[bfs2021]

Problem

lib/dictBuilder/cover.c fails to compile on OpenHarmony (musl libc): the OHOS libc exports qsort_r() with the GNU signature, but its headers do not declare it, so the call in stableSort() is an implicit function declaration, which newer OHOS SDK clang treats as an error. Fixes #4872.

Root cause

On OpenHarmony __linux__ is defined, so cover.c defines _GNU_SOURCE and the ZDICT_QSORT_GNU path is selected, but the OHOS musl headers omit the qsort_r() prototype. Unlike Android (which is excluded from the GNU path and falls back), there is no handling for this platform, and the C90 fallback (qsort() with a global context) would be a thread-safety regression.

Fix

Declare qsort_r() locally under #if defined(__OHOS__), right after the system includes. This is the Option B from the issue, which the reporter already validated on-device: OHOS's qsort_r is the GNU variant, so the re-entrant sort keeps being used and no behavior changes on any other platform. __OHOS__ is the macro predefined by the OHOS SDK toolchain.

Testing

• Reproduced the failure mode locally on a host whose libc also does not declare qsort_r: gcc -c -Ilib -D__linux__ -D__OHOS__ lib/dictBuilder/cover.c fails with error: implicit declaration of function 'qsort_r' before the change and compiles cleanly after it (also with -Wall -Wextra), same as the OHOS clang error class.
• Native build unaffected: gcc -c -Ilib lib/dictBuilder/cover.c clean before/after.
• Built the full static lib + CLI with CMake/Ninja and ran zstd --train-cover --split=4096 (which exercises stableSort() in cover.c) plus a compress/decompress round-trip with the trained dictionary: all pass.

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