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The _IO_FILE typedef

Posted on 1998-05-21
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In Kernighan and Ritchie's book on C, they say that the definition of FILE is to be found in <stdio.h>; they say it is defined to be an _iobuf struct. On my Linux system, though, FILE is defined differently: in <stdio.h> it's defined as an _IO_FILE struct. _IO_FILE in turn is defined as a structure in <libio.h>. My question is: there seems to be nothing corresponding here to the fd member of the _iobuf struct that Kernighan and Ritchie talk about. So how (for example) would fclose on my system be related to the close system call?
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Question by:vexil
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Expert Comment

by:minar
ID: 1637198
In general, it's better to assume that a FILE * pointer is
opaque: don't try to read inside it. Code that mucks inside
FILE * is hard to make portable.

If you need to get the file descriptor for a FILE * pointer,
use the fileno() macro. So for instance, fclose(FILE * foo)
is comparable to close(fileno(FILE * foo)). (But don't mix
the two styles of file I/O unless you know what you're doing).
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by:vexil
ID: 1637199
Thanks for answering, Minar, but I'm interested in the question in a theoretical rather than a practical sense. But I'll give you some points if you tell me where the fileno macro is.
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Accepted Solution

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minar earned 200 total points
ID: 1637200
Ah, you want theory! In practice, again, fileno() is the right way to get a file descriptor from a FILE *. But if you want to get into the guts, take a look at the source to libc.

In Linux libc5, the _IO_FILE struct has an integer member _fileno; This is the actual file descriptor; it's set up in libc's libio/fileops.c, the function _IO_file_fopen. The stdio library does some buffering but basically uses the underlying file descriptor for IO.

fileno() itself is a function in libio/stdio/fileno.c. It does some checks for EOF and then passes through to the _IO_fileno macro in libio/iolibio.h. That just returns (FP)->_fileno.

GNU libc6 will probably be different.

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Author Comment

by:vexil
ID: 1637201
Thanks!
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