Strange sine wave on output of an alsa linux c program

I'm studying alsa programming on ubuntu.

I'm trying to output a sine wave to line-out of my laptop soundcard, and then redirecting to line-in (microphone) via an audio cable.

  ___LINE_IN(microphone)\
 /                       \
|                         \ _____________
|                          |soundcard-pc |
cable                     /
|                        /
 \ ___LINE_OUT(speakers)/

I'm using this code

#include<stdio.h>
#include<stdlib.h>
#include<alsa/asoundlib.h>
#include<math.h>
#define SIZEBUF 2048
int main(void)
{
int i;
int err;
double x;
double cost;
double frequency=500;
unsigned int rate=44100;
short buf[SIZEBUF];
snd_pcm_t *phandle;
snd_pcm_hw_params_t *hw_params;
snd_pcm_open(&phandle,"default",SND_PCM_STREAM_PLAYBACK,0);
snd_pcm_hw_params_malloc(&hw_params);
snd_pcm_hw_params_any(phandle,hw_params);
if((err=snd_pcm_hw_params_set_access(phandle,hw_params,SND_PCM_ACCESS_RW_INTERLEAVED))<0)
    {
    printf("Cannot set access type.\n");
    exit(1);
    }
snd_pcm_hw_params_set_format(phandle,hw_params,SND_PCM_FORMAT_S16_LE);
snd_pcm_hw_params_set_rate_near(phandle,hw_params, &rate,0);
snd_pcm_hw_params_set_channels(phandle,hw_params,1);
snd_pcm_hw_params(phandle,hw_params);
snd_pcm_hw_params_free(hw_params);
snd_pcm_prepare(phandle);
cost=2.0*M_PI*frequency/(double)rate;
printf("cost=%f.\n",cost);
for(i=1;i<SIZEBUF;i++)
    {
    x=sin(i*cost);
    buf[i]=(short)(32767*x+32768);
    }
for(i=0;i<50;i++)
    {
    snd_pcm_writei(phandle,buf,SIZEBUF);
    }
snd_pcm_close(phandle);
exit(0);
}

I'm using audacity to see the wave, but it appears strange, like in this image

It seems not to have the behaviour of a sine wave. Why?