#include <stdio.h>
#include <fcntl.h>
#include <sys/soundcard.h>

#define BUF_SIZE 2

/*buffers for sound device*/
unsigned char devbuf[BUF_SIZE];

/*File descriptor for device*/
int audio_fd;

/*The sound card is known as the DSP*/
char DEVICE_NAME[]="/dev/dsp";

/*Device Settings*/
int format;
int channels;
int fs;
int len;
int frag;
int devcaps;

int main()
{
    unsigned int temp;
    unsigned int yout;

    /*Samples format is 16 bit unsigned*/
    format = AFMT_U16_LE;
    /*1 Channel, MONO*/
    channels = 1;
    /*Sampling Rate is 16 KHz*/
    fs = 16000;
    /*This is a parameter used to set the DSP for low latencies*/
    frag = 0x00020004;


        /******************************************************
          Set parameters in sound card device
    *****************************************************/

        /*Open sound card device*/
        if ((audio_fd = open(DEVICE_NAME,O_RDWR, 0)) == -1) {
                /* Open of device failed */
                perror(DEVICE_NAME);
                exit(1);
        }      

        if (ioctl (audio_fd, SNDCTL_DSP_SETFRAGMENT, &frag) == -1)
        {
            perror ("SNDCTL_DSP_SETFRAGMENT");
            exit (-1);
        }

        /*Set audio format*/
        if (ioctl(audio_fd, SNDCTL_DSP_SETFMT, &format) == -1) {
                /* fatal error */
                perror("SNDCTL_DSP_SETFMT");
                exit(1);
        }
        /*Set number of channels*/
        if (ioctl(audio_fd, SNDCTL_DSP_CHANNELS, &channels) == -1) {
                /* Fatal error */
                perror("SNDCTL_DSP_CHANNELS");
                exit(1);
        }
       
        /*Set sampling rate*/
        if (ioctl(audio_fd, SNDCTL_DSP_SPEED, &fs)==-1) {
                /* Fatal error */
                perror("SNDCTL_DSP_SPEED");
                exit(1);
        }

        if (ioctl(audio_fd, SNDCTL_DSP_SYNC) == -1) {
                /* fatal error */
                perror("SNDCTL_DSP_SYNC");
                exit(1);
        }      

        /******************************************************
          This is the infinite loop to:
          1. Read a sample
          2. Process the sample
          3. Write the sample to soundcard output
    *****************************************************/
    while(1) {
        /* This is a blocking read function, the application will wait
           until the sound card captures a sample
        */
        if ((len = read(audio_fd,devbuf, sizeof(devbuf))) == -1) {
                perror("audio read");
                exit(1);
            }

        /* We can pass this variable to a temporary value,
           in this case as unsigned 16 value, and then use this value
           for processing
        */  
            temp = (devbuf[0])|(devbuf[1]<<8);

        /* In this case no processing is done so the value is passed to the output. You can also use this sample to build an audio file, or send it trought a communications interface, etc.
        */
            yout = temp;

            devbuf[0] = yout&0xFF;
            devbuf[1] = (yout>>8)&0xFF;
               
        /* This is the way the output samples are sent to the soundcard            
        */
        if ((len = write(audio_fd,devbuf,sizeof(devbuf)) == -1)) {
                perror("audio write");
                exit(1);
        }
    }  
    return 0;
}