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Possible Duplicates:
Do-While and if-else statements in C/C++ macros
do { … } while (0) — what is it good for?

I'm reading the linux kernel and I found many macros like this:

#define INIT_LIST_HEAD(ptr) do { \
    (ptr)->next = (ptr); (ptr)->prev = (ptr); \
} while (0)

Why do they use this rather than define it simply in a {}?

John Kugelman
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amazingjxq
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2 Answers2

160

You can follow it with a semicolon and make it look and act more like a function. It also works with if/else clauses properly then.

Without the while(0), your code above would not work with

if (doit) 
   INIT_LIST_HEAD(x);
 else 
   displayError(x);

since the semicolon after the macro would "eat" the else clause, and the above wouldn't even compile.

SPWorley
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    But the OP's question stands. Why not just { (ptr)->next ... } instead of do { (ptr)->next ... } while (0);? – joshk0 May 29 '09 at 00:26
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    arno explained that. It would expand to "{ (ptr)->next ... };" thus, a statement followed by a second statement. If syntax however is "if ( expression ) statement else statement" . The else would not be associated with any if, since you would have written "if ( expression ) statement statement" (one "{ ... }" and one ";" statement). – Johannes Schaub - litb May 29 '09 at 00:37
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    As Amo said, it's a clever trick that allows a macro to *be* a C statement that must end with a semicolon. It makes the macro act exactly like a function call, so far as statement construction and termination (with ';') is concerned. – Eddie May 29 '09 at 01:53
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    Note, however, that in this case it's all completely unnecessary, as the body of the macro could be written much more cleanly as: `(ptr)->next=(ptr)->prev=(ptr)`. – Jerry Coffin Dec 02 '09 at 04:19
  • @JoshK the loop will be executed only once because while(0) is a false condition. However, an optimizing compiler will get ride of the `do {stat}while(0)` & replace with only `stat` because anyway, it's happening only once. – Karim Manaouil Sep 05 '17 at 19:01
48

It allows you to group several statements into one macro.

Assume you did something like:

if (foo) 
    INIT_LIST_HEAD(bar);

If the macro was defined without the encapsulating do { ... } while (0);, the above code would expand to

if (foo)
    (bar)->next = (bar);
    (bar)->prev = (bar);

This is clearly not what was intended, as only the first statement will be executed if foo holds. The second statement would be executed regardless of whether foo holds.

Edit: Further explanation at http://c-faq.com/cpp/multistmt.html and http://developer.apple.com/documentation/DeveloperTools/gcc-4.0.1/cpp/Swallowing-the-Semicolon.html#Swallowing-the-Semicolon

rodion
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