. Continued from a previous article.
The exact location of the infinite loop in GNU’s fseek,
which I wrote about in articles
14,
15, and
17, is now known.
It is in source file libio/wfileops.c, function
adjust_wide_data, and the repeated lines are
567, 568, 576, 582. I tested and debugged with GNU glibc
version 2.31, but these line numbers also apply to the sources of
version 2.34.
In the do-while loop there, the enum variable status
always remains __codecvt_partial, so the loop never
ends.
I cannot propose a fix, because I don’t understand what this part
of the code is doing, and why it is necessary in the first place.
As I remarked
before,
as I see it, fseek should not involve any file reading
and buffer filling, nor buffer flushing and file writing, and also
any conversion of wide or multibyte characters should be unnecessary
at this stage. All that fseek does in essence, is to set
a position for future reading or writing operations.
Whether those operations are wide-character oriented or not, should
not make any difference.
The implementation should and could be simple, efficient, fast,
error-free and easy to maintain.
As I also wrote before, it is easy to say that without fully understanding all of the code. I am aware of that.
Still, when looking at how fseek
is implemented in FreeBSD, I get the impression
it is much simpler and cleaner, and GNU in comparison is needlessly
complicated. Also, FreeBSD seems to do what it does for fseek,
without ever looking whether we are dealing with wide characters or not.
And that makes sense to me.
The slowness of ftell is also
telling in this respect (pun intended).
This is what I call clean code! The library
musl
by Rich Felker, and others. See also
this comparison.
Strangely, this musl library does consider an invalid
byte consumed.
So my trick using getc() cannot be used here, it
skips too much. So in Siworin, I now always do fseek(),
unless compiler symbol __GLIBC__ is defined, then
I do getc().
Later, but probably also before although I had forgotten about it, I found that the C library musl is also at the heart of busybox and Alpine Linux!
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