<body>
<div class="doc_title">System Library</div>
-
-<div class="doc_warning">
- <p>Warning: This document is a work in progress.</p>
-</div>
-
<ul>
<li><a href="#abstract">Abstract</a></li>
- <li><a href="#requirements">System Library Requirements</a>
+ <li><a href="#requirements">Keeping LLVM Portable</a>
<ol>
- <li><a href="#headers">Hide System Header Files</a></li>
+ <li><a href="#headers">Don't Include System Headers</a></li>
+ <li><a href="#expose">Don't Expose System Headers</a></li>
+ <li><a href="#c_headers">Allow Standard C Header Files</a></li>
+ <li><a href="#cpp_headers">Allow Standard C++ Header Files</a></li>
+ <li><a href="#highlev">High-Level Interface</a></li>
<li><a href="#nofunc">No Exposed Functions</a></li>
<li><a href="#nodata">No Exposed Data</a></li>
- <li><a href="#xcptns">No Exceptions</a></li>
- <li><a href="#errors">Standard Error Codes</a></li>
- <li><a href="#overhead">Minimize Overhead</a></li>
- </ol></li>
- <li><a href="#design">System Library Design</a>
- <ol>
- <li><a href="#opaque">Use Opaque Classes</a></li>
- <li><a href="#common">Common Implementations</a></li>
- <li><a href="#multi_imps">Multiple Implementations</a></li>
- <li><a href="#lowlevel">Use Low Level Interfaces</a></li>
- <li><a href="#memalloc">No Memory Allocation</a></li>
+ <li><a href="#nodupl">No Duplicate Implementations</a></li>
+ <li><a href="#nounused">No Unused Functionality</a></li>
<li><a href="#virtuals">No Virtual Methods</a></li>
- </ol></li>
- <li><a href="#detail">System Library Details</a>
- <ol>
+ <li><a href="#softerrors">Minimize Soft Errors</a></li>
+ <li><a href="#throw_spec">No throw() Specifications</a></li>
+ <li><a href="#organization">Code Organization</a></li>
+ <li><a href="#semantics">Consistent Semantics</a></li>
<li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
- <li><a href="#refimpl">Reference Implementatation</a></li>
</ol></li>
</ul>
<div class="doc_author">
- <p>Written by <a href="rspencer@x10sys.com">Reid Spencer</a></p>
+ <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="abstract">Abstract</a></div>
<div class="doc_text">
- <p>This document describes the requirements, design, and implementation
- details of LLVM's System Library. The library is composed of the header files
- in <tt>llvm/include/llvm/System</tt> and the source files in
- <tt>llvm/lib/System</tt>. The goal of this library is to completely shield
- LLVM from the variations in operating system interfaces. By centralizing
- LLVM's use of operating system interfaces, we make it possible for the LLVM
- tool chain and runtime libraries to be more easily ported to new platforms.
- The library also unclutters the rest of LLVM from #ifdef use and special
- cases for specific operating systems.</p>
- <p>The System Library was donated to LLVM by Reid Spencer who formulated the
- original design as part of the eXtensible Programming System (XPS) which is
- based, in part, on LLVM.</p>
+ <p>This document provides some details on LLVM's System Library, located in
+ the source at <tt>lib/System</tt> and <tt>include/llvm/System</tt>. The
+ library's purpose is to shield LLVM from the differences between operating
+ systems for the few services LLVM needs from the operating system. Much of
+ LLVM is written using portability features of standard C++. However, in a few
+ areas, system dependent facilities are needed and the System Library is the
+ wrapper around those system calls.</p>
+ <p>By centralizing LLVM's use of operating system interfaces, we make it
+ possible for the LLVM tool chain and runtime libraries to be more easily
+ ported to new platforms since (theoretically) only <tt>lib/System</tt> needs
+ to be ported. This library also unclutters the rest of LLVM from #ifdef use
+ and special cases for specific operating systems. Such uses are replaced
+ with simple calls to the interfaces provided in <tt>include/llvm/System</tt>.
+ </p>
+ <p>Note that the System Library is not intended to be a complete operating
+ system wrapper (such as the Adaptive Communications Environment (ACE) or
+ Apache Portable Runtime (APR)), but only provides the functionality necessary
+ to support LLVM.
