How could I create file in C when it is invoke from 4GL?

Question

I'm currently working on a C module of an Informix 4GL program. I was trying to put a debug log in the module like this:

int fgl_funcA(int i)
{
  FILE *pLog = NULL;
  pLog = fopen("debug.log", "a");
  fputs("fgl_funcA() called.", pLog);
  fclose(pLog);

  ...
  ...
}

But this function doesn't work at all. I mean it doesn't output debug.log. Even though I have manually create the file but somehow the content still not update yet. I was wonder whether I'm allow to create file in the C module when the code is call from 4GL?