Add the following:

// Note: the Posix API aims to capture functionality available on all Posix compliant
// platforms, but in practice the implementations may depend on non-Posix
// functionality. For example, the use of lseek64 and ftruncate64. This use
// of non-Posix API's is made possible by compiling/linking in a mode that
// is not restricted to being fully Posix complaint, such as by declaring
// -D_GNU_SOURCE. But be aware that in doing so we may enable non-Posix
// behaviour in API's that are defined by Posix. For example, that SIGSTKSZ
// is not defined as a constant as of Glibc 2.34.