Segmentation fault (core dumped) - C (СИ)

void fir(char ifile[], char ofile[], char ffile[])
{
 FILE *fk, *fin, *fout;
 float koef[20], in[300], out[300], max = 0;
 int i, j;
 fk = fopen(ffile, "r");
 fin = fopen(ifile, "r");
 fout = fopen(ofile, "w");
 for (i=0; i<300; i++) fscanf(fin, "%f", &in[i]);
 for (i=0; i<20; i++) fscanf(fk, "%f", &koef[i]);
 for (i=0; i<300; i++){
  out[i] = 0;
  for(j=0; j<20; j++)
   if (i-j>=0) out[i]+=koef[j]*in[i-j];
  max = fmax(fabs(out[i]) , max);
 }
 for (i=0; i<300; i++){
  out[i] = out[i]/max;
  fprintf(fout, "%f", out[i]);
 }
 graph(ifile, "GraphInput.txt");
 graph(ofile, "GraphOutput.txt");
 fclose(fk);
 fclose(fin);
 fclose(fout);
 return;
}