The primary motivation for beginning work on the Filesystem Library was
frustration with Boost administrative tools. Scripts were written in
Python, Perl, Bash, and Windows command languages. There was no single
scripting language familiar and acceptable to all Boost administrators. Yet they
were all skilled C++ programmers - why couldn't C++ be used as the scripting
The key feature C++ lacked for script-like applications was the ability to
perform portable filesystem operations on directories and their contents. The
Filesystem Library was developed to fill that void.
The intent is not to compete with traditional scripting languages, but to
provide a solution for situations where C++ is already the language
- Be able to write portable script-style filesystem operations in modern
Rationale: This is a common programming need. It is both an
embarrassment and a hardship that this is not possible with either the current
C++ or Boost libraries. The need is particularly acute
when C++ is the only toolset allowed in the tool chain. File system
operations are provided by many languages used on multiple platforms,
such as Perl and Python, as well as by many platform specific scripting
languages. All operating systems provide some form of API for filesystem
operations, and the POSIX bindings are increasingly available even on
operating systems not normally associated with POSIX, such as the Mac, z/OS,
- Work within the realities described below.
Rationale: This isn't a research project. The need is for something that works on
today's platforms, including some of the embedded operating systems
with limited file systems. Because of the emphasis on portability, such a
library would be much more useful if standardized. That means being able to
work with a much wider range of platforms that just Unix or Windows and their
- Avoid dangerous programming practices. Particularly, all-too-easy-to-ignore error notifications
and use of global variables. If a dangerous feature is provided, identify it as such.
Rationale: Normally this would be covered by "the usual Boost requirements...",
but it is mentioned explicitly because the equivalent native platform and
scripting language interfaces often depend on all-too-easy-to-ignore error
notifications and global variables like "current
- Structure the library so that it is still useful even if some functionality
does not map well onto a given platform or directory tree. Particularly, much
useful functionality should be portable even to flat
Rationale: Much functionality which does not
require a hierarchical directory structure is still useful on flat-structure
filesystems. There are many systems, particularly embedded systems,
where even very limited functionality is still useful.
- Interface smoothly with current C++ Standard Library input/output
facilities. For example, paths should be
easy to use in std::basic_fstream constructors.
Rationale: One of the most common uses of file system functionality is to
manipulate paths for eventual use in input/output operations.
Thus the need to interface smoothly with standard library I/O.
- Suitable for eventual standardization. The implication of this requirement
is that the interface be close to minimal, and that great care be take
Rationale: The lack of file system operations is a serious hole
in the current standard, with no other known candidates to fill that hole.
Libraries with elaborate interfaces and difficult to port specifications are much less likely to be accepted for
- The usual Boost requirements and
- Encourage, but do not require, portability in path names.
Rationale: For paths which originate from user input it is unreasonable to
require portable path syntax.
- Avoid giving the illusion of portability where portability in fact does not
Rationale: Leaving important behavior unspecified or "implementation defined" does a
great disservice to programmers using a library because it makes it appear
that code relying on the behavior is portable, when in fact there is nothing
portable about it. The only case where such under-specification is acceptable is when both users and implementors know from
other sources exactly what behavior is required, yet for some reason it isn't
possible to specify it exactly.
- Some operating systems have a single directory tree root, others have
- Some file systems provide both a long and short form of filenames.
- Some file systems have different syntax for file paths and directory
- Some file systems have different rules for valid file names and valid
- Some file systems (ISO-9660, level 1, for example) use very restricted
(so-called 8.3) file names.
- Some operating systems allow file systems with different
characteristics to be "mounted" within a directory tree. Thus an
ISO-9660 or Windows
file system may end up as a sub-tree of a POSIX directory tree.
- Wide-character versions of directory and file operations are available on some operating
systems, and not available on others.
- There is no law that says directory hierarchies have to be specified in
terms of left-to-right decent from the root.
- Some file systems have a concept of file "version number" or "generation
number". Some don't.
- Not all operating systems use single character separators in path names. Some use
paired notations. A typical fully-specified OpenVMS filename
might look something like this:
The general OpenVMS format is:
- For common file systems, determining if two descriptors are for same
entity is extremely difficult or impossible. For example, the concept of
equality can be different for each portion of a path - some portions may be
case or locale sensitive, others not. Case sensitivity is a property of the
pathname itself, and not the platform. Determining collating sequence is even
- Race-conditions may occur. Directory trees, directories, files, and file attributes are in effect shared between all threads, processes, and computers which have access to the
filesystem. That may well include computers on the other side of the
world or in orbit around the world. This implies that file system operations
may fail in unexpected ways. For example:
assert( exists("foo") == exists("foo") );
// may fail!
assert( is_directory("foo") == is_directory("foo");
// may fail!
