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<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>
<ol>
<li><a href="#headers">Hide System Header Files</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="#nofunc">No Exposed Functions</a></li>
<li><a href="#nodata">No Exposed Data</a></li>
<li><a href="#throw">Throw Only std::string</a></li>
<li><a href="#throw_spec">No throw() Specifications</a></li>
<li><a href="#nodupl">No Duplicate Implementations</a></li>
</ol></li>
<li><a href="#design">System Library Design</a>
<ol>
<li><a href="#nounused">No Unused Functionality</a></li>
<li><a href="#highlev">High-Level Interface</a></li>
<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="#virtuals">No Virtual Methods</a></li>
</ol></li>
<li><a href="#detail">System Library Details</a>
<ol>
<li><a href="#bug">Tracking Bugzilla Bug: 351</a></li>
<li><a href="#refimpl">Reference Implementation</a></li>
</ol></li>
</ul>
<div class="doc_author">
<p>Written by <a href="rspencer@x10sys.com">Reid Spencer</a></p>
</div>
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<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
since (theoretically) only <tt>llvm/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>llvm/include/llvm/System</tt>.</p> Note that
lib/System is not intended to be a complete operating system wrapper (such as
the Adaptive Communications Environment (ACE) or Apache Portable Runtime
(APR)), but only to provide the functionality necessary to support LLVM.
<p>The System Library was written by Reid Spencer who formulated the
design based on similar original work as part of the eXtensible Programming
System (XPS).</p>
</div>
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<div class="doc_section">
<a name="requirements">System Library Requirements</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. Of necessity, these requirements
must be strictly followed in order to ensure the library's goal is reached.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="headers">Hide System Header Files</a></div>
<div class="doc_text">
<p>The library must shield LLVM from <em>all</em> system libraries. To obtain
system level functionality, LLVM 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 except through the lib/System interface. Specifically this
means that header files like "unistd.h", "windows.h", "stdio.h", and
"string.h" are verbotten outside the implementation of lib/System.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="c_headers">Allow Standard C Headers</a>
</div>
<div class="doc_text">
<p>The <em>standard</em> C headers (the ones beginning with "c") are allowed
to be exposed through the lib/System 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="cpp_headers">Allow Standard C++ Headers</a>
</div>
<div class="doc_text">
<p>The <em>standard</em> C++ headers from the standard C++ library and
standard template library are allowed to be exposed through the lib/System
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="nofunc">No Exposed Functions</a></div>
<div class="doc_text">
<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. lib/System 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.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="nodata">No Exposed Data</a></div>
<div class="doc_text">
<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_subsection"><a name="throw">Throw Only std::string</a></div>
<div class="doc_text">
<p>If an error occurs that lib/System cannot handle, the only action taken by
lib/System is to throw an instance of std:string. The contents of the string
must explain both what happened and the context in which it happened. The
format of the string should be a (possibly empty) list of contexts each
terminated with a : and a space, followed by the error message, optionally
followed by a reason, and optionally followed by a suggestion.</p>
<p>For example, failure to open a file named "foo" could result in a message
like:</p>
<ul><li>foo: Unable to open file because it doesn't exist."</li></ul>
<p>The "foo:" part is the context. The "Unable to open file" part is the error
message. The "because it doesn't exist." part is the reason. This message has
no suggestion. Where possible, the implementation of lib/System should use
operating system specific facilities for converting the error code returned by
a system call into an error message. This will help to make the error message
more familiar to users of that type of operating system.</p>
<p>Note that this requirement precludes the throwing of any other exceptions.
For example, various C++ standard library functions can cause exceptions to be
thrown (e.g. out of memory situation). In all cases, if there is a possibility
that non-string exceptions could be thrown, the lib/System library must ensure
that the exceptions are translated to std::string form.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="throw_spec">No throw Specifications</a>
</div>
<div class="doc_text">
<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 the many call chains and as such can be frequently called. We
need them to be as efficient as possible.</p>
</div>
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<div class="doc_subsection"><a name="nodupl">No Duplicate Implementations</a>
</div>
<div class="doc_text">
<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.</p>
</div>
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<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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="nounused">No Unused Functionality</a></div>
<div class="doc_text">
<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="highlev">High Level Interface</a></div>
<div class="doc_text">
<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="highlev">Minimize Soft Errors</a></div>
<div class="doc_text">
<p>Operating system interfaces will generally provide errors 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". Well call the
first group "soft" errors and the second group "hard" errors.<p>
<p>lib/System must always attempt to minimize soft errors and always just
throw a std::string on hard 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 "can't" 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>
<pre><tt>
Notes:
10. 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.
11. Implementations are separated first by the general class of operating system
as provided by the configure script's $build variable. This variable is used
to create a link from $BUILD_OBJ_ROOT/lib/System/platform to a directory in
$BUILD_SRC_ROOT/lib/System directory with the same name as the $build
variable. This provides a retargetable include mechanism. By using the link's
name (platform) we can actually include the operating specific
implementation. For example, support $build is "Darwin" for MacOS X. If we
place:
#include "platform/File.cpp"
into a a file in lib/System, it will actually include
lib/System/Darwin/File.cpp. What this does is quickly differentiate the basic
class of operating system that will provide the implementation.
12. Implementation files in lib/System need may only do two things: (1) define
functions and data that is *TRULY* generic (completely platform agnostic) and
(2) #include the platform specific implementation with:
#include "platform/Impl.cpp"
where Impl is the name of the implementation files.
13. Platform specific implementation files (platform/Impl.cpp) may only #include
other Impl.cpp files found in directories under lib/System. The order of
inclusion is very important (from most generic to most specific) so that we
don't inadvertently place an implementation in the wrong place. For example,
consider a fictitious implementation file named DoIt.cpp. Here's how the
#includes should work for a Linux platform
lib/System/DoIt.cpp
#include "platform/DoIt.cpp" // platform specific impl. of Doit
DoIt
lib/System/Linux/DoIt.cpp // impl that works on all Linux
#include "../Unix/DoIt.cpp" // generic Unix impl. of DoIt
#include "../Unix/SUS/DoIt.cpp // SUS specific impl. of DoIt
#include "../Unix/SUS/v3/DoIt.cpp // SUSv3 specific impl. of DoIt
Note that the #includes in lib/System/Linux/DoIt.cpp are all optional but
should be used where the implementation of some functionality can be shared
across some set of Unix variants. We don't want to duplicate code across
variants if their implementation could be shared.
</tt></pre>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="opaque">Use Opaque Classes</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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="common">Common Implementations</a></div>
<div class="doc_text">
<p>To be written.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="multi_imps">Multiple Implementations</a>
</div>
<div class="doc_text">
<p>To be written.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="memalloc">No Memory Allocation</a></div>
<div class="doc_text">
<p>To be written.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="virtuals">No Virtual Methods</a></div>
<div class="doc_text">
<p>To be written.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="detail">System Library Details</a></div>
<div class="doc_text">
<p>To be written.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="bug">Bug 351</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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="bug">Rationale For #include Hierarchy</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
applicable 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
implemented 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>
<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
}
</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 chosen 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 &lt;@OS@/File.cpp&gt;" this would get
transformed to "#include &lt;actual/File.cpp&gt;" 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
&lt;platform/File.cpp&gt;</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 &lt;&gt; 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 &lt;File.cpp&gt;, 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>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="refimpl">Reference Implementation</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>
</div>
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