Recording using sox in c/c++

Question

I am trying to record sound using microphone and sox library in C/C++.

sox_open_read("default", &_input->signal, NULL, NULL)

I am trying to use default input device. I am getting the error

formats: can't open input file `default': No such file or directory

Which I guess is caused because I did not pass the last argument: filetype and sox tries to find a file with name 'default'.
Sox lists:

Audio file formats: 8svx aif aifc aiff aiffc al amb au avr caf cdda cdr cvs cvsd cvu dat dvms f32 f4 f64 f8 fap flac fssd gsm gsrt hcom htk ima ircam la lpc lpc10 lu mat mat4 mat5 maud mp2 mp3 nist ogg opus paf prc pvf raw s1 s16 s2 s24 s3 s32 s4 s8 sb sd2 sds sf sl sln smp snd sndfile sndr sndt sou sox sph sw txw u1 u16 u2 u24 u3 u32 u4 u8 ub ul uw vms voc vorbis vox w64 wav wavpcm wv wve xa xi
audio devices drivers: alsa, oss, ossdsp

What should I pass to the sox_open_read function as the last parameter to use a microphone as an input?

The file type would be something like "WAV" or "MP3", it seems. The lack of documentation for even such basic parameters is telltale, though. The latest release of sox itself is from 2015; libsox appears to be equally abandoned. The man page has a copyright date in 2011. — MSalters, Sep 10 '20 at 11:50
I looked into sox sources and I believe I should put "alsa" as the last parameter. I cannot confirm it, because the app crashes immediately after sox_open_read call, but I might be due to the system faulty setup and not the wrong parameter. I don't know yet. — Chris, Sep 10 '20 at 21:39

score 1 · Accepted Answer · answered Sep 15 '20 at 13:23

As the last parameter to sox_open_read function for microphone input, one of the audio devices drivers should be passed. In my case, it is 'alsa'.
Example:

#include <sox.h>
#include <memory>

sox_signalinfo_t _intermediateSignal;
sox_format_t* input;
sox_format_t* output;
sox_effects_chain_t* effectsChain;

void addEffect(std::string effectName, sox_format_t* options) {
    std::unique_ptr<sox_effect_t> effect(sox_create_effect(sox_find_effect(effectName.c_str())));
    char *args[] = {reinterpret_cast<char *>(options)};
    sox_effect_options(effect.get(), 1, args);
    sox_add_effect(effectsChain, effect.get(), &_intermediateSignal, &input->signal);
}

int main() {
    if (sox_init() != SOX_SUCCESS)
        throw std::runtime_error("Could not initialise SOX.");

    input = sox_open_read("default", NULL, NULL, "alsa");
    output = sox_open_write("recorded.wav", &input->signal, NULL, NULL, NULL, NULL);
    if (!input || !output)
        throw std::runtime_error("SOX I/O error");

    _intermediateSignal = input->signal;

    effectsChain = sox_create_effects_chain(&input->encoding, &output->encoding);

    if (!effectsChain)
        throw std::runtime_error("SOX could not initialize effects chain.");

    addEffect("input", input);
    addEffect("output", output);

    sox_flow_effects(effectsChain, NULL, NULL);
    sox_quit();
}

This example will never finish as sox_flow_effects call blocks the execution. Once the program is killed with ctrl+c, recorded.wav contains recorded audio.

Recording using sox in c/c++

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