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This noncompliant code example contains multiple, unbound statements.
| Code Block |
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| bgColor | #FFcccc |
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| lang | c#ffcccc |
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/*
* Swaps two values.
* Requires tmp variable to be defined.
*/
#define SWAP(x, y) \
tmp = x; \
x = y; \
y = tmp
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This macro will expand correctly in a normal sequence of statements, but not as the then-clause in an if statement:
| Code Block |
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| bgColor | #FFcccc |
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| lang | c#ffcccc |
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|
int x, y, z, tmp;
if (z == 0)
SWAP( x, y);
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This will expand to
| Code Block |
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| bgColor | #FFcccc |
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| lang | #ffccccc |
|---|
|
int x, y, z, tmp;
if (z == 0)
tmp = x;
x = y;
y = tmp;
|
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This noncompliant code example inadequately bounds multiple statements.
| Code Block |
|---|
| bgColor | #FFcccc |
|---|
| lang | #ffccccc |
|---|
|
/*
* Swaps two values.
* Requires tmp variable to be defined.
*/
#define SWAP(x,y) { 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 |
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| bgColor | #FFcccc |
|---|
| lang | c#ffcccc |
|---|
|
if (x > y)
SWAP(x,y); /* Branch 1 */
else
do_something(); /* Branch 2 */
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Following macro expansion, however, this code is interpreted as an if-statement with only one branch:
| Code Block |
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| bgColor | #FFcccc |
|---|
| lang | c#ffcccc |
|---|
|
if (x > y) { /* Single-branch if-statement!!! */
tmp = x; /* The one and only branch consists */
x = y; /* of the block. */
y = tmp;
}
; /* empty statement */
else /* ERROR!!! "parse error before else" */
do_something();
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Wrapping the macro inside a do-while loop mitigates the problem.
| Code Block |
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| bgColor | #ccccFF |
|---|
| lang | c#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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