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@@ -51,3 +51,85 @@
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is required to check if propagating the attribute is
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appropriate. Incorrectly applied attributes can result in
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undesired behavioral changes in the compiled code.
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+
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+ [[sect-Defensive_Coding-C-Common-Mistakes]]
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+ === Common mistakes
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+
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+ ==== Mistakes in macros
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+ A macro is a name given to a block of C statements as a pre-processor
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+ directive. Being a pre-processor the block of code is transformed by
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+ the compiler before being compiled.
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+
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+ A macro starts with the preprocessor directive, #define. It can
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+ define a single value or any 'substitution', syntactically valid or
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+ not.
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+
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+ A common mistake when working with macros is that programmers treat
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+ arguments to macros like they would functions. This becomes an issue
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+ when the argument may be expanded multiple times in a macro.
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+
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+ For example:
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+
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+ macro-misuse.c
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+ [source,C]
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+ ----
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+ #define simple(thing) do { \
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+ if (thing < 1) { \
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+ y = thing; \
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+ } \
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+ else if (thing > 100) { \
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+ y = thing * 2 + thing; \
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+ } \
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+ else { \
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+ y = 200; \
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+ } \
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+ } while (0)
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+
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+ int main(void) {
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+ int x = 200;
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+ int y = 0;
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+ simple(x++);
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+
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+ return 0;
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+ }
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+ ----
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+
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+ Each pass through the simple() macro would mean that x could be
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+ expanded in-place each time 'thing' was mentioned.
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+
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+ The 'main' function would be processed and expanded as follows:
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+
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+ macro-misuse-post-processing.c
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+ [source,C]
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+ ----
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+
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+ int main(void) {
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+ int x = 200;
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+ int y = 0;
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+ do {
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+ if ( x++ < 1) {
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+ y = x++;
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+ }
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+ else if (thing > 100) {
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+ y = x++ * 2 + x++;
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+ }
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+ else {
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+ x = 200;
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+ }
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+ } while (0)
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+
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+ return 0;
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+ }
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+ ----
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+
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+ Each evaluation of the argument to 'simple' (x++) would be executed
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+ each time it was referenced.
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+
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+ While this may be 'expected' behaviour by the original creator, large
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+ projects may have programmers who were unaware of how the macro may
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+ expand and this may introduce unexpected behaviour, especially if the
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+ value is later used as indexing into an array or able to be
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+ overflowed.
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+
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+
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+
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wmealing
commented 4 years ago
First pull ive done for this project, let me know if it should be in another location. This outlines the possible misuse of macros and the automated expansion by the c pre-processor.