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    Subject[PATCH] Documentation: Explain a second alternative for multi-line macros.

    Add an explanation for defining multi-line macros using the ({ })
    notation to CodingStyle.

    Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>

    ---

    diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
    index 9069189..1d0ddb8 100644
    --- a/Documentation/CodingStyle
    +++ b/Documentation/CodingStyle
    @@ -549,13 +549,24 @@ may be named in lower case.

    Generally, inline functions are preferable to macros resembling functions.

    -Macros with multiple statements should be enclosed in a do - while block:
    +There are two techniques for defining macros that contain multiple
    +statements.

    -#define macrofun(a, b, c) \
    - do { \
    + (a) Enclose those statements in a do - while block:
    +
    + #define macrofun(a, b, c) \
    + do { \
    + if (a == 5) \
    + do_this(b, c); \
    + } while (0)
    +
    + (b) Use the gcc extension that a compound statement enclosed in
    + parentheses represents an expression:
    +
    + #define macrofun(a, b, c) ({ \
    if (a == 5) \
    do_this(b, c); \
    - } while (0)
    + })

    Things to avoid when using macros:

    -
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