[i098]

What and why

Fixes #4756.

In ZSTD_seekable_seek_buff() (contrib/seekable_format/zstdseek_decompress.c), the default: case of switch (origin) is only assert(0). With -DNDEBUG the assert is compiled out, so newOffset is read uninitialized and gcc warns. This PR adds return -1; after the assert. -1 is what the function already returns when the new offset is out of range. assert(0) is kept, so debug builds still catch the case.

seekBuffWithTotal() in contrib/seekable_format/tests/seekable_tests.c is a copy of the same function with the same defect, so it gets the same one-line change.

origin is always SEEK_SET, SEEK_CUR or SEEK_END at the library's call sites, so no runtime input reaches default:. The fix only removes the compiler diagnostic.

The warning is a compile-time diagnostic, so the reproducer is a compiler command (gcc 15.2.0):

gcc -DNDEBUG -O2 -Wall -Wextra -Wmaybe-uninitialized -Werror=maybe-uninitialized \
    -Ilib -Ilib/common -Icontrib/seekable_format \
    -c contrib/seekable_format/zstdseek_decompress.c -o /dev/null

Before:

zstdseek_decompress.c: In function 'ZSTD_seekable_seek_buff':
zstdseek_decompress.c:190:8: error: 'newOffset' may be used uninitialized [-Werror=maybe-uninitialized]
  190 |     if (newOffset > buff->size) {

After: no output, exit 0. The same command with -DXXH_NAMESPACE=ZSTD_ on contrib/seekable_format/tests/seekable_tests.c no longer reports newOffset either.

make -C contrib/seekable_format/tests test passes (all 5 seekable tests), and so do make check and make test.

Checklist

• I searched open pull requests and issues: this problem is not already being addressed.
• Bug fix: it comes with a reproducer or a test that fails without the fix.
• Performance change: numbers from repeated runs are included, with CPU, compiler, flags and data.
  A gain in one configuration is not a general gain: see the Performance section of CONTRIBUTING.md.
• I stated below whether AI or other automated tools were used.

Tools used

An AI coding assistant (Claude Code) wrote the change, ran the checks above, and wrote this description.

I am new to open source, and this pull request was made with AI assistance.