Programming

Hi!

I'm on section 8.5., implementation of fopen(). I can't access members of the FILE structure, my compiler simply doesn't recognize them, along with some other things, like flags. According to K&R, all these elements should be defined in <stdio.h>.

Any suggestions?
p.s. I'm using Ubuntu.

The reason for your inability is because it's designed to prevent you from doing so. That structure's opaque and system-dependent.

K&R's utterly ancient, that implementation probably has little to nothing in common with yours now, aside from actually having an fopen().

So, if some value is opaque, there is no way we can directly access it?

Opaque means the header file just defines it as a 'struct FILE'. It won't complain about the incomplete definition when you use it as FILE *, since it never needs to know its size or its members to pass it around in that fashion.

You could dig into the source code of glibc and find out where it really is defined, and typecast a FILE * into a pointer to a structure you define yourself. Remember though, the reason you're not supposed to do this is because

1) This will break if you move this code to another platform. We've had people bring code depending on this from Solaris and wonder why it doesn't work in Linux when it says IN THE CODE 'platform dependent'.
2) This could easily break when you update glibc, too.
3) Modifying the contents of this structure may have unforeseen consequences.

I found this recent thread:

unix.com/programming/150687-file-structure-stdio-h.html

It describes a problem very similar to mine, so I just wanted to post it here.

=================

I'm currently examining stdio.h on Ubuntu, and I found the following:

struct _IO_FILE;

typedef struct _IO_FILE FILE;

typedef struct _IO_FILE __FILE;

Also there is (among others) #include <libio.h>, and in it there is

struct _IO_FILE;

I've tried looking into most of the other headers included in stdio.h, but I just can't find the definition of _IO_FILE anywhere.

In GNU/Linux the definition is in the libio.h header. As Corona688 said, it is not a portable documented structure.

I beg your pardon, the structure IS defined.

Things make sense one again

Thank you for your help, gentleman.