WFDB-x

A fast, lightweight implementation of the WFDB library and command-line tools, with identical behavior across seven programming languages

The WFDB Software Package (by George B. Moody and contributors) is the reference C library and set of command-line tools for reading, writing and converting physiologic signal and annotation files, such as the PhysioNet databases. WFDB-x is a faithful reimplementation of its version 10.7.0 in C++, Python, MATLAB/Octave, Rust, Julia, R and Kotlin, so that developers can use the same files, the same functions and the same tools without the C library.

WFDB-x is an independent project. It is not part of the WFDB project or of PhysioNet, and is not endorsed by them. It follows the WFDB format and the behavior of the original C package; see Related projects for the official WFDB software.

Each implementation:

• reads and writes exactly the same files as the original C library: headers (.hea), signal files in every WFDB storage format (8, 16, 61, 80, 160, 212, 310, 311, 24, 32, and the FLAC formats 508/516/524), annotation files (MIT and AHA formats), EDF/EDF+, calibration and info files, and multi-segment records;
• provides the same command-line tools (33 of them, listed below), with the same options;
• provides the C library API, with the C function names (isigopen, getvec, getann, putann, strtim, ...), and a high-level API in the idiom of its language;
• is lightweight: no dependencies beyond the language's standard library (except NumPy for Python). FLAC is supported without libFLAC. Reading records from URLs uses the system curl program in C++ and Rust (and in R and MATLAB/Octave when it is installed).

The original C sources are kept, unmodified, in third_party/wfdb-main.zip, and serve as the reference implementation for testing.

WFDB-x at a glance

(Also as a PNG image: docs/overview.png.)

Why WFDB-x

For more than four decades, the WFDB Software Package has been the common ground of physiologic signal processing. PhysioNet distributes its databases in WFDB format, and these databases are among the most widely used in the field. Thousands of published studies, open-source tools and machine-learning benchmarks rest on what the WFDB library returns when it reads a record, and on what it writes.

Researchers work with these records in many languages. WFDB-x offers one behavior across seven of them: it takes a single specification, the original C package, and reimplements it in Python, C++, MATLAB/Octave, Rust, Julia, R and Kotlin, tested to be identical, byte for byte, to the original and to each other, for reading and for writing. A record read in Julia gives exactly the samples it gives in C++, and a file written in R is exactly the file the original tools write. Researchers can choose a language for their work, and move between languages, with the same results. The Kotlin implementation brings the same behavior to mobile applications (Android, and iOS through Kotlin Multiplatform) and to the JVM.

Deliberate differences from the original

WFDB-x behaves exactly as the original package, with a few deliberate exceptions. They are the same in all seven languages and tested like everything else:

• No built-in signal parameters. WFDB-x never assumes a sampling frequency, gain, ADC resolution or units that a record does not give. Tools that write them require them, for example wrsamp -F 250 -G 200 -R 16 -U mV.

• Files are read and written only where they are named. A full path, a ~ path or a URL is used as given. Any other name is read from the input directory and written to the output directory, which are both the current directory unless they are set. Nothing is searched.

• Network access is off unless it is turned on. To read from PhysioNet, set the input directory to the database's URL and turn network access on. For example, to print the first 10 seconds of record 100 of the MIT-BIH Arrhythmia Database:

  export WFDBX_INPUT_DIR=https://physionet.org/files/mitdb/1.0.0/
  export WFDBX_NETWORK=1
  rdsamp -r 100 -t 10

• Annotation files are sorted without losing annotations, inside the library rather than by running a separate program.

• A tool never writes a signal file over a signal file it is reading.

• A write error gives exit status 1. When an output file or the standard output cannot be written, a tool says so and exits with status 1, as the standard Unix tools do; the original exited with status 0.

docs/DIVERGENCES.md explains each difference and the reasons for it. Legacy mode reproduces the original package exactly, for reproducing old results: --legacy as the first argument of a tool, or the legacy option of the library and of the high-level functions.

What "identical behavior" means, and how it is checked

For the same input, a WFDB-x tool produces byte for byte the same standard output, standard error (including error messages), exit status and output files as the original C tool. The library functions return the same values as their C counterparts. (The exceptions are the deliberate divergences above; the reference results are produced by the C code with these changes applied, so they are tested in the same way.)

