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List:       linux-man
Subject:    file descriptor flag retrieval
From:       Tycho Kirchner <tychokirchner () mail ! de>
Date:       2019-03-18 12:09:45
Message-ID: 415175d5-7cf3-65d4-ac6a-3237b04b29e4 () mail ! de
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Hi,
in proc(5)
it says for the fdinfo-entries regarding the flags:

> This is an octal number that displays the file access
> mode and file status flags (see open(2)).  If the
> close-on-exec file descriptor flag is set, then flags
> will also include the value O_CLOEXEC.


That is true, but not complete. The 'file creation flags'

O_DIRECTORY, O_NOFOLLOW, O_TMPFILE

are also returned.
Further I think it is worth mentioning that those flags are equal to the 
flags received via

fcntl(fd , F_GETFL);

*except* O_CLOEXEC, which is retrieved separately (and indirectly) via

fcntl(fd , F_GETFD);

as FD_CLOEXEC.

Similarly, in fnctl(2), it should say for F_GETFL:
"
Return (as the function result) the file access mode and the file status 
flags as well as the file creation flags O_DIRECTORY, O_NOFOLLOW and 
O_TMPFILE; arg is ignored.
"

Consequently the file creation flags O_CREAT, O_EXCL, O_TRUNC, O_NOCTTY 
are not returned by either call. Despite the inconsistency with 
O_CLOEXEC (a mistake, in my opinion), it seems that the reason for 
omitting those is to being able to retrieve them and reopening a file 
with the same flags, except those, that would destroy it (that does 
*not* apply to O_NOCTTY, which is not returned for reasons I don't 
understand).	


Test-system:
$ uname -a
Linux debian-foo 4.9.0-8-amd64 #1 SMP Debian 4.9.144-3.1 (2019-02-19) 
x86_64 GNU/Linux


Thanks
Tycho

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