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PDCLib - the Public Domain C Library

What is it

This is a C Standard Library - what's defined in ISO/IEC 9899 "Information technology — Programming languages — C" or extensions to the above defined in ISO/IEC 14882 "Information technology — Programming languages — C++". A few extensions may optionally be provided.


Written in
  • 2003-2012 by Martin "Solar" Baute,
  • 2012- by Owen Shepherd

To the extent possible under law, the author(s) have dedicated all copyright and related and neighboring rights to this software to the public domain worldwide. This software is distributed without any warranty.

You should have received a copy of the CC0 Public Domain Dedication along with this software. If not, see <>.


Unicode Character Data

PDCLib necessarily includes Unicode character data derived from that provided by Unicode, Inc in its' implementation of the localization and wide character support (in particular for use by the ctype.h and wctype.h functions.)

Unicode, Inc licenses that data under a license agreement which can be found at <>, or in the file UNICODE_DATA_LICENSE.txt. found in the same directory as this file.

Test Suite

Portions of the test suite are under different licenses. Where this is the case, it is clearly noted in the relevant location.

The license of this code has no bearing upon the licensing of the built library (as it does not comprise part of it).

At the time this was written, this exception only applies to portions of the printf test suite, which are released under the terms of the 2-clause BSD license (see testing/printf_testcases.h for full details)

Terms for extensions

Extensions are permitted only if they pass the following tests:

Pre-existing wide usage
On most systems, the system C library must maintain its application binary interface for long periods of time (potentially eternity). Existing wide usage demonstrates utility
In keeping with the spirit of the standard
The extension should respect the design, intentions and conventions of the C standard, and feel like a natural extension to the offered capability.
Not system dependent
The extension should not add any additional dependencies on the underlying system
Extensions should not duplicate functionality already provided by the standard
Disabled by default
PDCLib will always default to a "strictly conforming" mode exposing only functionality offered by the version of the standard specified by the __STDC_VERSION__, __STDC__ or __cplusplus macro; extensions will only be exposed when requested.

Additionally, extra consideration will be given to extensions which are difficult or impossible to implement without access to internal structures of the C library.

Conrete Examples:

Included. strndup is easily defined in terms of existing standard functions, follows the standard's naming conventions, is in wide usage, and does not duplicate features already provided.
Rejected. Has existing wide usage, is not system dependent (can be implemented, albeit inefficiently, on top of malloc), but naming is not consistent with the naming used by the standard (posix_ prefix) and duplicates functionality provided by the C11 standard
open, close, read, write, ...
Rejected. Widely used, but duplicates functionality provided by the standard (FILE objects set to be unbuffered), and not able to implement full semantics (e.g. in relation to POSIX fork and other functionality from the same defining standard) in a platform-neutral way
Rejected. Used somewhat widely, in keeping with the standard, not system dependent, but duplicative of functionality provided by (optional) Annex K of the C standard.
flockfile, funlockfile, getc_unlocked, putc_unlocked, fwrite_unlocked, ...
Accepted. Provide functionality not provided by the standard (and useful in light of the C11 addition of threading). Can be trivially implemented in terms of the <threads.h> mutex functions and the bodies of the existing I/O functions, and impossible to implement externally


As a namespace convention, everything (files, typedefs, functions, macros) not defined in ISO/IEC 9899 is prefixed with _PDCLIB. The standard defines any identifiers starting with '_' and a capital letter as reserved for the implementation, and since the chances of your compiler using an identifier in the _PDCLIB range are slim, any strictly conforming application should work with this library.

PDCLib consists of several parts:

  1. standard headers;
  2. implementation files for standard functions;
  3. internal header files keeping complex stuff out of the standard headers;
  4. the central, platform-specific file _PDCLIB_config.h;
  5. platform-specific implementation files;

The standard headers (in ./includes/) only contain what they are defined to contain. Where additional logic or macro magic is necessary, that is deferred to the internal files. This has been done so that the headers are actually educational as to what they provide (as opposed to how the library does it).

There is a seperate implementation file (in ./function/{header}/) for every function defined by the standard, named {function}.c. Not only does this avoid linking in huge amounts of unused code when you use but a single function, it also allows the optimization overlay to work (see below).

(The directory ./functions/_PDCLIB/ contains internal and helper functions that are not part of the standard.)

Then there are internal header files (in ./internal/), which contain all the "black magic" and "code fu" that was kept out of the standard headers. You should not have to touch them if you want to adapt PDCLib to a new platform. Note that, if you do have to touch them, I would consider it a serious design flaw, and would be happy to fix it in the next PDCLib release. Any adaption work should be covered by the steps detailed below.

For adapting PDCLib to a new platform (the trinity of CPU, operating system, and compiler), make a copy of ./platform/example/ named ./platform/{your_platform}/, and modify the files of your copy to suit the constraints of your platform. When you are done, copy the contents of your platform directory over the source directory structure of PDCLib (or link them into the appropriate places). That should be all that is actually required to make PDCLib work for your platform.

Future directions

Obviously, full C89, C99 and C11 conformance; and full support for the applicable portions of C++98, C++03 and C++11 (the version which acomplishes this will be christened "1.0").

Support for "optimization overlays." These would allow efficient implementations of certain functions on individual platforms, for example memcpy, strcpy and memset. This requires further work to only compile in one version of a given function.

Post 1.0, support for C11 Annexe K "Bounds checking interfaces"

Development Status

v0.1 - 2004-12-12
Freestanding-only C99 implementation without any overlay, and missing the INTN_C() / UINTN_C() macros. <float.h> still has the enquire.c values hardcoded into it; not sure whether to include enquire.c in the package, to leave <float.h> to the overlay, or devise some parameterized macro magic as for <limits.h> / <stdint.h>. Not thoroughly tested, but I had to make the 0.1 release sometime so why not now.
v0.2 - 2005-01-12
Adds implementations for <string.h> (excluding strerror()), INTN_C() / UINTN_C() macros, and some improvements in the internal headers. Test drivers still missing, but added warnings about that.
v0.3 - 2005-11-21
Adds test drivers, fixes some bugs in <string.h>.
v0.4 - 2005-02-06
Implementations for parts of <stdlib.h>. Still missing are the floating point conversions, and the wide-/multibyte-character functions.
v0.4.1 - 2006-11-16

With v0.5 (<stdio.h>) taking longer than expected, v0.4.1 was set up as a backport of bugfixes in the current development code.

  • #1 realloc( NULL, size ) fails
  • #2 stdlib.h - insufficient documentation
  • #4 Misspelled name in credits
  • #5 malloc() splits off too-small nodes
  • #6 qsort() stack overflow
  • #7 malloc() bug in list handling
  • #8 strncmp() does not terminate at '0'
  • #9 stdint.h dysfunctional
  • #10 NULL redefinition warnings
v0.5 - 2010-12-22
Implementations for <inttypes.h>, <errno.h>, most parts of <stdio.h>, and strerror() from <string.h>. Still no locale / wide-char support. Enabled all GCC compiler warnings I could find, and fixed everything that threw a warning. (You see this, maintainers of Open Source software? No warnings whatsoever. Stop telling me it cannot be done.) Fixed all known bugs in the v0.4 release.

Near Future

Current development directions are:

Implement portions of the C11 standard that have a direct impact on the way that PDCLib itself is built. For example, in order to support multithreading, PDCLib needs a threading abstraction; therefore, C11's thread library is being implemented to provide the backing for this (as there is no purpose in implementing two abstractions)

Modularize the library somewhat. This can already be seen with components under "opt/". This structure is preliminary; it will likely change as the process continues.