__USE(3)

__USE(3) | Library Functions Manual | __USE(3)

# NAME

__USE — compile time macro that marks a variable as being used

# SYNOPSIS

#include <sys/cdefs.h>

void
__USE(x);

# DESCRIPTION

The __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.

# IMPLEMENTATION NOTES

__USE is implemented as:

    #define __USE(a)	((void)(a))

# SEE ALSO

cc(1), cdefs(3)

# CAVEATS

__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 7.1