How do I get in python the maximum filesystem path length in unix?

Question

In the code I maintain I run across:

from ctypes.wintypes import MAX_PATH

I would like to change it to something like:

try:
    from ctypes.wintypes import MAX_PATH
except ValueError: # raises on linux
    MAX_PATH = 4096 # see comments

but I can't find any way to get the value of max filesystem path from python (os, os.path, sys...) - is there a standard way or do I need an external lib ?

Or there is no analogous as MAX_PATH in linux, at least not a standard among distributions ?

Answer

try:
    MAX_PATH = int(subprocess.check_output(['getconf', 'PATH_MAX', '/']))
except (ValueError, subprocess.CalledProcessError, OSError):
    deprint('calling getconf failed - error:', traceback=True)
    MAX_PATH = 4096

It's 4096 on Linux - http://unix.stackexchange.com/questions/32795/what-is-the-maximum-allowed-filename-and-folder-size-with-ecryptfs — jonrsharpe, Sep 27 '15 at 11:56

Antti Haapala -- Слава Україні · Answer 1 · 2022-10-19T05:31:42.910

15

The way to do it correctly is to use the os.pathconf or os.fpathconf with PC_ prefixed names:

>>> os.pathconf('/', 'PC_PATH_MAX')
4096
>>> os.pathconf('/', 'PC_NAME_MAX')
255

Notice that the maximum length of a path component may vary from directory to another, as it is filesystem dependent, so you might have os.pathconf('/', 'PC_NAME_MAX') as 255 and os.pathconf('/', 'PC_NAME_MAX') as 12, say.

edited Oct 19 '22 at 05:31

answered Jul 08 '20 at 13:51

Antti Haapala -- Слава Україні

129,958
22
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Thanks what is the difference with the accepted answer - why is `PC_` prefixed properties more correct? – Mr_and_Mrs_D Jul 08 '20 at 14:04
2

@Mr_and_Mrs_D this calls a python function in a couple of nanoseconds, and the accepted answer executes a **unix shell** (bash) that executes a program that gets the value and returns that value as a string. – Antti Haapala -- Слава Україні Jul 08 '20 at 14:17
Thanks - I mean `PC_PATH_MAX` vs `PATH_MAX`. I guess PC stands for pathconfig? Seems strange that I query different variables – Mr_and_Mrs_D Jul 08 '20 at 14:37
1

@Mr_and_Mrs_D those are (unfortunately) the C library constant names, even though such is not necessary in Python. I do not know who is to blame. – Antti Haapala -- Слава Україні Jul 08 '20 at 17:19

WojtekCh · Accepted Answer · 2015-09-28T07:30:45.757

4

You can read this values from files:

* PATH_MAX (defined in limits.h)
* FILENAME_MAX (defined in stdio.h)

Or use subprocess.check_output() with getconf function:

$ getconf NAME_MAX /
$ getconf PATH_MAX /

as in the following example:

name_max = subprocess.check_output("getconf NAME_MAX /", shell=True)
path_max = subprocess.check_output("getconf PATH_MAX /", shell=True)

to get values and fpath to set different values for files.

edited Sep 28 '15 at 07:30

answered Sep 27 '15 at 19:56

WojtekCh

90
3

First things first - is `shell=True` used to avoid passing absolute paths ? And is this getconf call portable ? May I need root rights using it ? – Mr_and_Mrs_D Sep 28 '15 at 14:16
1. `shell=True` makes that the specified command will be executed through the shell 2. getconf call is portable in unix/linux systems, not in windows 3. root is not required – WojtekCh Sep 29 '15 at 07:54
Alright +1 for additional effort - 1. shell=True - I know what it does but what difference does it make here (I tested without - I get `OSError: [Errno 2] No such file or directory` - why?)? In other words could you rewrite this with Popen ? 4. Apparently I should catch CalledProcessError __and__ OSError __and__ ....? – Mr_and_Mrs_D Sep 29 '15 at 16:11
1. Here you are: `name_max = subprocess.check_output(["getconf", "NAME_MAX", "/"], shell=False)` = `name_max = subprocess.Popen("getconf NAME_MAX /", stdout=subprocess.PIPE, shell=True)` = `name_max = subprocess.Popen(["getconf", "NAME_MAX", "/"], stdout=subprocess.PIPE, shell=False)` When you pass shell=True, Popen expects a single string argument, not a list. And if args is a sequence['1', , ], the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. – WojtekCh Sep 30 '15 at 23:12
4. The minor difference is that check_output will raise a Python error if the subprocess returns a non-zero exit status. With Popen you can control that: `name_max = subprocess.Popen("getconf NAME_MAX /", stdout=subprocess.PIPE, shell=True) output, error = process.communicate() retcode = process.poll()` – WojtekCh Sep 30 '15 at 23:13

How do I get in python the maximum filesystem path length in unix?

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