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The procps manpage states that the flags field in files in /proc/PID/fdinfo/ is an octal number indicating the file access mode and file status flags. The open manpage gives explanations of various flags (O_APPEND, O_ASYNC, etc.) but no corresponding octal values. Are these octal values listed anywhere, or do I have to search through the linux source code? (and if so then where?).

Ben
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1 Answers1

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I’m not aware of documentation listing the values of the flag constants, but you don’t need to search through source code or even header files, you can ask the compiler:

echo O_APPEND | gcc -include fcntl.h -E -

Going over all the flags in open(2) on x86-64 Linux:

$ for flag in APPEND ASYNC CLOEXEC CREAT DIRECT DIRECTORY DSYNC EXCL LARGEFILE NOATIME NOCTTY NOFOLLOW NONBLOCK PATH SYNC TMPFILE TRUNC; do printf '%s: ' O_$flag; echo O_$flag | gcc -D_GNU_SOURCE -include fcntl.h -E - | tail -n 1; done
Flag Value
O_APPEND 02000
O_ASYNC 020000
O_CLOEXEC 02000000
O_CREAT 0100
O_DIRECT 040000
O_DIRECTORY 0200000
O_DSYNC 010000
O_EXCL 0200
O_LARGEFILE 0
O_NOATIME 01000000
O_NOCTTY 0400
O_NOFOLLOW 0400000
O_NONBLOCK 04000
O_PATH 010000000
O_SYNC 04010000
O_TMPFILE (020000000 | 0200000)
O_TRUNC 01000

(Some of these are architecture-specific; for example O_LARGEFILE is 0100000 on i386.)

Stephen Kitt
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  • Thanks, that's very helpful.. however I still can't understand how to interpret the fdinfo flags; for example the fdinfo for the 0 symlink (i.e. STDIN) has flags of 02, which doesn't correspond to any of the flags you listed, and its access permissions are 700 for the symlink, and 620 for the pty slave it links to. What's going on? – Ben Mar 19 '21 at 22:44
  • OK, so I found the O_RDONLY (00), O_WRONLY (01) & O_RDWR (02) flags. However I have a file descriptor pointing to /dev/ptmx with flags = 0100002?! – Ben Mar 20 '21 at 01:45
  • OK, after some more googling I discovered that you can also use `lsof +fg ` to get a list of the fdinfo flags codes, with descriptions listed in the `lsof` manpage. From this I figured out that 0100002 = O_LARGEFILE|O_RDRW, which means that O_LARGEFILE = 0100000 even though your one-liner printed it as 0 on my x86_64 system, strange. – Ben Mar 20 '21 at 01:55