macOS C++ FLTK File IO issues

[slooksterPSV]

void _open(Fl_Widget *f, void *)
{
  FILE *fptr = new FILE;
  char *flch;//[80];
  char *flch2 = new char;
  fb = new Fl_File_Chooser("HTML", NULL, 0, NULL);
  fb->show();
  while(fb->visible())
   Fl::wait();
  fptr = fopen(fb->value(), "rb");
  while(!feof(fptr))
  {
  fgets(flch, NULL, fptr);//fread(flch, NULL, NULL, fptr);
  sprintf(flch2, "%s\n%s", stktextbuffer->text(),flch);
  stktextbuffer->text(flch2);
  }
  fclose(fptr);
}

There's the code, here's the problem. I've tried doing this many ways, but I can only open one file before the program crashes. If I open a file and it reads it, and click the open button again and choose another file, it crashes. I know this doesn't work with how I have it setup, but could someone help me with this. I've tried a lot of different things. Thanx - if you need all the source lemme know.

 

[WebMongol]

void _open(Fl_Widget *f, void *)
{
  FILE *fptr = new FILE;
  char *flch;//[80];
  char *flch2 = new char;
  fb = new Fl_File_Chooser("HTML", NULL, 0, NULL);
  fb->show();
  while(fb->visible())
   Fl::wait();
  fptr = fopen(fb->value(), "rb");
  while(!feof(fptr))
  {
  fgets(flch, NULL, fptr);//fread(flch, NULL, NULL, fptr);
  sprintf(flch2, "%s\n%s", stktextbuffer->text(),flch);
  stktextbuffer->text(flch2);
  }
  fclose(fptr);
}

There's the code, here's the problem. I've tried doing this many ways, but I can only open one file before the program crashes. If I open a file and it reads it, and click the open button again and choose another file, it crashes. I know this doesn't work with how I have it setup, but could someone help me with this. I've tried a lot of different things. Thanx - if you need all the source lemme know.

I would recommend to start with reading about memory allocation and management in C. Next check man pages for fgets and sprintf functions. In Terminal type:
man fgets
man sprintf

The general problem with your code snippet is that you pass uninitialized pointer (flch) or pointer to 1-char buffer to functions that expect pointers to preallocated buffers (allocated on stack or heap). Following statement
fgets(flch, NULL, fptr);
does not lead to a crash because buffer size is set to 0 (NULL) otherwise it'll cause a crash
and this line
sprintf(flch2, "%s\n%s", stktextbuffer->text(),flch);
writes formatted string to some heap objects (C++ objects allocated with new or malloc) and your program crashes soon after this call.
In following snippet I corrected these problems:

void _open(Fl_Widget *f, void *)
{
  FILE *fptr = NULL;  // (new FILE) is wrong
  char buf[1024];     // allocate memory on stack for tmp buffers
  char buf2[1024];
  fb = new Fl_File_Chooser("HTML", NULL, 0, NULL);
  fb->show();
  while(fb->visible())
   Fl::wait();
  fptr = fopen(fb->value(), "rb");
  while( fgets(buf, sizeof(buf), fptr) ) // read up to sizeof(buf)-1 bytes
  {
  // use snprintf instead of sprintf
  snprintf(buf2, sizeof(buf2), "%s\n%s", stktextbuffer->text(),buf);
  stktextbuffer->text(buf);
  }
  fclose(fptr);
}