Re: FILE * is opaque so now what ?

On Mon, Jul 20, 2026 at 11:10:59AM -0400, enh wrote:
> if you decide to go in the direction of adding api, note that
> glibc/musl/bionic all have <stdio_ext.h> with this kind of thing in
> it. specifically in this case:
> 
> /**
>  * [__fpending(3)](https://man7.org/linux/man-pages/man3/__fpending.3.html)
> returns the number of
>  * bytes in the output buffer. See __freadahead() for the input buffer.
>  */
> size_t __fpending(FILE* _Nonnull __fp);
> 
> /**
>  * __freadahead(3) returns the number of bytes in the input buffer.
>  * See __fpending() for the output buffer.
>  */
> size_t __freadahead(FILE* _Nonnull __fp);
> 
> in the absence of these, projects like gnulib copy & paste your struct
> FILE internals into their code and read/write your fields directly,
> making evolution harder.

Yes, we saw that for bionic in gnulib. We added these functions for 
precisely that reason:

https://man.openbsd.org/__freading

In our testing things worked without gnulib needing to reach into struct
FILE (at least in the three dozen copies we have in our ports tree), but
for some reason they still decided to do so at least for part of the
struct:

https://github.com/coreutils/gnulib/blob/e6bb21523280811c17e2c45ebaa75fe5ddef1fda/lib/stdio-impl.h#L146-L155
https://github.com/coreutils/gnulib/commit/e1d738170920b51cb0035bf939dd29e4f7c6d6fb