Did you read the first sentence in that paragraph? That entire paragraph is talking about when _POSIX_SYNCHRONIZED_IO is not defined: > If _POSIX_SYNCHRONIZED_

Did you read the first sentence in that paragraph? That entire paragraph is talking about when _POSIX_SYNCHRONIZED_IO is not defined: > If _POSIX_SYNCHRONIZED_IO is not defined, the wording relies heavily on the conformance document to tell the user what can be expected from the system. [...] If you check your Linux system, you will see _POSIX_SYNCHRONIZED_IO is defined: printf "%s\n" "#include <unistd.h>" "#ifdef _POSIX_SYNCHRONIZED_IO" "#error _POSIX_SYNCHRONIZED_IO is defined" "#endif" | gcc -fsyntax-only -x c - and hence the rest of that paragraph isn't talking about this situation.