Open Sound System
OSS 4.x Programmer's Guide

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A simple sample program for doing dull duplex using mmap


This is a sample program for doing full duplex using mmap.

Unfortunately this program doesn't work at this moment (under construction).

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/time.h>
#include <sys/types.h>
#include <soundcard.h>

Note that the mmap approach bypasses the OSS drivers completely so you are on your own. You will need to write the application so that it can work with any sample rate or sample format one can imagine.

We will try to use 48 kHz / 16 bits / stereo which is the most likely format supported by the consumer audio devices.

#define SAMPLE_RATE	48000
#define NUM_CHANNELS	2


The mmap() feature is not supported by all devices. Using it will make the application to fail with some devices (professional ones in particular).

When using the mmap mode it's very important to disable any format conversions done by OSS. This can be done by calling SNDCTL_DSP_COOKEDMODE _IMMEDIATELY_ after opening the device.

int sample_rate, num_channels, sample_format;

main (int argc, char *argv[])
  int fd, sz, i, tmp, n, l, have_data = 0;

/dev/dsp_mmap is the best default device. Another alternative is /dev/dsp but in some systems the user may want to use different devices with mmap applications than with the normal ones.

/dev/dsp_mmap will not be present in pre OSS 4.0 systems. The application may automatically try to open /dev/dsp if /dev/dsp_mmap is missing. Another approach is asking the user to create /dev/dsp_mmap.

ln -s /dev/dsp /dev/dsp_mmap

It's recommended that there is some method for configuring or selecting the audio device (such as a command line option or an environment variable.

  char *audio_dev = "/dev/dsp_mmap";

  struct buffmem_desc imemd, omemd;
  caddr_t buf;

  char *ip, *op;

  struct audio_buf_info info;

  int frag = 0x00200008;	/* 32 fragments of 2^8=256 bytes */

  fd_set reads, writes;

Getting the device name to open. First the command line method (simplified).

  if (argc >= 2)
      audio_dev = argv[1];

No device given on command line. Try to see if an environment variable is set.

We will use MYAPP_AUDIODEV as the variable name. Replace the MYAPP_ prefix with your application name.

      char *p;

      if ((p = getenv ("MYAPP_AUDIODEV")) != NULL)
	audio_dev = p;

  if ((fd = open (audio_dev, O_RDWR, 0)) == -1)
      perror (audio_dev);
      exit (-1);

Disable cooked mode to permit mmap() with some devices. Don't do any error checking since usually this call will fail. There is no need to care about the return value.

  tmp = 0;
  ioctl (fd, SNDCTL_DSP_COOKEDMODE, &tmp);	/* Don't check the error return */

  ioctl (fd, SNDCTL_DSP_SETFRAGMENT, &frag);	/* No error checking needed */

Set the sample format. AFMT_S16_NE is best because it is supported by practically all devices. Also it equals to the "short" data type.

  sample_format = SAMPLE_FORMAT;
  if (ioctl (fd, SNDCTL_DSP_SETFMT, &sample_format) == -1)
    {				/* Something really fatal occurred. */
      perror ("SNDCTL_DSP_SETFMT");
      exit (-1);

Check that we can support the returned sample format.

  switch (sample_format)
    case AFMT_S16_NE:

Everything is OK. The sample format equals to 16 bit native integer (short in C/C++) so we do not need to do any format conversions.


#if 0

We do not support any other formats. However you must be prepared to do so. Please take a look at the formats section of the OSS API manual.

Here are the most common other formats.

    case AFMT_S16_OE:

16 bits but with alien endianess. We can use short as the data type but the samples must be "byte swapped" prior writing them to the device.


    case AFMT_U8:

16 bit samples can be converted to this format by doing u8_sample = (s16_ne_sample >> 8) - 128.


    case AFMT_S24_NE:
    case AFMT_S24_OE:
    case AFMT_S32_NE:
    case AFMT_S32_OE:



      fprintf (stderr, "The device doesn't support the requested format\n");
      fprintf (stderr, "0x%08x != 0x%08x\n", tmp, SAMPLE_FORMAT);
      exit (-1);

Set up the number of channels. This program will automatically support any number of channels by writing the same sample value to each of the channels (mono).

