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