What is a casted parameter statement in C?

Question

in the following function

int f (some_struct* p)
{
    (void) p;
    /* something else */
    return 0;
}

what does the statement

(void) p; 

mean?

Answer 1

The statement does nothing at runtime, and results in no machine code.

It suppresses a compiler warning that p is unused in the body of the function. This is a portable and safe way to suppress this warning across a variety of different compilers, including GCC, Clang, and Visual C++.

Answer 2

“Cast to void” is a C language idiom that, by convention, suppresses compiler and lint warnings about unused variables or return values.

In this case, as Dietrich Epp correctly points out, it tells the compiler that you know that you're not using the argument p, and not to give you “unused argument” warnings about it.

The other use of this idiom, casting the return value of a function to void, is the traditional way of telling lint or, more importantly, other programmers that you'd made a conscious decision not to bother checking the return value of a function. For example:

(void)printf("foo")

Would mean “I know printf() returns a value, and I should really check it, but I've decided not to bother”.