Question

I was trying to PCM sound a little bit in Python, but all the packages I’ve tried were too much or were undocumented or dead, so I decided to make a simple one with libao.

I used as start point this source code by xiph.org that plays 440Hz for 1 second, and I compiled it with gcc -o ao_example ao_example.c -lao -ldl -lm and I run successfully this code, immediately hearing a 440Hz sine for 1 second in both channels.

So far, so good.

So I $ cp ao_exemple.c mySoundAo.c and I edited mySoundAo.c to be compiled as Python module. The full code follows:

 #include <math.h>
 #include <stdio.h>
 #include <string.h>
 #include <ao/ao.h>
 #include <Python.h>
 #define BUF_SIZE 4096

 static PyObject* py_soundAo(PyObject* self, PyObject* args)
 {
     ao_device *device;
     ao_sample_format format;
     int default_driver;
     char *buffer;
     int buf_size;
     int sample;
     float freq = 440.0;
     int i;
     /* -- Initialize -- */
     fprintf(stderr, "libao example program\n");
     ao_initialize();
     /* -- Setup for default driver -- */
     default_driver = ao_default_driver_id();
     memset(&format, 0, sizeof(format));
     format.bits = 16;
     format.channels = 2;
     format.rate = 44100;
     format.byte_format = AO_FMT_LITTLE;
     /* -- Open driver -- */
     device = ao_open_live(default_driver, &format, NULL /* no options */);
     if (device == NULL) {
         fprintf(stderr, "Error opening device.\n");
         return Py_BuildValue("", 0);
     }
     /* -- Play some stuff -- */
     buf_size = format.bits/8 * format.channels * format.rate;
     buffer = calloc(buf_size,
             sizeof(char));
     for (i = 0; i < format.rate; i++) {
         sample = (int)(0.75 * 32768.0 * sin(2 * M_PI * freq * ((float) i/format.rate)));
         /* Put the same stuff in left and right channel */
         buffer[4*i] = buffer[4*i+2] = sample & 0xff;
         buffer[4*i+1] = buffer[4*i+3] = (sample >> 8) & 0xff;
     }
     ao_play(device, buffer, buf_size);
     /* -- Close and shutdown -- */
     ao_close(device);
     ao_shutdown();
  return Py_BuildValue("", 0);
 }

 static PyMethodDef mySoundAo_methods[] = {
    {"soundAo", py_soundAo, METH_VARARGS},
    {NULL, NULL}
 };

 void initmySoundAo()
 {
    (void) Py_InitModule("mySoundAo", mySoundAo_methods);
 }

So I compiled as gcc -shared -I/usr/include/python2.7/ -o mySoundAo.so mySoundAo.c -lpython2.7 -lm -lsndfile -lao -ldl and I had this warning:

In file included from /usr/include/python2.7/Python.h:8:0,
             from mySoundAo.c:5:
/usr/include/python2.7/pyconfig.h:1158:0: warning: "_POSIX_C_SOURCE" redefined [enabled by default]
/usr/include/features.h:214:0: note: this is the location of the previous definition

Doesn’t sound much dangerous, so I moved on.

In python, I did the following:

$ python
Python 2.7.2+ (default, Oct  4 2011, 20:03:08) 
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mySoundAo
>>> mySoundAo.soundAo()
libao example program
Error opening device.
>>> 

With no sound. Inspecting the code a little bit, I discovered the function ao_initialize(); hangs for about 4 second and the following line default_driver = ao_default_driver_id(); sets this variable to -1 (error).

This behavior is strange, since it’s almost the same code.

So, any ideas to make this work?

Thank you!

Answer 1

The warning you get is harmless, simply moving #include <Python.h> to the top should let the standard library correctly recognize the macro is already defined.

The problem is probably caused by a miscompiled /usr/lib/ao/plugins-4/libalsa.so (this file is mentioned if you set debug in ~/.libao.conf). Since the ao’s alsa plugin fails to load, ao tries all other options, and exhausts the nas timeout of 4 seconds (that’s the cause for the delay).

To check whether the miscompiled (or mislinked) libalsa.so is the problem, run

$ ldd -r /usr/lib/ao/plugins-4/libalsa.so > /dev/null
undefined symbol: ao_is_big_endian      (/usr/lib/ao/plugins-4/libalsa.so)

The error in the output should points to a problem with the symbol. You can simply download libao yourself, and patch out the line in libao-*/src/plugins/alsa/ao_alsa.c, or copy the definition from ao_is_big_endian, or fix the linking.