npio: Numpy Input and Output for C

Features

• A tiny (~ 20 kB) C library for reading and writing version 1 Python/numpy .npy files.
• No dependencies.

Limitations

• Only tested on little endian machines (AARCH64, x86_64).
• Not everything is supported, see some plans in the CHANGELOG list.

Minimal example

Loading data from a pre-defined filename and showing what it contains:

#include <npio.h>
#include <stdlib.h>

int main(int argc, char ** argv)
{
    npio_t * np = npio_load("numpy_io_ut_2x2.npy");
    npio_print(stdout, np);
    npio_free(np);
    return EXIT_SUCCESS;
}

Which will give the following output:

$ gcc minimal.c -lnpio
$ ./a.out
filename: numpy_io_ut_2x2.npy
descr: '<f4' (little endian, float, 4 bytes)
np_byte_order: '<'
np_type: 'f'
fortran_order: 0
ndim: 2
shape_str: '(2, 2,)'
shape: [2, 2]
nel: 4
size of data: 4 x 4 = 16 B

Actual data can be found in the npio_t object.

Build and install

mkdir build
cd build
cmake ../
sudo make install

Validation

• Passes valgrind.
• Passes -fanalyzer.
• Some cases are covered by the Python script /npio_test_suite.py (which calls ./npio).
• Self tests are run by ./npio --unittest.
• Fuzzing?
• Other evil edge cases.

Command Line Utility

The command line utility exists mostly to demonstrate how to use the library. However it can also be used to quickly inspect .npy files:

./npio numpy_io_ut_2x2.npy
Loading numpy_io_ut_2x2.npy
filename: numpy_io_ut_2x2.npy
descr: '<f8' (little endian, float, 8 bytes)
np_byte_order: '<'
np_type: 'f'
fortran_order: 0
ndim: 2
shape_str: '(2, 2,)'
shape: [2, 2]
nel: 4
size of data: 4 x 8 = 32 B

build it from the makefile.

References

• NEP 1 — A Simple File Format for NumPy Arrays
• numpy/format.py