| __USE(3) | Library Functions Manual | __USE(3) | 
__USE —
#include <sys/cdefs.h>
void
  
  __USE(x);
__USE macro can be used to omit warnings produced by
  certain compilers when variables are being set, but not used in a function.
There are cases where it is simpler to mark a variable as used, as opposed to ifdef out its use:
#ifdef DEBUG_FOO
#define DPRINTF(a) printf a
#else
#define DPRINTF(a)
void
foo(void) {
	int var;
	var = getval();
	DPRINTF(("val is %d0, var));
}
In this case, ifdefing the code would make it:
void
foo(void) {
#ifdef DEBUG_FOO
	int var;
	var = getval();
	DPRINTF(("val is %d0, var));
#else
	(void)getval();
#endif
}
This is not desirable because it duplicates code. With the
    __USE macro this can be written as:
void
foo(void) {
	int var;
	var = getval();
#ifdef DEBUG_FOO
	DPRINTF(("val is %d0, var));
#else
	__USE(var);
#endif
}
without producing compiler warnings.
Although it is simple to write:
(void)var;
abstracting this into the macro allows for alternate implementations, as well as changing it to an empty implementation so that the liveness of the variable can be re-evaluated.
__USE is implemented as:
#define __USE(a) ((void)(a))
__USE should be used sparingly as it can cause valid
  warnings to be hidden.
Use of this macro is non-portable; this is part of the implementation namespace and should only be used in NetBSD code.
| October 17, 2013 | NetBSD 10.0 |