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Asked: 2026-05-13T10:49:41+00:00

I’ve noticed that sometimes, C macros are written as something like this: #define foo(bar)

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I’ve noticed that sometimes, C macros are written as something like this:

#define foo(bar) ({ ++bar; })

After some experimentation, I’ve found that:

  1. ({}); will compile, but do nothing. (As expected.)
  2. Leaving the ; off will cause a syntax error. A side effect of this is ensuring that foo() looks like a function in your code. (Although, if you leave the semicolon off, the error isn’t very useful for diagnosing the problem!)
  3. return ({}); complains about a void value not being ignored, just like if I had tried to use a void function.

Is this just to make developers add a semicolon to their macros, or does it have another purpose? I’ve tried Google, but it fails miserably with punctuation. Is there a name for this?

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1 Answer

  1. Editorial Team

    This is a GNU extension called statement expressions.

    When declaring macros in standard-C, you often see do...while(0) loops used for similar purposes (ie creating a block scope). A statement expression is superior to the loop hack because it can return a value. If you want to do something similar in standard-C, you’d have to define an additional function and lose the convenience of lexical scoping.

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