BUG: Remove unnecessary global fflush(nullptr) causing MetaIO read deadlocks - #146
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…adlocks MetaObject::ReadStream() and MetaForm::ReadStream() unconditionally call fflush(nullptr) before parsing, a leftover from the original MetaIO commit that predates any multithreading concerns for this library. fflush(NULL) flushes every open FILE* stream in the entire process, not just the one being read. On platforms such as macOS, this requires walking the process-wide list of all open FILE* streams and locking each one in turn. Under heavy concurrent MetaImage/MHA reads (many threads each opening/reading/closing their own files), this global flush-all creates severe lock contention that can grind concurrent reads to a halt indefinitely, since each thread's fflush(nullptr) call competes to lock every other thread's in-flight FILE* handle. Reading does not require flushing any previously written data, so this call serves no purpose and can simply be removed. Ported from InsightSoftwareConsortium/ITK#6833. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
dzenanz
approved these changes
Sep 4, 2026
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Summary
MetaObject::ReadStream() and MetaForm::ReadStream() unconditionally call fflush(nullptr) before parsing header fields. This line dates back to MetaIO's very first file-IO commit, long before multithreading was a design concern for this library.
fflush(NULL) flushes every open FILE* stream in the entire process, not just the one being read. On platforms such as macOS, this requires walking the process-wide list of all open FILE* streams and locking each one in turn (_fwalk -> sflush_locked -> flockfile).
Under heavy concurrent MetaImage/.mha reads (many threads, each opening/reading/closing its own file), this global flush-all creates severe lock contention: every thread's fflush(nullptr) call competes to lock every other thread's in-flight FILE* handle. In practice this can grind concurrent reads to a halt indefinitely.
Reading a file does not require flushing any previously written data (nothing was just written), so this call serves no purpose and can simply be removed.
Ported from InsightSoftwareConsortium/ITK#6833.
Test plan
-DBUILD_TESTING=ONninjabuild succeeds cleanlyctest— all 14 tests pass, including testMeta2Object, testMeta3Image, and testMeta11Form which exercise the ReadStream() code paths touched by this change