Unfortunately, you misunderstand how CLOEXEC works and how the Go runtime implements the feature you think libc lacks.

This syscall level behavior is POSIX-specified[1] since at least the 2008 edition[2]:

> O_CLOEXEC > If set, the FD_CLOEXEC flag for the new file descriptor shall be set.

What that means is, any C program or Go program that passes the O_CLOEXEC flag to open(2) on a POSIX 2008 conforming system (including Linux and the BSDs, for example), will atomically create the fd without inherit behavior. There is no "short window" and hasn't been for more than a decade. The Go runtime must use that flag to provide that property; there is no other way on these systems. Libc users are of course able to use the same flag.

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/o...

[2]: https://pubs.opengroup.org/onlinepubs/9699919799.2008edition...