In the first example, the file may have been deleted between calls to
exists(). In the second example, the file may have been deleted and then
replaced by a directory of the same name between the calls to is_directory().
- Even though an application may be portable, it still will have to traffic
in system specific paths occasionally; user provided input is a common
- Symbolic links cause canonical and
normal form of some paths to represent different files or directories. For
example, given the directory hierarchy
/a/b/c, with a symbolic
x pointing to
then under POSIX Pathname Resolution rules a path of
should resolve to
"/a/x/.." were first
"/a", it would resolve incorrectly. (Case supplied
by Walter Landry.)
The Requirements and
Realities above drove much of the C++ interface design. In particular,
the desire to make script-like code straightforward caused a great deal of
effort to go into ensuring that apparently simple expressions like exists( "foo"
) work as expected.
See the FAQ for the rationale behind many detailed
Several key insights went into the path class design:
- Decoupling of the input formats, internal conceptual (vector<string>
or other sequence)
model, and output formats.
- Providing two input formats (generic and O/S specific) broke a major
- Providing several output formats solved another set of previously
- Several non-obvious functions (particularly decomposition and composition)
are required to support portable code. (Peter Dimov, Thomas Witt, Glen
Error checking was a particularly difficult area. One key insight was that
with file and directory names, portability isn't a universal truth.
Rather, the programmer must think out the question "What operating systems do I
want this path to be portable to?" By providing support for several
answers to that question, the Filesystem Library alerts programmers of the need
to ask it in the first place.
Dietmar Kühl's original
directory_iterator design and implementation supported
wide-character file and directory names. It was abandoned after extensive
discussions among Library Working Group members failed to identify portable
semantics for wide-character names on systems not providing native support. See
Previous iterations of the interface design used explicitly named functions providing a
large number of convenience operations, with no compile-time or run-time
options. There were so many function names that they were very confusing to use,
and the interface was much larger. Any benefits seemed theoretical rather than
Designs based on compile time (rather than runtime) flag and option selection
(via policy, enum, or int template parameters) became so complicated that they
were abandoned, often after investing quite a bit of time and effort. The need
to qualify attribute or option names with namespaces, even aliases, made use in
template parameters ugly; that wasn't fully appreciated until actually writing
Yet another set of convenience functions ( for example, remove with
permissive, prune, recurse, and other options, plus predicate, and possibly
other, filtering features) were abandoned because the details became both
complex and contentious.
What is left is a toolkit of low-level operations from which the user can
create more complex convenience operations, plus a very small number of
convenience functions which were found to be useful enough to justify inclusion.
There were so many abandoned path designs, I've lost track. Policy-based
class templates in several flavors, constructor supplied runtime policies,
operation specific runtime policies, they were all considered, often
implemented, and ultimately abandoned as far too complicated for any small
Additional design considerations apply to Internationalization.
A number of designs for the error checking machinery were abandoned, some
after experiments with implementations. Totally automatic error checking was
attempted in particular. But automatic error checking tended to make the overall
library design much more complicated.
Some designs associated error checking mechanisms with paths. Some with
operations functions. A policy-based error checking template design was
partially implemented, then abandoned as too complicated for everyday
The final design, which depends partially on explicit error checking function
calls, is much simpler and straightforward, although it does depend to
some extent on programmer discipline. But it should allow programmers who
are concerned about portability to be reasonably sure that their programs will
work correctly on their choice of target systems.
||IBM Corporation, z/OS V1R3.0 C/C++ Run-Time
Library Reference, SA22-7821-02, 2001,
||International Standards Organization, 1988
||UTF-8 and Unicode FAQ for Unix/Linux,
||Microsoft Platform SDK for Windows, Storage Start
||IEEE Std 1003.1-2001, ISO/IEC 9945:2002, and The Open Group Base Specifications, Issue 6. Also known as The
Single UNIX® Specification, Version 3.
Available from each of the organizations involved in its creation. For
example, read online or download from
www.unix.org/single_unix_specification/. The ISO JTC1/SC22/WG15 - POSIX
||RFC-2396, Uniform Resource Identifiers (URI): Generic
||William Wulf, Mary Shaw, Global
Variable Considered Harmful, ACM SIGPLAN Notices, 8, 2, 1973, pp. 23-34
© Copyright Beman Dawes, 2002
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