This is verified by tests whose expected results were produced by the original C code, not written by hand:

• Conformance suite (tests/conformance/, described in docs/conformance.md): 1098 command-line test cases, run by a language-independent harness. Their expected results were produced by the C programs, built from third_party/ with FLAC support and with tests/reference/divergences.patch applied (tests/reference/build_reference.sh). The exceptions are 20 cases whose input makes the C programs crash; their expected results are the behavior that WFDB-x defines for that input (see docs/DIVERGENCES.md). Every implementation passes all of them on the reference platform, x86-64 Linux with glibc (Python 3.13, GCC 13, Rust 1.97, Julia 1.13, R 4.3, GNU Octave 8.4 and OpenJDK 21 for Kotlin). The MATLAB/Octave code is tested with GNU Octave, also through the MATLAB driver (matlab/bin/wfdbx_matlab.sh); it is written for MATLAB too, but MATLAB itself has not been available to run the tests. Other platforms can differ where the C behavior depends on the platform (see docs/KNOWN_C_BEHAVIORS.md).
• Test records (tests/data/): written by the C library itself (tests/reference/gen_records.c). They cover every storage format, multi-frequency and multi-segment (fixed and variable layout) records, skewed signals, byte offsets, block-structured files, EDF, and annotation files using every feature of the annotation format.
• Library unit tests in each language directory, with expected values printed by the C library (tests/reference/api_expect.c).

A few behaviors of the C code are undefined (crashes or reads of uninitialized memory), and many are surprising but well-defined. Both kinds are documented in docs/KNOWN_C_BEHAVIORS.md, with the behavior WFDB-x chose where C's is undefined.

Implementations

All seven implement the complete library and all 33 tools (see docs/INVENTORY.md for the details). Each directory has a README with installation instructions, usage examples and requirements.

Language	Directory and README	Install (from a copy of this repository)	Run a tool
Python	python/	pip install wfdb-x (PyPI), or pip install ./python	wfdbx rdsamp -r 100s
C++	cpp/	cmake -S cpp -B cpp/build && cmake --build cpp/build	rdsamp -r 100s
MATLAB / GNU Octave	matlab/	addpath('/path/to/wfdb-x/matlab') (tested with GNU Octave; MATLAB untested)	wfdbx rdsamp -r 100s
Rust	rust/	cargo install --path rust	rdsamp -r 100s
Julia	julia/	using Pkg; Pkg.develop(path="/path/to/wfdb-x/julia")	wfdbx rdsamp -r 100s
R	r/	R CMD INSTALL r	wfdbx rdsamp -r 100s
Kotlin (Android, iOS, JVM)	kotlin/	cd kotlin && ./gradlew publishToMavenLocal, then implementation("io.github.alphanumericslab:wfdbx:1.1.0") (Gradle, with mavenLocal())	kotlin/bin/wfdbx rdsamp -r 100s

Record 100s is the sample record in tests/data. Run the tools there, or point WFDBX_INPUT_DIR at that directory; the Python package also includes it (see python/README.md).

In Julia, R and MATLAB/Octave, wfdbx is a short alias for the language's own launcher of the tools; each README shows how to set it up. The Python package is distributed on PyPI as wfdb-x, with example notebooks and a sample record (wfdbx --examples copies the notebooks). In code, the packages are named wfdbx (Python, MATLAB, Rust, R, Kotlin, and the C++ namespace) or WFDBX (Julia).

Three ways to use WFDB-x

Every language offers the same three ways in. They are written natively in the language, and none of them calls the C library or another language.

	What it is	Use it for
1. The high-level API (recommended)	A few functions in the idiom of the language: read_record, read_header, read_annotations, write_record, write_annotations and check_record (readRecord, ... in Kotlin). One call reads a whole record or annotation file into arrays (NumPy, Vector, matrix, ...), with named options, errors reported as exceptions or error values, and fast block reads.	New code, analysis, notebooks, machine learning: anything that wants a record as arrays. It has no counterpart in the C package.
2. The WFDB library functions	The C library API under its C names (isigopen, getvec, getann, putann, strtim, ...), with the C behavior, frame by frame and annotation by annotation.	Porting a program written against the C library, or processing a record as a stream with the exact C semantics.
3. The command-line tools	The 33 tools (rdsamp, rdann, wrsamp, xform, ...) with the options, output, messages and exit status of the C tools, implemented in each language.	Shell scripts and pipelines, and reproducing work done with the original tools, without installing the C package.

The high-level API is built on the library functions, and returns exactly the samples and annotations that they (and the C library) give. The tools are built on the library functions too. Each language's README shows all three; see the table under Implementations for how each language runs the tools.

Performance

docs/BENCHMARKS.md compares the speed and memory use of the seven implementations, of the original C library and tools, and of wfdb-python, on the same records and the same machine, verifies that they all give identical results, and explains how to get the most speed from WFDB-x in application code. In short:

• Rust, Julia and C++ are the fastest: an hour of a two-signal ECG is read in 4 to 6 ms, 4 to 7 times faster than with the C library (26 ms), which reads one frame per getvec call. The C library reads annotation files fastest.
• The C++ and Rust rdsamp print that hour 4 to 5 times as fast as the C rdsamp; the C tools start fastest (1.3 ms, against about 2 ms).
• Kotlin reads records 1.2 to 2.1 times slower than C++ (annotation files 4 to 8 times).
• Python reads records at speeds close to those of wfdb-python and the C library (from 1.7 times slower than C to 2.4 times faster, depending on the record), and writes them 9 times faster than wfdb-python.
• R and MATLAB/Octave take 46 to 67 ms for that hour, fast enough for most records.

