Sunday, September 9, 2012

libav in C++

libav, which is intended for codecs, also serves as a nice signal processing library for scientific applications as it has assembly FFT routines optimized for various processors including x86 SSE and ARM NEON.  Written in C for C, it can be a little tricky to use in C++.

The first trick is to wrap the #includes within extern "C":

extern "C" {
    #include <libavutil/avutil.h>
    #include <libavcodec/avfft.h>

The second trick is that to use the nice C++ array types std::vector or boost::multi_array, it is necessary to use a custom allocator class that calls the libav av_malloc() and av_free().  The reason is that these ensure the 32-byte alignment that libav assumes (without checking every time) is present when it utilizes SIMD instructions.  Without a custom allocator, std::vector and boost::multi_array just use new[], which does not allocate aligned, and libav in the process of ANDing addresses ends up running past the end of the buffer and generating a segmentation fault.

The code below uses a custom allocator adapted from The C++ Standard Library -- a Tutorial and Reference.  The advantage of using std::vector or boost::multi_array, of course, is automated memory management using the Resource Acquisition Is Initialization pattern/idiom, similar to C++ auto_ptr (which can't be used for arrays because it is hard-coded to delete instead of delete[]).  Although std::vector is used below, the same allocator works equally well with boost::multi_array.


#include <cstring>
#include <vector>

extern "C" {
#include <libavutil/avutil.h>
#include <libavcodec/avfft.h>

template <typename T> class allocator_av {
    typedef T               value_type;
    typedef T*              pointer;
    typedef const T*        const_pointer;
    typedef T&              reference;
    typedef const T&        const_reference;
    typedef std::size_t     size_type;

    template <typename U>
    struct rebind {
        typedef allocator_av<U> other;

    allocator_av() {}
    template <typename U> allocator_av(const allocator_av<U>&) {}
    size_type max_size () const { return 1 << 16; }

    pointer allocate (size_type num, const void* = 0) {
        return  static_cast<pointer>(av_malloc(num*sizeof(T)));

    void construct (pointer p, const T& value) {}
    void destroy (pointer p) {}
    void deallocate (pointer p, size_type num) { av_free(p); }

int main(int argc, char** argv) {
    std::vector<FFTComplex, allocator_av<FFTComplex> > z(256);
    FFTContext* c = av_fft_init(8, 0);

1 comment:

njp said...

Thanks for posting this. It was a great help, allocator_av in particular.