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Re: Callbacks in the abstact data types and extra contextual data
From: |
Bruno Haible |
Subject: |
Re: Callbacks in the abstact data types and extra contextual data |
Date: |
Thu, 09 Jul 2020 22:16:09 +0200 |
User-agent: |
KMail/5.1.3 (Linux/4.4.0-179-generic; KDE/5.18.0; x86_64; ; ) |
Hi Marc,
> a number of modules (like the hash module or the list module) allow
> the user to specify callbacks (e.g. a comparison function).
> Unfortunately, these procedures do not take a context parameter, which
> can be a problem because C lacks closures.
Is this a practical, actual problem, or only a theoretical one?
I would hesitate to change (or duplicate) public API, when there is no
practical need.
Note that the lack of context can be remedied by
- use of per-thread variables, or
- use of nested functions [1], or
- storing a pointer to the necessary context in the list elements.
> The original qsort function in stdlib.h has the same problem. Glibs
> has remedied the problem by introducing qsort_r.
qsort_r is only portable to glibc systems, FreeBSD, and macOS. And yet,
no one has requested a substitute for it in gnulib. IMO this indicates
that few programs need this function.
Bruno
[1] https://gcc.gnu.org/onlinedocs/gcc-10.1.0/gcc/Nested-Functions.html