Bad file descriptor error when trying to associate fmemopen file descriptor with stdin

Question

I'm trying to interpose fscanf function that originally reads from stdin like fscanf(stdin,"%ms", &secret). In my interposed function, I'm trying to make it to read from an input file that I created with character string. Following is what I'm doing:

int fscanf(FILE *stream, const char *format, ...){

   FILE * in ;
   char * buffer = "secretString";
   size_t length = strlen(buffer);

  in = fmemopen(buffer, length, "r");
  if (in== NULL){
  handle_error("fmemopen");}
  // This is the part where I'm trying to feed my file as stdin
  // since original fscanf expects stdin which I'm trying to hack
  int dptr = fileno(in);
  dup2(dptr, 0);
  va_list args;
  va_start(args, format);

  int p = vfscanf(in, format, args);
    va_end(ap);
    fclose(in);
    return p;
}

However, I'm getting Bad file descriptor error. I would appreciate if someone can tell me what am I doing wrong.

Answer 1

I'm not clear where you got the idea that you should try to replace the file descriptor for stdin like this. It can't work that way, because the whole point of fmemopen is that there is no underlying file (and thus no file descriptor) behind it; it's merely an adaptation of an in-memory buffer to the stdio API.

Fortunately, I see no reason you need or want to remap file descriptors to do what you want. If you're intercepting fscanf and re-calling vfscanf with a different FILE* argument, stdin is not getting used, so any changes to it would have no effect.