+ <p>The System Library was written by Reid Spencer who formulated the
+ design based on similar work originating from the eXtensible Programming
+ System (XPS). Several people helped with the effort; especially,
+ Jeff Cohen and Henrik Bach on the Win32 port.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
- <a name="requirements">System Library Requirements</a>
+ <a name="requirements">Keeping LLVM Portable</a>
</div>
<div class="doc_text">
- <p>The System library's requirements are aimed at shielding LLVM from the
- variations in operating system interfaces. The following sections define the
- requirements needed to fulfill this objective.</p>
+ <p>In order to keep LLVM portable, LLVM developers should adhere to a set of
+ portability rules associated with the System Library. Adherence to these rules
+ should help the System Library achieve its goal of shielding LLVM from the
+ variations in operating system interfaces and doing so efficiently. The
+ following sections define the rules needed to fulfill this objective.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="headers">Hide System Header Files</a></div>
-<div class="doc_text">
- <p>To be written.</p>
+<div class="doc_subsection"><a name="headers">Don't Include System Headers</a>
</div>
-
-<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>Except in <tt>lib/System</tt>, no LLVM source code should directly
+ <tt>#include</tt> a system header. Care has been taken to remove all such
+ <tt>#includes</tt> from LLVM while <tt>lib/System</tt> was being
+ developed. Specifically this means that header files like "unistd.h",
+ "windows.h", "stdio.h", and "string.h" are forbidden to be included by LLVM
+ source code outside the implementation of <tt>lib/System</tt>.</p>
+ <p>To obtain system-dependent functionality, existing interfaces to the system
+ found in <tt>include/llvm/System</tt> should be used. If an appropriate
+ interface is not available, it should be added to <tt>include/llvm/System</tt>
+ and implemented in <tt>lib/System</tt> for all supported platforms.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
-<div class="doc_text">
- <p>To be written.</p>
+<div class="doc_subsection"><a name="expose">Don't Expose System Headers</a>
</div>
-
-<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="xcptns">No Exceptions</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>The System Library must shield LLVM from <em>all</em> system headers. To
+ obtain system level functionality, LLVM source must
+ <tt>#include "llvm/System/Thing.h"</tt> and nothing else. This means that
+ <tt>Thing.h</tt> cannot expose any system header files. This protects LLVM
+ from accidentally using system specific functionality and only allows it
+ via the <tt>lib/System</tt> interface.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="errors">Standard Error Codes</a></div>
+<div class="doc_subsection"><a name="c_headers">Use Standard C Headers</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>The <em>standard</em> C headers (the ones beginning with "c") are allowed
+ to be exposed through the <tt>lib/System</tt> interface. These headers and
+ the things they declare are considered to be platform agnostic. LLVM source
+ files may include them directly or obtain their inclusion through
+ <tt>lib/System</tt> interfaces.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="overhead">Minimize Overhead</a></div>
-<div class="doc_text">
- <p>To be written.</p>
+<div class="doc_subsection"><a name="cpp_headers">Use Standard C++ Headers</a>
</div>
-
-<!-- *********************************************************************** -->
-<div class="doc_section"><a name="design">System Library Design</a></div>
<div class="doc_text">
- <p>In order to fulfill the requirements of the system library, strict design
- objectives must be maintained in the library as it evolves. The goal here
- is to provide interfaces to operating system concepts (files, memory maps,
- sockets, signals, locking, etc) efficiently and in such a way that the
- remainder of LLVM is completely operating system agnostic.</p>
+ <p>The <em>standard</em> C++ headers from the standard C++ library and
+ standard template library may be exposed through the <tt>lib/System</tt>
+ interface. These headers and the things they declare are considered to be
+ platform agnostic. LLVM source files may include them or obtain their
+ inclusion through lib/System interfaces.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="opaque">Use Opaque Classes</a></div>
+<div class="doc_subsection"><a name="highlev">High Level Interface</a></div>
<div class="doc_text">
- <p>no public data</p>
- <p>onlyprimitive typed private/protected data</p>
- <p>data size is "right" for platform, not max of all platforms</p>
- <p>each class corresponds to O/S concept</p>
+ <p>The entry points specified in the interface of lib/System must be aimed at
+ completing some reasonably high level task needed by LLVM. We do not want to
+ simply wrap each operating system call. It would be preferable to wrap several
+ operating system calls that are always used in conjunction with one another by
+ LLVM.</p>
+ <p>For example, consider what is needed to execute a program, wait for it to
+ complete, and return its result code. On Unix, this involves the following
+ operating system calls: <tt>getenv, fork, execve,</tt> and <tt>wait</tt>. The
+ correct thing for lib/System to provide is a function, say
+ <tt>ExecuteProgramAndWait</tt>, that implements the functionality completely.
