Passing two parameters (int and array) to embedded Python function

Question

I need to call Python function from my module and set two parameters for it: int and array.

For a now I get segfault during calling this function and I have no idea what I'm doing wrong. Could someone specify where my mistake is?

Function in my Python module app.py. It works if I call it from Python code:

def get_model(rate, signal):
    mfcc_train = MFCC().compute(rate, signal) 
    with open('mfcc_test', 'wb') as f:
        pickle.dump(mfcc_train, f)
    return clf()._fit(mfcc_train)

My C code that calls the function above. The last pring is "Before calling"

#include <Python.h>
#include <stdio.h>
#include "wav.h"
#include <numpy/arrayobject.h>

int main(int argc, char *argv[])
{
    PyObject *pName, *pModule, *pDict, *pFunc, *pValue, *pArgs;
    uint8_t *samples = NULL;

    wavread("test.wav", &samples);

    printf("No. of channels: %d\n",     header->num_channels);
    printf("Sample rate:     %d\n",     header->sample_rate);
    printf("Bit rate:        %dkbps\n", header->byte_rate*8 / 1000);
    printf("Bits per sample: %d\n\n",     header->bps);
    printf("Sample 0:        %d\n", samples[0]);
    printf("Sample 1:        %d\n", samples[1]);
    // Initialize the Python Interpreter
    printf("Before init\n");
    Py_Initialize();
    PyObject *sysPath = PySys_GetObject("path");
    const char *scriptDirectoryName = ".";
    PyObject *path = PyUnicode_FromString(scriptDirectoryName);
    int result = PyList_Insert(sysPath, 0, path);
    printf("after init\n");
    // Build the name object
    pName = PyUnicode_DecodeFSDefault(argv[1]);
    printf("after pname %s %d\n", argv[1], pName == NULL ? 1 : 0);

    // Load the module object
    pModule = PyImport_Import(pName);
    printf("after pmodule %d\n", pModule == NULL ? 1 : 0);

    // pFunc is also a borrowed reference 
    pFunc = PyObject_GetAttrString(pModule, "get_model");
    printf("after pfunc\n");

    if (PyCallable_Check(pFunc)) 
    {
        pArgs = PyTuple_New(2);
        printf("after pytuple\n");
        PyTuple_SetItem(pArgs, 0, PyLong_FromLong(header->sample_rate));
        printf("after set item\n");
        uint8_t* array = malloc(header->datachunk_size);
        int dims[1];
        dims[0] = header->datachunk_size;
        printf("alloc\n");
        import_array();
        PyObject* pSamples = PyArray_SimpleNewFromData(1, dims, NPY_INT8, (void*)samples);
        printf("pSamples\n");
        PyArray_ENABLEFLAGS((PyArrayObject*)pSamples, NPY_ARRAY_OWNDATA);
        PyTuple_SetItem(pArgs, 1, pSamples);
        printf("Before calling\n");
        pValue = PyObject_CallObject(pFunc, pArgs);
        printf("After calling\n");
    } else 
    {
        PyErr_Print();
    }

    printf("pValue:        %d\n", pValue);
    // Clean up
    Py_DECREF(pModule);
    Py_DECREF(pFunc);
    Py_DECREF(pName);

    // Finish the Python Interpreter
    Py_Finalize();

    free(header);
    free(samples);
}

UPD: updated code where one issue was fixed. But another problem still exists. It's in line PyObject* pSamples = PyArray_SimpleNewFromData(1, dims, NPY_INT8, (void*)samples);. And I can't find out what is wrong with it.

And wav.h just in case:

#include <inttypes.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <err.h>
typedef struct {
    char     chunk_id[4];
    uint32_t chunk_size;
    char     format[4];
    char     fmtchunk_id[4];
    uint32_t fmtchunk_size;
    uint16_t audio_format;
    uint16_t num_channels;
    uint32_t sample_rate;
    uint32_t byte_rate;
    uint16_t block_align;
    uint16_t bps;
    char     datachunk_id[4];
    uint32_t datachunk_size;
}WavHeader;
WavHeader *header;
void wavread(char *file_name, int16_t **samples)
{
    int fd;
    if (!file_name)
        errx(1, "Filename not specified");
    if ((fd = open(file_name, O_RDONLY)) < 1)
        errx(1, "Error opening file");
    if (!header)
        header = (WavHeader*)malloc(sizeof(WavHeader));
    if (read(fd, header, sizeof(WavHeader)) < sizeof(WavHeader))
        errx(1, "File broken: header");
    if (strncmp(header->chunk_id, "RIFF", 4) ||
        strncmp(header->format, "WAVE", 4))
        errx(1, "Not a wav file");
    if (header->audio_format != 1)
        errx(1, "Only PCM encoding supported");
    if (*samples) free(*samples);
    *samples = (int16_t*)malloc(header->datachunk_size);
    if (!*samples)
        errx(1, "Error allocating memory");
    if (read(fd, *samples, header->datachunk_size) < header->datachunk_size)
        errx(1, "File broken: samples");
    close(fd);
}

Donz · Accepted Answer · 2017-08-21 14:02:27Z

2

It's difficult to tell without the definition of header but I believe the issue is in the line

PyTuple_SetItem(pArgs, 0, header->sample_rate);

PyTuple_SetItem expects a Python object and you're passing it what I think is an integer, which is being misinterpreted as a PyObject*.

I suspect you want

PyTuple_SetItem(pArgs, 0, PyInt_FromLong(header->sample_rate));

(PyLong_FromLong in Python3)

Second issue: you free samples twice. First you pass it to numpy and tell numpy that it owns the data:

PyObject* pSamples = PyArray_SimpleNewFromData(1, dims, NPY_INT8, (void*)samples);
PyArray_ENABLEFLAGS((PyArrayObject*)pSamples, NPY_ARRAY_OWNDATA);

then at the end of your code you free it

free(samples);

I suspect that you meant to pass your newly allocated array to numpy instead of samples. (You still need to copy the data between them too, if this is the case)

UPD: One more right solution from comments is to change type of dims from int to npy_intp

edited Aug 21, 2017 at 14:02

Donz

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answered Aug 17, 2017 at 17:52

DavidW

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7 Comments

Donz Over a year ago

Yes, you're right. I forgot to update this post yesterday when I found out this issue. But unfortunately this is not only problem in my code. Now I have trouble with this line: PyObject* pSamples = PyArray_SimpleNewFromData(1, dims, NPY_INT8, (void*)samples); It works but not in right way because sometime I got segfault at the end of execution and some another side effects.

Donz Over a year ago

And don't mind about the header wav.h. It's just for reading wav file. But just in case I added it to post.

Donz Over a year ago

Thank you, now I recognized that I don't need last free call. But my main issue is not connected with this. I get segfault every second time during PyObject_CallObject. And I don't understand what you mean in last sentence. I put wrapper pSamples into arguments. And it already contains necessary data, doesn't it?

DavidW Over a year ago

The last line was guessing that you do uint8_t* array = malloc(header->datachunk_size); with the intention of passing it to numpy instead of samples (in which case you'd need to copy the data). I could be guessing wrongly though

DavidW Over a year ago

I'm struggling to see anything else obviously wrong right now I'm afraid. Two small suggestions: 1) make dims a npy_intp just to check that you haven't got an integer type mismatch. 2) Most of your Python functions return NULL if there's an error - add checks after each one for that

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