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The two main functions used for dictionary training using the COVER algorithm require initialization of a COVER_ctx_t where a call to qsort() is performed. The issue is that the standard C99 qsort() function doesn't offer a way to pass an extra parameter for the comparison function callback (e.g. a pointer to a context) and currently zstd relies on a *global* static variable to hold a pointer to a context needed to perform the sort operation. If a zstd library user invokes either ZDICT_trainFromBuffer_cover or ZDICT_optimizeTrainFromBuffer_cover from multiple threads, the global context may be overwritten before/during the call/execution to qsort() in the initialization of the COVER_ctx_t, thus yielding to crashes and other bad things (Tm) as reported on issue #4045. Enters qsort_r(): it was designed to address precisely this situation, to quote from the documention [1]: "the comparison function does not need to use global variables to pass through arbitrary arguments, and is therefore reentrant and safe to use in threads." It is available with small variations for multiple OSes (GNU, BSD[2], Windows[3]), and the ISO C11 [4] standard features on annex B-21 qsort_s() as part of the <stdlib.h>. Let's hope that compilers eventually catch up with it. For now, we have to handle the small variations in function parameters for each platform. The current fix solves the problem by allowing each executing thread pass its own COVER_ctx_t instance to qsort_r(), removing the use of a global pointer and allowing the code to be reentrant. Unfortunately for *BSD, we cannot leverage qsort_r() given that its API has changed on newer versions of FreeBSD (14.0) and the other BSD variants (e.g. NetBSD, OpenBSD) don't implement it. For such cases we provide a fallback that will work only requiring support for compilers implementing support for C90. [1] https://man7.org/linux/man-pages/man3/qsort_r.3.html [2] https://man.freebsd.org/cgi/man.cgi?query=qsort_r [3] https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/qsort-s?view=msvc-170 [4] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf