How does a FILE pointer in C work?

Question

If we write:

//in main

FILE *p = fopen("filename", "anymode");

My question is: to what is p pointing?

Answer 1

The file pointer p is pointing a structure handled by the C library that manages I/O functionality for the named file in the given open mode.

You can't tell, a priori, whether what it points at is statically allocated memory or dynamically allocated memory; you don't need to know. You treat it as an opaque pointer.

Note that the standard says:

ISO/IEC 9899:2011 7.21.3 Files

The address of the FILE object used to control a stream may be significant; a copy of a FILE object need not serve in place of the original.

This says (roughly): Don't futz around with the pointer; pass it to the functions that need it and otherwise leave well alone.

Answer 2

p points to a memory location holding a FILE structure. That's all you really need to know, the contents of that structure are entirely implementation-specific.

If it's implemented correctly in terms of encapsulation (i.e., as an opaque structure), you shouldn't even be able to find out what it contains.

All you should be doing with it (unless you're the implementer of the standard library) is receive it from fopen and pass it to other functions requiring a FILE *, such as fwrite, fread and fclose.