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which is certainly not what the programmer intended.
Noncompliant Code Example
This noncompliant code example also contains multiple, unbound statements.
| Code Block | ||
|---|---|---|
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/*
* Swaps two values.
*/
#define SWAP(x,y) { int tmp; tmp=x; x=y; y=tmp; }
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This macro fails to expand correctly in some case such as the following example which is meant to be an if-statement with two branches:
| Code Block | ||
|---|---|---|
| ||
if (x > y) SWAP(x,y); /* Branch 1 */ else do_something(); /* Branch 2 */ {code:bgColor=#ffcccc} Following macro expansion, however, this code is interpreted as an if-statement with only one branch: if (x > y) { /* Single-branch if-statement!!! */ int tmp; /* The one and only branch consists */ tmp = x; /* of the block. */ x = y; y = tmp; } ; /* empty statement */ else /* ERROR!!! "parse error before else" */ do_something(); The problem is the semi-colon ';' following the block. h2. Compliant Solution |
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Wrapping the macro inside a do-while loop mitigates the problem. |
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{code | ||
:bgColor | =#ccccff | }
/*
* Swaps two values.
* Requires tmp variable to be defined.
*/
#define SWAP(x, y) \
do { \
tmp = x; \
x = y; \
y = tmp; } \
while (0)
|
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| Wiki Markup |
|---|
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "NMP Pre-processor Directions"
[Linux Kernel Newbies FAQ|http://kernelnewbies.org/FAQ] [FAQ/DoWhile0|http://kernelnewbies.org/FAQ/DoWhile0] |
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01. Preprocessor (PRE) PRE11-C. Do not conclude a single statement macro definition with a semicolon