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Make leap seconds threadsafe #108

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Make leap seconds threadsafe #108

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f8ec5de
make leap seconds thread safe
ColmTalbot Mar 29, 2026
c74dbc7
leave leap seconds initialized as null
ColmTalbot Mar 30, 2026
952d180
revert moving leapseconds into header
ColmTalbot Mar 30, 2026
a599c86
documentation fix
ColmTalbot Mar 30, 2026
618946e
remove unused variable
ColmTalbot Mar 30, 2026
0960e47
Add some more comments about thread local methods
ColmTalbot Apr 6, 2026
468d4c9
Update TLS for C23
ColmTalbot Apr 6, 2026
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5 changes: 3 additions & 2 deletions src/erfadatextra.c
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Original file line number Diff line number Diff line change
Expand Up @@ -17,9 +17,10 @@
*/
#include "erfa.h"
#include "erfaextra.h"
#include "erfadatextra.h"

static eraLEAPSECOND *changes;
static int NDAT = -1;
static ERFA_THREAD_LOCAL eraLEAPSECOND *changes;
static ERFA_THREAD_LOCAL int NDAT = -1;


int eraGetLeapSeconds(eraLEAPSECOND **leapseconds)
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26 changes: 26 additions & 0 deletions src/erfadatextra.h
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Expand Up @@ -23,4 +23,30 @@
int eraDatini(const eraLEAPSECOND *builtin, int n_builtin,
eraLEAPSECOND **leapseconds);

/*
** For thread safety we want a way to define thread-local variables.
** If no TLS is available, fall back to ordinary storage and disable
** thread safety for those variables. This mostly follows the numpy
** specification (as of 8203da6e4cdad722664fa740c7a9514104483f07)
**
** The order is:
** C++ >= 11 and GCC C >= 23 uses thread_local
** GCC: 11 <= C < 23 uses _Thread_local
** Clang uses __thread
** MSVC uses _declspec(thread)
*/
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@mhvk mhvk Apr 5, 2026

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It would be good to include in the comment where these definitions come from, in case things break later and we have to update or so. I notice that they are somewhat different from what is used by numpy

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@ColmTalbot ColmTalbot Apr 6, 2026

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I hadn't seen the numpy version, thanks for linking.

The differences in the conditions are that I missed C23 also has thread_local rather than _Thread_local and for cpp there is an explicit check of the version being > = C++11. I extended the comment to include more of this.

The main difference I see is the main checks are added in meson.build.
I tested this with erfa and it wasn't clear where this should go, I think for safety it would need to be in meson.build, configure.ac, and setup.py.
I'm happy to push a version with that, but it is quite a lot of repetition.
Do you think that would be better?

#if defined(__cplusplus) && __cplusplus >= 201103L
# define ERFA_THREAD_LOCAL thread_local
#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
# define ERFA_THREAD_LOCAL thread_local
#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
# define ERFA_THREAD_LOCAL _Thread_local
#elif defined(__GNUC__) || defined(__clang__)
# define ERFA_THREAD_LOCAL __thread
#elif defined(_MSC_VER)
# define ERFA_THREAD_LOCAL __declspec(thread)
#else
# define ERFA_THREAD_LOCAL
#endif

#endif