This document provides some details on LLVM's System Library, located in the
source at lib/System and include/llvm/System. 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.
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 lib/System 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 include/llvm/System.
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.
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.
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.
Except in lib/System, no LLVM source code should directly #include a
system header. Care has been taken to remove all such #includes from LLVM
while lib/System 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
lib/System.
To obtain system-dependent functionality, existing interfaces to the system
found in include/llvm/System should be used. If an appropriate interface is
not available, it should be added to include/llvm/System and implemented in
lib/System for all supported platforms.
The System Library must shield LLVM from all system headers. To obtain
system level functionality, LLVM source must #include "llvm/System/Thing.h"
and nothing else. This means that Thing.h cannot expose any system header
files. This protects LLVM from accidentally using system specific functionality
and only allows it via the lib/System interface.
The standard 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 directly or obtain their inclusion through lib/System interfaces.
The standard C++ headers from the standard C++ library and standard
template library may 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.
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.
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: getenv, fork, execve, and wait. The
correct thing for lib/System to provide is a function, say
ExecuteProgramAndWait, that implements the functionality completely. what
we don't want is wrappers for the operating system calls involved.
There must not be a one-to-one relationship between operating system calls and the System library's interface. Any such interface function will be suspicious.
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.
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).
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 #include mechanism works just fine.
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.
For example, the stat system call is notorious for having variations in the
data it provides. lib/System must not declare stat 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 stat but that is strictly an implementation detail. The interface
provided by the System Library must be implemented on all platforms (even those
without stat).
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.
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 "soft" errors and the second group "hard" errors.
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 can't fail, except when
faced with hard errors.
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.
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.
Rules of thumb:
- Don't throw soft errors, only hard errors.
- If you're tempted to throw a soft error, re-think the interface.
- Handle internally the most common normal/good/soft error conditions so the rest of LLVM doesn't have to.
None of the lib/System interface functions may be declared with C++
throw() 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.
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
#if defined(LLVM_ON_XYZ) directives. For example, if we had
lib/System/File.cpp, we'd expect to see in that file:
#if defined(LLVM_ON_UNIX)
#include "Unix/File.cpp"
#endif
#if defined(LLVM_ON_WIN32)
#include "Win32/File.cpp"
#endifThe 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 #ifdef.
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.