[systemd-devel] Why do we must use the close_all_fds(3) function in some files?

[Lennart Poettering]

On Tue, 17.06.14 07:59, Geunsik Lim (geunsik.lim at gmail.com) wrote:

> 
> It seems that a goal of close_all_fds() is garbage collector to guarantee
> available file descriptors  for new fopen() system call.
> Nowadays, the modern computer system is multi-process/multi-thread scheme
> more than single-process/multi-thread.
> Does systemd have to care the number of "open file size"? Actually, Is this
> function need for safety net?

Yes, because 1024 is the default limit per process and that is very
small. Also, file descriptors are reference to resources, most
importantly files but also devices, file locks, and other objects. If we
keep them open, these resources stay busy, and possibly unavailable to
others.

> For example,
> invain at u1204lgs:/opt/git-systemd$ ulimit  -n
> 1024
> 
> If Systemd does not execute the close_all_fds() functions,
> What will be happened in real environment?

Depends on the system and what you do. It's just a matter of
cleanliness, and it's pretty much for free anyway as the thing basically
becomes a NOP if no fd is open.

> On the other hand, if systemd have to  not close more than two file
> descriptors(e.g. 5, 17, 19) for some case to release Linux distribution
> based on Systemd, can we use close_all_fds(except, ***)? In this case, Do
> we have to specify  withclose_all_fds(except, 3 + 19)?

I cannot parse this, sorry...

I am really not sure why you want to get rid of close_all_fds()?

Lennart

-- 
Lennart Poettering, Red Hat