C attribute: maybe_unused (since C23)

Suppresses warnings on unused entities.

Syntax

Explanation

This attribute can appear in the declaration of the following entities:

• struct/union: struct [[maybe_unused]] S;,
• typedef name: [[maybe_unused]] typedef S* PS;,
• object: [[maybe_unused]] int x;,
• struct/union member: union U { [[maybe_unused]] int n; };,
• function: [[maybe_unused]] void f(void);,
• enumeration: enum [[maybe_unused]] E {};,
• enumerator: enum { A [[maybe_unused]], B [[maybe_unused]] = 42 };.

If the compiler issues warnings on unused entities, that warning is suppressed for any entity declared maybe_unused.

Example

#include <assert.h>
 
[[maybe_unused]] void f([[maybe_unused]] _Bool cond1, [[maybe_unused]] _Bool cond2)
{
   [[maybe_unused]] _Bool b = cond1 && cond2;
   assert(b); // in release mode, assert is compiled out, and b is unused
              // no warning because it is declared [[maybe_unused]]
} // parameters cond1 and cond2 are not used, no warning
 
int main(void)
{
    f(1, 1);
}

See also