C ++从'char *'到'unsigned char *'的无效转换？

Question

I have a code for playing audio in raspberry pi. My compiler is GCC.

When I compile the code, I get the error:

invalid conversion from ‘char*’ to ‘unsigned char*’.

The error is triggered at the call to ao_play(dev, buffer, done); .

How I can fix that? I know that the problem is for casting. but how I can casting in the mentioned line? ( ao_play(dev, buffer, done); )

/*
# install libraries :
sudo apt-get install libmpg123-dev
sudo apt-get install libao-dev

gcc -O2 -o mp3_cpp_audio mp3_cpp_audio.cpp -lmpg123 -lao

./mp3_cpp_audio /var/www/html/mp3_cpp_audio.mp3
*/

#include <ao/ao.h>
#include <mpg123.h>

#define BITS 8


int main(int argc, char *argv[]){
    mpg123_handle *mh;
    unsigned char *buffer;
    size_t buffer_size;
    size_t done;
    int err;

    int driver;
    ao_device *dev;

    ao_sample_format format;
    int channels, encoding;
    long rate;

    if(argc < 2)
        exit(0);

    /* initializations */
    ao_initialize();
    driver = ao_default_driver_id();
    mpg123_init();
    mh = mpg123_new(NULL, &err);
    buffer_size = mpg123_outblock(mh);
    buffer = (unsigned char*) malloc(buffer_size * sizeof(unsigned char));

    /* open the file and get the decoding format */
    mpg123_open(mh, argv[1]);
    mpg123_getformat(mh, &rate, &channels, &encoding);

    /* set the output format and open the output device */
    format.bits = mpg123_encsize(encoding) * BITS;
    format.rate = rate;
    format.channels = channels;
    format.byte_format = AO_FMT_NATIVE;
    format.matrix = 0;
    dev = ao_open_live(driver, &format, NULL);

    /* decode and play */
    while (mpg123_read(mh, buffer, buffer_size, &done) == MPG123_OK)
        ao_play(dev, buffer, done);  // error belong to this line : invalid conversion from ‘char*’ to ‘unsigned char*’

    /* clean up */
    free(buffer);
    ao_close(dev);
    mpg123_close(mh);
    mpg123_delete(mh);
    mpg123_exit();
    ao_shutdown();

    return 0;
}

Answer 1

Here is the prototype as given on Xiph :

int ao_play(ao_device *device, char *output_samples, uint_32 num_bytes);

This indicates that buffer should be a char* , not an unsigned char* .

Now the issue is that mpg123 indicates that the call to mpg123_read requires unsigned char* :

int     mpg123_read (mpg123_handle *mh, unsigned char *outmemory, size_t outmemsize, size_t *done)

According to basic.lval , it is safe/not undefined behavior to convert from char* to unsigned char* , so pass reinterpret_cast<char*>(buffer) to ao_play instead of plain buffer .