openat is missing from Posix
The syscall openat(2)
(POSIX.1-2008.) is missing from Posix. Relevant man page section:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int openat(int dirfd, const char *pathname, int flags);
int openat(int dirfd, const char *pathname, int flags, mode_t mode);