CTASSERT —
compile time assertion macro
void
CTASSERT(expression);
The CTASSERT() macro evaluates
  expression at compile time and causes a compiler error
  if it is false.
The CTASSERT() macro is useful for
    asserting the size or alignment of important data structures and variables
    during compilation, which would otherwise cause the code to fail at run
    time.
The CTASSERT() macro should not be used in a header
  file. It is implemented using a dummy typedef, with a name (based on line
  number) that may conflict with a CTASSERT() in a
  source file including that header.
Assert that the size of the uuid structure is 16 bytes.
CTASSERT(sizeof(struct uuid) ==
  16);
This manual page was written by Hiten M. Pandya
  ⟨hmp@FreeBSD.org⟩.