`` interface, even if the header file 151for that function is not exposed. As with functions, this prevents inadvertent 152use of data that might not exist on all platforms. 153 154Minimize Soft Errors 155-------------------- 156 157Operating system interfaces will generally provide error results for every 158little thing that could go wrong. In almost all cases, you can divide these 159error results into two groups: normal/good/soft and abnormal/bad/hard. That is, 160some of the errors are simply information like "file not found", "insufficient 161privileges", etc. while other errors are much harder like "out of space", "bad 162disk sector", or "system call interrupted". We'll call the first group "*soft*" 163errors and the second group "*hard*" errors. 164 165``lib/Support`` must always attempt to minimize soft errors. This is a design 166requirement because the minimization of soft errors can affect the granularity 167and the nature of the interface. In general, if you find that you're wanting to 168throw soft errors, you must review the granularity of the interface because it 169is likely you're trying to implement something that is too low level. The rule 170of thumb is to provide interface functions that **can't** fail, except when 171faced with hard errors. 172 173For a trivial example, suppose we wanted to add an "``OpenFileForWriting``" 174function. For many operating systems, if the file doesn't exist, attempting to 175open the file will produce an error. However, ``lib/Support`` should not simply 176throw that error if it occurs because its a soft error. The problem is that the 177interface function, ``OpenFileForWriting`` is too low level. It should be 178``OpenOrCreateFileForWriting``. In the case of the soft "doesn't exist" error, 179this function would just create it and then open it for writing. 180 181This design principle needs to be maintained in ``lib/Support`` because it 182avoids the propagation of soft error handling throughout the rest of LLVM. 183Hard errors will generally just cause a termination for an LLVM tool so don't 184be bashful about throwing them. 185 186Rules of thumb: 187 188#. Don't throw soft errors, only hard errors. 189 190#. If you're tempted to throw a soft error, re-think the interface. 191 192#. Handle internally the most common normal/good/soft error conditions 193 so the rest of LLVM doesn't have to. 194 195No throw Specifications 196----------------------- 197 198None of the ``lib/Support`` interface functions may be declared with C++ 199``throw()`` specifications on them. This requirement makes sure that the 200compiler does not insert additional exception handling code into the interface 201functions. This is a performance consideration: ``lib/Support`` functions are 202at the bottom of many call chains and as such can be frequently called. We 203need them to be as efficient as possible. However, no routines in the system 204library should actually throw exceptions. 205 206Code Organization 207----------------- 208 209Implementations of the Support Library interface are separated by their general 210class of operating system. Currently only Unix and Win32 classes are defined 211but more could be added for other operating system classifications. To 212distinguish which implementation to compile, the code in ``lib/Support`` uses 213the ``LLVM_ON_UNIX`` and ``_WIN32`` ``#defines``. Each source file in 214``lib/Support``, after implementing the generic (operating system independent) 215functionality needs to include the correct implementation using a set of 216``#if defined(LLVM_ON_XYZ)`` directives. For example, if we had 217``lib/Support/Path.cpp``, we'd expect to see in that file: 218 219.. code-block:: c++ 220 221 #if defined(LLVM_ON_UNIX) 222 #include "Unix/Path.inc" 223 #endif 224 #if defined(_WIN32) 225 #include "Windows/Path.inc" 226 #endif 227 228The implementation in ``lib/Support/Unix/Path.inc`` should handle all Unix 229variants. The implementation in ``lib/Support/Windows/Path.inc`` should handle 230all Windows variants. What this does is quickly inc the basic class 231of operating system that will provide the implementation. The specific details 232for a given platform must still be determined through the use of ``#ifdef``. 233 234Consistent Semantics 235-------------------- 236 237The implementation of a ``lib/Support`` interface can vary drastically between 238platforms. That's okay as long as the end result of the interface function is 239the same. For example, a function to create a directory is pretty straight 240forward on all operating system. System V IPC on the other hand isn't even 241supported on all platforms. Instead of "supporting" System V IPC, 242``lib/Support`` should provide an interface to the basic concept of 243inter-process communications. The implementations might use System V IPC if 244that was available or named pipes, or whatever gets the job done effectively 245for a given operating system. In all cases, the interface and the 246implementation must be semantically consistent. 247