+ what we don't want is wrappers for the operating system calls involved.</p>
+ <p>There must <em>not</em> be a one-to-one relationship between operating
+ system calls and the System library's interface. Any such interface function
+ will be suspicious.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="common">Common Implementations</a></div>
+<div class="doc_subsection"><a name="nounused">No Unused Functionality</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>There must be no functionality specified in the interface of lib/System
+ that isn't actually used by LLVM. We're not writing a general purpose
+ operating system wrapper here, just enough to satisfy LLVM's needs. And, LLVM
+ doesn't need much. This design goal aims to keep the lib/System interface
+ small and understandable which should foster its actual use and adoption.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="multi_imps">Multiple Implementations</a>
+<div class="doc_subsection"><a name="nodupl">No Duplicate Implementations</a>
</div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>The implementation of a function for a given platform must be written
+ exactly once. This implies that it must be possible to apply a function's
+ implementation to multiple operating systems if those operating systems can
+ share the same implementation. This rule applies to the set of operating
+ systems supported for a given class of operating system (e.g. Unix, Win32).
+ </p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="low_level">Use Low Level Interfaces</a>
-</div>
+<div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>The System Library interfaces can be called quite frequently by LLVM. In
+ order to make those calls as efficient as possible, we discourage the use of
+ virtual methods. There is no need to use inheritance for implementation
+ differences, it just adds complexity. The <tt>#include</tt> mechanism works
+ just fine.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="memalloc">No Memory Allocation</a></div>
+<div class="doc_subsection"><a name="nofunc">No Exposed Functions</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>Any functions defined by system libraries (i.e. not defined by lib/System)
+ must not be exposed through the lib/System interface, even if the header file
+ for that function is not exposed. This prevents inadvertent use of system
+ specific functionality.</p>
+ <p>For example, the <tt>stat</tt> system call is notorious for having
+ variations in the data it provides. <tt>lib/System</tt> must not declare
+ <tt>stat</tt> nor allow it to be declared. Instead it should provide its own
+ interface to discovering information about files and directories. Those
+ interfaces may be implemented in terms of <tt>stat</tt> but that is strictly
+ an implementation detail. The interface provided by the System Library must
+ be implemented on all platforms (even those without <tt>stat</tt>).</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
+<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>Any data defined by system libraries (i.e. not defined by lib/System) must
+ not be exposed through the lib/System interface, even if the header file for
+ that function is not exposed. As with functions, this prevents inadvertent use
+ of data that might not exist on all platforms.</p>
</div>
-<!-- *********************************************************************** -->
-<div class="doc_section"><a name="detail">System Library Details</a></div>
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="softerrors">Minimize Soft Errors</a></div>
<div class="doc_text">
- <p>To be written.</p>
+ <p>Operating system interfaces will generally provide error results for every
+ little thing that could go wrong. In almost all cases, you can divide these
+ error results into two groups: normal/good/soft and abnormal/bad/hard. That
+ is, some of the errors are simply information like "file not found",
+ "insufficient privileges", etc. while other errors are much harder like
+ "out of space", "bad disk sector", or "system call interrupted". We'll call
+ the first group "<i>soft</i>" errors and the second group "<i>hard</i>"
+ errors.<p>
+ <p>lib/System must always attempt to minimize soft errors.
+ This is a design requirement because the
+ minimization of soft errors can affect the granularity and the nature of the
+ interface. In general, if you find that you're wanting to throw soft errors,
+ you must review the granularity of the interface because it is likely you're
+ trying to implement something that is too low level. The rule of thumb is to
+ provide interface functions that <em>can't</em> fail, except when faced with
+ hard errors.</p>
+ <p>For a trivial example, suppose we wanted to add an "OpenFileForWriting"
+ function. For many operating systems, if the file doesn't exist, attempting
+ to open the file will produce an error. However, lib/System should not
+ simply throw that error if it occurs because its a soft error. The problem
+ is that the interface function, OpenFileForWriting is too low level. It should
+ be OpenOrCreateFileForWriting. In the case of the soft "doesn't exist" error,
+ this function would just create it and then open it for writing.</p>
+ <p>This design principle needs to be maintained in lib/System because it
+ avoids the propagation of soft error handling throughout the rest of LLVM.