Equally well stereo samples can be mixed for a 1 channel device by summing the channel values together. Stereo samples can be played with multi channel devices by setting the remaining channel samples to a silent value (0 with signed formats).

  num_channels = NUM_CHANNELS;
  if (ioctl (fd, SNDCTL_DSP_CHANNELS, &num_channels) == -1)
      perror ("SNDCTL_DSP_CHANNELS");
      exit (-1);

Set the sample rate.

  tmp = SAMPLE_RATE;
  if (ioctl (fd, SNDCTL_DSP_SPEED, &tmp) == -1)
      perror ("SNDCTL_DSP_SPEED");
      exit (-1);

  if (tmp != SAMPLE_RATE)

In some cases the device is not capable to support exactly the requested sampling rate.

We will tolerate a 5% error in between the requested and granted sampling rates.

If the error is larger then we cannot continue.

NOTE! Applications written for the mass market must be prepared to support every possible sampling rate locally.

      int v;

      v = abs (tmp - SAMPLE_RATE);

      if (v > ((SAMPLE_RATE * 5) / 100))
	  fprintf (stderr,
		   "The device doesn't support sampling rate of %d Hz.\n",
	  fprintf (stderr, "The nearest rate it supports is %d Hz\n", tmp);
	  exit (-1);

  if (ioctl (fd, SNDCTL_DSP_GETISPACE, &info) == -1)
      perror ("GETISPACE");
      exit (-1);

  sz = info.fragstotal * info.fragsize;

  if ((buf =
       mmap (NULL, sz, PROT_READ, MAP_FILE | MAP_PRIVATE, fd,
	     0)) == (caddr_t) - 1)
      perror ("mmap (read)");
      exit (-1);
  printf ("mmap (in) returned %08x\n", buf);
  ip = buf;

  if ((buf =
       mmap (NULL, sz, PROT_WRITE, MAP_FILE | MAP_PRIVATE, fd,
	     0)) == (caddr_t) - 1)
      perror ("mmap (write)");
      exit (-1);
  printf ("mmap (out) returned %08x\n", buf);
  op = buf;

Prepare for launch. Set the trigger bits to 0

  tmp = 0;
  ioctl (fd, SNDCTL_DSP_GETTRIGGER, &tmp);
  printf ("Trigger was %08x\n", tmp);

  ioctl (fd, SNDCTL_DSP_SETTRIGGER, &tmp);
  printf ("Trigger set to %08x\n", tmp);

  for (i = 0; i < sz; i++)
    op[i] = 0x00;

  while (1)
      struct timeval time;

      FD_ZERO (&reads);
      FD_ZERO (&writes);

      /*  FD_SET(fd, &writes); */
      FD_SET (fd, &reads);

      time.tv_sec = 1;
      time.tv_usec = 0;
      if (select (fd + 1, &reads, &writes, NULL, &time) == -1)
	  perror ("select");
	  exit (-1);

      if (FD_ISSET (fd, &reads))
	  count_info count, ocount;
	  int l, p;

	  if (ioctl (fd, SNDCTL_DSP_GETIPTR, &count) == -1)
	    perror ("GETIPTR");

	  if (ioctl (fd, SNDCTL_DSP_GETOPTR, &ocount) == -1)
	    perror ("GETOPTR");

	  printf ("read(%08x/%08x/%d)\n", count.bytes, count.ptr,
	  printf ("write(%08x/%08x/%d)\n", ocount.bytes, ocount.ptr,

	  l = count.ptr;
	  p = ocount.ptr;

	  if (l > (sz - ocount.ptr))
	    l = sz - ocount.ptr;

	  for (i = 0; i < l; i++)
	    op[i + p] = ip[i];

      if (FD_ISSET (fd, &writes))
	  printf ("write()\n");
	  have_data = 0;

  exit (0);

Copyright (C) 4Front Technologies, 2007. All rights reserved.

Back to index OSS web site

Copyright (C) 4Front Technologies, 2007. All rights reserved.
Back to index OSS web site