What does (void) 'variable name' do at the beginning of a C function? [duplicate]

Answer 1

It works around some compiler warnings. Some compilers will warn if you don't use a function parameter. In such a case, you might have deliberately not used that parameter, not be able to change the interface for some reason, but still want to shut up the warning. That (void) casting construct is a no-op that makes the warning go away. Here's a simple example using clang:

int f1(int a, int b)
{
  (void)b;
  return a;
}

int f2(int a, int b)
{
  return a;
}

Build using the -Wunused-parameter flag and presto:

$ clang -Wunused-parameter   -c -o example.o example.c
example.c:7:19: warning: unused parameter 'b' [-Wunused-parameter]
int f2(int a, int b)
                  ^
1 warning generated.

Answer 2

It does nothing, in terms of code.

It's here to tell the compiler that those variables (in that case parameters) are unused, to prevent the -Wunused warnings.

Another way to do this is to use:

#pragma unused