+ Hard errors will generally just cause a termination for an LLVM tool so don't
+ be bashful about throwing them.</p>
+ <p>Rules of thumb:</p>
+ <ol>
+ <li>Don't throw soft errors, only hard errors.</li>
+ <li>If you're tempted to throw a soft error, re-think the interface.</li>
+ <li>Handle internally the most common normal/good/soft error conditions
+ so the rest of LLVM doesn't have to.</li>
+ </ol>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="bug">Bug 351</a></div>
+<div class="doc_subsection"><a name="throw_spec">No throw Specifications</a>
+</div>
<div class="doc_text">
- <p>See <a href="http://llvm.cs.uiuc.edu/PR351">bug 351</a>
- for further details on the progress of this work</p>
+ <p>None of the lib/System interface functions may be declared with C++
+ <tt>throw()</tt> specifications on them. This requirement makes sure that the
+ compiler does not insert additional exception handling code into the interface
+ functions. This is a performance consideration: lib/System functions are at
+ the bottom of many call chains and as such can be frequently called. We
+ need them to be as efficient as possible. However, no routines in the
+ system library should actually throw exceptions.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection"><a name="bug">Rationale For #include Hierarchy</a>
-</div>
+<div class="doc_subsection"><a name="organization">Code Organization</a></div>
<div class="doc_text">
- <p>In order to provide different implementations of the lib/System interface
- for different platforms, it is necessary for the library to "sense" which
- operating system is being compiled for and conditionally compile only the
- applicabe parts of the library. While several operating system wrapper
- libraries (e.g. APR, ACE) choose to use #ifdef preprocessor statements in
- combination with autoconf variable (HAVE_* family), lib/System chooses an
- alternate strategy. <p>
- <p>To put it succinctly, the lib/System strategy has traded "#ifdef hell" for
- "#include hell". That is, a given implementation file defines one or more
- functions for a particular operating system variant. The functions defined in
- that file have no #ifdef's to disambiguate the platform since the file is only
- compiled on one kind of platform. While this leads to the same function being
- imlemented differently in different files, it is our contention that this
- leads to better maintenance and easier portability.</p>
- <p>For example, consider a function having different implementations on a
- variety of platforms. Many wrapper libraries choose to deal with the different
- implementations by using #ifdef, like this:</p>
+ <p>Implementations of the System Library interface are separated by their
+ general class of operating system. Currently only Unix and Win32 classes are
+ defined but more could be added for other operating system classifications.
+ To distinguish which implementation to compile, the code in lib/System uses
+ the LLVM_ON_UNIX and LLVM_ON_WIN32 #defines provided via configure through the
+ llvm/Config/config.h file. Each source file in lib/System, after implementing
+ the generic (operating system independent) functionality needs to include the
+ correct implementation using a set of <tt>#if defined(LLVM_ON_XYZ)</tt>
+ directives. For example, if we had lib/System/File.cpp, we'd expect to see in
+ that file:</p>
<pre><tt>
- void SomeFunction(void) {
- #if defined __LINUX
- // .. Linux implementation
- #elif defined __WIN32
- // .. Win32 implementation
- #elif defined __SunOS
- // .. SunOS implementation
- #else
- #warning "Don't know how to implement SomeFunction on this platform"
- #endif
- }
+ #if defined(LLVM_ON_UNIX)
+ #include "Unix/File.cpp"
+ #endif
+ #if defined(LLVM_ON_WIN32)
+ #include "Win32/File.cpp"
+ #endif
</tt></pre>
- <p>The problem with this is that its very messy to read, especially as the
- number of operating systems and their variants grow. The above example is
- actually tame compared to what can happen when the implementation depends on
- specific flavors and versions of the operating system. In that case you end up
- with multiple levels of nested #if statements. This is what we mean by "#ifdef
- hell".</p>
- <p>To avoid the situation above, we've choosen to locate all functions for a
- given implementation file for a specific operating system into one place. This
- has the following advantages:<p>
- <ul>
- <li>No "#ifdef hell"</li>
- <li>When porting, the strategy is quite straight forward: copy the
- implementation file from a similar operating system to a new directory and
- re-implement them.<li>
- <li>Correctness is helped during porting because the new operating system's
- implementation is wholly contained in a separate directory. There's no
- chance to make an error in the #if statements and affect some other
- operating system's implementation.</li>
- </ul>
- <p>So, given that we have decided to use #include instead of #if to provide
- platform specific implementations, there are actually three ways we can go
- about doing this. None of them are perfect, but we believe we've chosen the
- lesser of the three evils. Given that there is a variable named $OS which
- names the platform for which we must build, here's a summary of the three
- approaches we could use to determine the correct directory:</p>
- <ol>
- <li>Provide the compiler with a -I$(OS) on the command line. This could be
- provided in only the lib/System makefile.</li>
- <li>Use autoconf to transform #include statements in the implementation
- files by using substitutions of @OS@. For example, if we had a file,
- File.cpp.in, that contained "#include <@OS@/File.cpp>" this would get
- transformed to "#include <actual/File.cpp>" where "actual" is the
- actual name of the operating system</li>
- <li>Create a link from $OBJ_DIR/platform to $SRC_DIR/$OS. This allows us to
- use a generic directory name to get the correct platform, as in #include
- <platform/File.cpp></li>
- </ol>
- <p>Let's look at the pitfalls of each approach.</p>
- <p>In approach #1, we end up with some confusion as to what gets included.
