What does "before-main stdio file descriptors are not guaranteed to be open on unix platforms" mean?

[nabijaczleweli]

Location

library/std/src/lib.rs, module-level rustdoc

• before-main stdio file descriptors are not guaranteed to be open on unix platforms

Summary

What does this mean? I'm assuming this was supposed to be "before main, " since file descriptors are a property of the process at all times, but even then... this implies that fd0/1/2 may be... closed? between exec returning and the runtime calling main? and then become open somehow? on main entry? I don't believe there to be any UNIX system ever in history where this is true?

[nabijaczleweli]

I don't see this discussed in #115247 or #110708 or #110897 where this language originates, so I don't know what it's trying to say.

[zopsicle]

this implies that fd0/1/2 may be... closed?

close(0);
close(1);
close(2);
exec("a-rust-program");

and then become open somehow?

rust/library/std/src/sys/pal/unix/mod.rs

Lines 36 to 39 in b522e7c

	// The standard streams might be closed on application startup. To prevent
	// std::io::{stdin, stdout,stderr} objects from using other unrelated file
	// resources opened later, we reopen standards streams when they are closed.
	sanitize_standard_fds();

[nabijaczleweli]

Thanks, this indicates what this should say instead.