NULL guard in _preview_write_image (fixes GCC 16 -Werror=nonnull)#20895
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TurboGit merged 1 commit intodarktable-org:masterfrom Apr 28, 2026
Merged
NULL guard in _preview_write_image (fixes GCC 16 -Werror=nonnull)#20895TurboGit merged 1 commit intodarktable-org:masterfrom
TurboGit merged 1 commit intodarktable-org:masterfrom
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Add a NULL guard at the top of
_preview_write_image()to fix the build under GCC 16+ with-Werror=nonnull.Fixes #20894
Details
GCC 16's interprocedural NULL analysis follows function pointers and detects that
dt_imageio_export()passesNULLfor theinargument when the format mime is"x-copy"(imageio.c:1010-1014). Since_preview_write_imageis registered as awrite_imagecallback, GCC concludes that NULL can reach thememcpycall inside it. The diagnostic is technically correct:memcpyfrom a NULL source is undefined behaviour even whennis zero.In practice the runtime path doesn't fire for the preview format, but the static analysis can't see that.
The fix is a one-line
if(!in) return 1;at the top of the function – return non-zero so a future caller that does pass NULL gets a clear error rather than UB.The bug has been latent since b49bf38062 (Oct 2022); only newer GCC versions (16+) catch it because of stricter NULL propagation across indirect calls.