Command-line tools

Signal and annotation tools from app/: rdsamp, wrsamp, rdann, wrann, sortann, ann2rr, rr2ann, wfdbdesc, wfdbtime, time2sec, sampfreq, signame, signum, sigamp, snip, xform, wfdbcat, wfdbwhich.

Format converters from convert/: edf2mit, mit2edf, rdedfann, wav2mit, mit2wav, wfdb2mat, a2m, m2a, ad2m, md2a, ahaecg2mit, makeid, readid, revise, parsescp.

The interactive viewer (WAVE), the plotting tools, and the analysis programs of psd/ are outside the scope of this project.

Documentation

• docs/DIVERGENCES.md: the deliberate differences from the original package, the reasons for them, and legacy mode
• docs/INVENTORY.md: every tool and library function, with its status in each language
• docs/conformance.md: the conformance suite, and how to run it for any implementation
• docs/BENCHMARKS.md: speed and memory use in each language, and how to read records fastest
• docs/RELEASING.md: how to release the Python package on PyPI (accounts, API tokens, TestPyPI, PyPI)
• docs/KNOWN_C_BEHAVIORS.md: reproduced C behaviors, and the handling of undefined ones
• docs/porting-guide.md: architecture, and the C semantics that every implementation must reproduce
• docs/AUTHORS.md: the authors of the C code that each part of WFDB-x ports
• The WFDB Applications Guide (third_party/wag.pdf) describes the tools, and the WFDB Programmer's Guide (in third_party/wfdb-main.zip) the library API; WFDB-x reproduces both.

How WFDB-x was developed

WFDB-x was built through AI-assisted development, cross-language verification and unit testing.

• AI-assisted development. We designed the project and its requirements: faithfulness to the original C package, identical behavior across languages, the testing methodology, and deliberate design decisions such as removing built-in signal parameters. We directed and reviewed the work. The code, tests and documentation were written with Claude Code, Anthropic's AI coding assistant, working from the original C sources, and were reviewed and tested by human engineers.
• Cross-language verification. Every implementation is checked against the outputs of the original C tools and library on the conformance suite: 1098 cases, plus 877 for legacy mode. Because all seven must pass the same cases, each implementation is also checked against the other six.
• Unit testing. Each implementation has its own unit tests of the library and of the high-level functions, with expected values printed by the C library.

Reports of any difference from the original package are welcome as GitHub issues.

Versions

WFDB-x has its own version numbers, shared by the seven implementations and following semantic versioning; the current version is 1.1.0. They are independent of the version of the WFDB Software Package that WFDB-x implements, 10.7.0, which the library still reports as in C (wfdbversion() returns "10.7.0", and wfdberror() names it).

Related projects

These projects and initiatives are related to WFDB-x, which does not use or depend on any of them. WFDB-x is a port of the original C package to other languages, and the C programs produce the expected results of its tests. The other projects are acknowledged as related work.

• The WFDB project maintains the WFDB format specification and the official WFDB software, and coordinates their development through its working group. WFDB was originally developed at MIT by George B. Moody in 1989, and builds on decades of work by him, Roger G. Mark and the broader research community.
• The WFDB Software Package, by George B. Moody, Benjamin Moody and contributors, distributed by PhysioNet: the original C library and tools. WFDB-x reimplements its version 10.7.0.
• WFDB Python and the WFDB Toolbox for MATLAB: the Python and MATLAB implementations of the WFDB project, alongside its C package.
• wfdbcheck, by Benjamin Moody: a tool that finds common problems in WFDB records, from metadata errors to out-of-range samples and annotation errors.

Authors and license

WFDB-x was developed by Reza Sameni (GitHub) (2026), through AI-assisted development, cross-language verification and unit testing (see How WFDB-x was developed). It is an independent reimplementation of the WFDB Software Package: its code in each language is a translation of the package's C code, written for WFDB-x, not code by the original WFDB developers. The C library and tools were written by George B. Moody, Benjamin Moody and the other contributors credited, file by file, in each WFDB-x source file and in docs/AUTHORS.md.

WFDB-x is distributed under the GNU General Public License, version 3 or (at your option) any later version (LICENSE). The original C sources in third_party/ keep their own licenses: the GNU Library General Public License, version 2 or later, for the library, and the GNU General Public License, version 2 or later, for the tools.

The original WFDB Software Package is Copyright (C) 1983-2022 George B. Moody and the WFDB contributors.