c++ - C++ 从 'char*' 到 'unsigned char*' 的无效转换?
问题描述
我有一个在树莓派中播放音频的代码。我的编译器是 GCC。
当我编译代码时,我得到了错误:
invalid conversion from ‘char*’ to ‘unsigned char*’.
该错误在调用 时触发ao_play(dev, buffer, done);
。
我该如何解决?我知道问题出在铸造上。但是我如何才能在提到的行中进行投射?( 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;
}
解决方案
这是Xiph上给出的原型:
int ao_play(ao_device *device, char *output_samples, uint_32 num_bytes);
这表明buffer
应该是一个char*
,而不是一个unsigned char*
。
现在的问题是mpg123表明调用mpg123_read
requires unsigned char*
:
int mpg123_read (mpg123_handle *mh, unsigned char *outmemory, size_t outmemsize, size_t *done)
根据basic.lvalchar*
,从to转换是安全/不是未定义的行为unsigned char*
,所以传递reinterpret_cast<char*>(buffer)
toao_play
而不是 plain buffer
。
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