64 lines
2.1 KiB
ReStructuredText
64 lines
2.1 KiB
ReStructuredText
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.. title:: clang-tidy - bugprone-implicit-widening-of-multiplication-result
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bugprone-implicit-widening-of-multiplication-result
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===================================================
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The check diagnoses instances where a result of a multiplication is implicitly
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widened, and suggests (with fix-it) to either silence the code by making
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widening explicit, or to perform the multiplication in a wider type,
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to avoid the widening afterwards.
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This is mainly useful when operating on very large buffers.
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For example, consider:
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.. code-block:: c++
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void zeroinit(char* base, unsigned width, unsigned height) {
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for(unsigned row = 0; row != height; ++row) {
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for(unsigned col = 0; col != width; ++col) {
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char* ptr = base + row * width + col;
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*ptr = 0;
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}
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}
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}
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This is fine in general, but if ``width * height`` overflows,
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you end up wrapping back to the beginning of ``base``
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instead of processing the entire requested buffer.
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Indeed, this only matters for pretty large buffers (4GB+),
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but that can happen very easily for example in image processing,
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where for that to happen you "only" need a ~269MPix image.
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Options
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-------
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.. option:: UseCXXStaticCastsInCppSources
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When suggesting fix-its for C++ code, should C++-style ``static_cast<>()``'s
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be suggested, or C-style casts. Defaults to ``true``.
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.. option:: UseCXXHeadersInCppSources
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When suggesting to include the appropriate header in C++ code,
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should ``<cstddef>`` header be suggested, or ``<stddef.h>``.
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Defaults to ``true``.
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Examples:
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.. code-block:: c++
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long mul(int a, int b) {
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return a * b; // warning: performing an implicit widening conversion to type 'long' of a multiplication performed in type 'int'
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}
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char* ptr_add(char *base, int a, int b) {
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return base + a * b; // warning: result of multiplication in type 'int' is used as a pointer offset after an implicit widening conversion to type 'ssize_t'
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}
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char ptr_subscript(char *base, int a, int b) {
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return base[a * b]; // warning: result of multiplication in type 'int' is used as a pointer offset after an implicit widening conversion to type 'ssize_t'
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}
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