- Suppose we have lib/System/File.cpp that includes just File.cpp to get the
- platform specific part of the implementation. In this case, the include
- directive with the <> syntax will include the right file but the include
- directive with the "" syntax will recursively include the same file,
- lib/System/File.cpp. In the case of #include <File.cpp>, the -I options
- to the compiler are searched first so it works. But in the #include "File.cpp"
- case, the current directory is searched first. Furthermore, in both cases,
- neither include directive documents which File.cpp is getting included.</p>
- <p>In approach #2, we have the problem of needing to reconfigure repeatedly.
- Developer's generally hate that and we don't want lib/System to be a thorn in
- everyone's side because it will constantly need updating as operating systems
- change and as new operating systems are added. The problem occurs when a new
- implementation file is added to the library. First of all, you have to add a
- file with the .in suffix, then you have to add that file name to the list of
- configurable files in the autoconf/configure.ac file, then you have to run
- AutoRegen.sh to rebuild the configure script, then you have to run the
- configure script. This is deemed to be a pretty large hassle.</p>
- <p>In approach #3, we have the problem that not all platforms support links.
- Fortunately the autoconf macro used to create the link can compensate for
- this. If a link can't be made, the configure script will copy the correct
- directory from $BUILD_SRC_DIR to $BUILD_OBJ_DIR under the new name. The only
- problem with this is that if a copy is made, the copy doesn't get updated if
- the programmer adds or modifies files in the $BUILD_SRC_DIR. A reconfigure or
- manual copying is needed to get things to compile.<p>
- <p>The approach we have taken in lib/System is #3. Here's why:<p>
- <ul>
- <li>Approach #1 is rejected because it doesn't document what's actually
- getting included and the potential for mistakes with alternate include
- directive forms is high.</li>
- <li>Approach #2 are both viable and only really impact development when new
- files are added to the library.</li>
- <li>However, approach #2 impacts every new file on every platform all the
- time. With approach #3, only those platforms not supporting links will be
- affected. The number of platforms not supporting links is very small and
- they are generally archaic.</li>
- <li>Given the above, approach #3 seems to have the least impact.</li>
- </ul>
+ <p>The implementation in lib/System/Unix/File.cpp should handle all Unix
+ variants. The implementation in lib/System/Win32/File.cpp should handle all
+ Win32 variants. What this does is quickly differentiate the basic class of
+ operating system that will provide the implementation. The specific details
+ for a given platform must still be determined through the use of
+ <tt>#ifdef</tt>.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="refimpl">Reference Implementation</a>
+<div class="doc_subsection"><a name="semantics">Consistent Semantics</a></div>
+<div class="doc_text">
+ <p>The implementation of a lib/System interface can vary drastically between
+ platforms. That's okay as long as the end result of the interface function
+ is the same. For example, a function to create a directory is pretty straight
+ forward on all operating system. System V IPC on the other hand isn't even
+ supported on all platforms. Instead of "supporting" System V IPC, lib/System
+ should provide an interface to the basic concept of inter-process
+ communications. The implementations might use System V IPC if that was
+ available or named pipes, or whatever gets the job done effectively for a
+ given operating system. In all cases, the interface and the implementation
+ must be semantically consistent. </p>
</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection"><a name="bug">Bug 351</a></div>
<div class="doc_text">
- <p>The <tt>linux</tt> implementation of the system library will always be the
- reference implementation. This means that (a) the concepts defined by the
- linux must be identically replicated in the other implementations and (b) the
- linux implementation must always be complete (provide implementations for all
- concepts).</p>
+ <p>See <a href="http://llvm.org/PR351">bug 351</a>
+ for further details on the progress of this work</p>
</div>
<!-- *********************************************************************** -->
<hr>
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- <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
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