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Faiss C API

Faiss provides a pure C interface, which can subsequently be used either in pure C programs or to produce bindings for programming languages with Foreign Function Interface (FFI) support. Although this is not required for the Python interface, some other programming languages (e.g. Rust and Julia) do not have SWIG support.

Compilation instructions

The full contents of the pure C API are in the "c_api" folder. Please be sure to follow the instructions on building the main C++ library first. Include -DFAISS_ENABLE_C_API=ON to the cmake command.

make -C build

This builds the dynamic library "faiss_c", containing the full implementation of Faiss and the necessary wrappers for the C interface. It does not depend on libfaiss.a or the C++ standard library.

To build the example program, you should run make -C build example_c at the top level of the faiss repo. The example program will be in build/c_api/example_c .

Using the API

The C API is composed of:

  • A set of C header files comprising the main Faiss interfaces, converted for use in C. Each file follows the format «name»_c.h, where «name» is the respective name from the C++ API. For example, the file Index_c.h file corresponds to the base Index API. Functions are declared with the faiss_ prefix (e.g. faiss_IndexFlat_new), whereas new types have the Faiss prefix (e.g. FaissIndex, FaissMetricType, ...).
  • A dynamic library, compiled from the sources in the same folder, encloses the implementation of the library and wrapper functions.

The index factory is available via the faiss_index_factory function in AutoTune_c.h:

FaissIndex* index = NULL;
int c = faiss_index_factory(&index, 64, "Flat", METRIC_L2);
if (c) {
    // operation failed
}

Most operations that you would find as member functions are available with the format faiss_«classname»_«member».

idx_t ntotal = faiss_Index_ntotal(index);

Since this is C, the index needs to be freed manually in the end:

faiss_Index_free(index);

Error handling is done by examining the error code returned by operations with recoverable errors. The code identifies the type of exception that rose from the implementation. Fetching the corresponding error message can be done by calling the function faiss_get_last_error() from error_c.h. Getter functions and free functions do not return an error code.

int c = faiss_Index_add(index, nb, xb);
if (c) {
    printf("%s", faiss_get_last_error());
    exit(-1);
}

An example is included, which is built automatically for the target all. It can also be built separately:

make bin/example_c

Building with GPU support

For GPU support, a separate dynamic library in the "c_api/gpu" directory needs to be built.

make

The "gpufaiss_c" dynamic library contains the GPU and CPU implementations of Faiss, which means that it can be used in place of "faiss_c". The same library will dynamically link with the CUDA runtime and cuBLAS.

Using the GPU with the C API

A standard GPU resources object can be obtained by the name FaissStandardGpuResources:

FaissStandardGpuResources* gpu_res = NULL;
int c = faiss_StandardGpuResources_new(&gpu_res);
if (c) {
    printf("%s", faiss_get_last_error());
    exit(-1);
}

Similarly to the C++ API, a CPU index can be converted to a GPU index:

FaissIndex* cpu_index = NULL;
int c = faiss_index_factory(&cpu_index, d, "Flat", METRIC_L2);
if (c) { /* ... */ }
FaissGpuIndex* gpu_index = NULL;
c = faiss_index_cpu_to_gpu(gpu_res, 0, cpu_index, &gpu_index);
if (c) { /* ... */ }

A more complete example is available by the name bin/example_gpu_c.