FILE Pointer

[Eljay Love-Jensen]

Hi Bharathi,

 >The sizeof(FILE) is 148 & sizeof(void) is 1. Does it mean anything ?

It means the struct FILE is 148 bytes long.  And sizeof(void) is probably 
calculated as if sizeof(char) -- probably something in the standard about that.

They could have done...

struct FILE {
   char opaque[148];
};

...and thus...

sizeof(FILE) == 148

What's the sizeof(FILE*) & sizeof(void*)?

After all, you don't "malloc(sizeof(FILE))" or "new FILE" in the use of a 
FILE*.  You just use the FILE* returned by the fopen.

 >and System dependent lib only can process the file descriptor ?

Which is why fileno(fp) will tell you the file descriptor.  Along with all 
the other functions I listed, which are the accessors to the FILE structure 
information.

 >I try to pass a void ptr to the ftell(), then I got the compilation error 
"invalid use of void expression".

FILE* fp = fopen("foo.txt", "r");
void* vp = fp;
size_t position = ftell((FILE*)vp); // Don't forget your cast.

 >With out casting, How it  found that, ptr is NOT a type FILE ?

I'm not sure why you'd need to care, but you can use GCC's C++ language 
extensions to test for type-ness.

Sincerely,
--Eljay