__ver must be 3 or the behavior of these functions is undefined.
__filename is as specified in POSIX.
__filedesc is as specified in POSIX.
__stat_buf is as specified in POSIX.
__xstat(3, __filename, __stat_buf) has the same specification as stat(__filename, __stat_buf) as specified by POSIX.
__lxstat(3, __filename, __stat_buf) has the same specification as lstat(__filename, __stat_buf) as specified by POSIX.
__fxstat(3, __filedesc, __stat_buf) has the same specification as fstat(__filedesc, __stat_buf) as specified by POSIX.
Note that the struct stat used by these functions is not the one that the kernel uses.
__xstat, __lxstat, and __fxstat are not in the source standard; they are only in the binary standard.
stat, lstat, and fstat are not in the binary standard; they are only in the source standard.