oss-sine.c

// modprobe snd_pcm_oss

#include <fcntl.h>
#include <linux/soundcard.h>
#include <math.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <unistd.h>

static inline void err(char *str)
{ perror(str); exit(1); }

static inline void errif(bool condition, char *str)
{ if (condition) err(str); }

int main(int argc, char **argv)
{ int dsp = 0;

  { // open and configure DSP device

    char dsp_device[] = "/dev/dsp2";
    int
      channels = 2
    , frequency = 48000
    , format = AFMT_S16_LE
    ;

    dsp = open(dsp_device, O_WRONLY);
    errif (!dsp, dsp_device);
    errif (!!ioctl(dsp, SNDCTL_DSP_SETFMT, &format), "sndctl_dsp_setfmt");
    errif (!!ioctl(dsp, SNDCTL_DSP_CHANNELS, &channels), "sndctl_dsp_channels");
    errif (!!ioctl(dsp, SNDCTL_DSP_SPEED, &frequency), "dnsctl_dsp_speed");
  }

  { // generate sine wave

    int period = 48000 / 261.63; // Middle C in A440

    for (int i = 0; ; i = (i+1) % period)
    { float x = 2 * M_PI * i / period;
      float y = sin(x);
      int16_t sample[2] = { INT16_MAX * y, INT16_MAX * y };
      write(dsp, sample, sizeof(sample));
    }
  }

  return 0;
}