[issue42580] ctypes.util.find_library("libc") fails

[Eryk Sun]

Eryk Sun  added the comment:

Can't there be a library named "libc" with the shared module "liblibc.so"? It 
seems to me that the _is_elf() function added in issue 41976 is breaking the 
contract that "[i]f no library can be found, returns None". It's not supposed 
to leak arbitrary exceptions from the implementation if the library isn't found.

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[issue42580] ctypes.util.find_library("libc") fails

[STINNER Victor]

STINNER Victor  added the comment:

The documentation is explicit: you must not include the "lib" prefix.
https://docs.python.org/dev/library/ctypes.html#ctypes.util.find_library

See also examples:
https://docs.python.org/dev/library/ctypes.html#finding-shared-libraries

If it worked with "lib" prefix in Python 3.8, it wasn't on purpose.

You should fix your code, but Python works as expected. I close the issue.

--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue42580] ctypes.util.find_library("libc") fails

[Vadym Stupakov]

Vadym Stupakov  added the comment:

I mean, find_library relies on gcc linker, when you pass library name to the 
linker, it automatically ads "lib" prefix to the library
so, when you pass "libc", linker ads "lib" so u have an error with "liblibc" 
name.

just man ld and see "-l" option:

"
-l namespec
   --library=namespec
   Add the archive or object file specified by namespec to the
   list of files to link.  This option may be used any number of
   times.  If namespec is of the form :filename, ld will search
   the library path for a file called filename, otherwise it
   will search the library path for a file called libnamespec.a.
"

as you can see, you pass not a library name, but a "namespec"
which then transforms to "libnamespec"

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[issue42580] ctypes.util.find_library("libc") fails

[Vadym Stupakov]

Vadym Stupakov  added the comment:

> ctypes.util.find_library("libc") used to work in 3.8, not working in 3.9. As 
> I said before, ctypes.util.find_library("c") works in both 3.8 and 3.9.

no, it doesn't work (and it shouldn't) neither in python 3.8 nor 3.7

Python 3.8.5 (default, Sep  4 2020, 07:30:14) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.19.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import ctypes.util
In [2]: a = ctypes.util.find_library("libc")
In [3]: print(a)
None

Python 3.7.6 (default, Jan  8 2020, 19:59:22) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.16.1 -- An enhanced Interactive Python. Type '?' for help.
Python 3.7.6 (default, Jan  8 2020, 19:59:22) 

import ctypes.util
a = ctypes.util.find_library("libc")
print(a)
None

as I said, adding prefix "lib" is wrong

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[issue42580] ctypes.util.find_library("libc") fails

[Eryk Sun]

Eryk Sun  added the comment:

Issue 41976 added ctypes.util._is_elf() to filter out linker scripts such as 
"libc.so". The PR was backported to 3.7. _is_elf() assumes the trace result has 
absolute paths that can be opened, but Matthias is getting the relative 
filename "liblibc.a" in the result. Whatever the reason, I think if the file 
can't be opened for reading, then _is_elf() should just return False. For 
example:

def _is_elf(filename):
"Return True if the given file is an ELF file"
elf_header = b'\x7fELF'
try:
with open(filename, 'br') as thefile:
return thefile.read(4) == elf_header
except OSError:
return False

--
nosy: +eryksun

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[issue42580] ctypes.util.find_library("libc") fails

[Matthias Klose]

Matthias Klose  added the comment:

ctypes.util.find_library("libc") used to work in 3.8, not working in 3.9. As I 
said before, ctypes.util.find_library("c") works in both 3.8 and 3.9.

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[issue42580] ctypes.util.find_library("libc") fails

[Vadym Stupakov]

Vadym Stupakov  added the comment:

So, can we close it?

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[issue42580] ctypes.util.find_library("libc") fails

[STINNER Victor]

STINNER Victor  added the comment:

Oh right, I always use find_library("c"), not find_library("libc").

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[issue42580] ctypes.util.find_library("libc") fails

[Vadym Stupakov]

Vadym Stupakov  added the comment:

Note, that adding "lib" prefix to any library is wrong, that's why it returns 
"None" or raises the exception.

And it works fine when you call find_library("c")

I guess the issue, that was reported here, is about raising and exception 
instead of returning "None", but not about finding library, isn't it?

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[issue42580] ctypes.util.find_library("libc") fails

[STINNER Victor]

STINNER Victor  added the comment:

> _findLib_gcc() uses "gcc -Wl,-t -o tmp -lc" command and search for "lib" in 
> the created "tmp" file.

Oh, it looks for the "lib" in the process stdout (not in the created "tmp" 
file).

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[issue42580] ctypes.util.find_library("libc") fails

[STINNER Victor]

STINNER Victor  added the comment:

This function is quite complicated on Linux:

def find_library(name):
# See issue #9998
return _findSoname_ldconfig(name) or \
   _get_soname(_findLib_gcc(name)) or 
_get_soname(_findLib_ld(name))

_findSoname_ldconfig() uses "/sbin/ldconfig -p" and searchs for 'libc6,x86-64' 
in the output (on x86-64).

_findLib_gcc() uses "gcc -Wl,-t -o tmp -lc" command and search for "lib" in the 
created "tmp" file.

_findLib_ld() uses "ld -t -lc" command and searchs for "lib" pattern.

The exact code is more complicated :-) You should debug this issue by running 
these commands manually on Debian.

Note: python3.9 -c 'import ctypes.util; print(ctypes.util.find_library("c"))' 
commands displays libc.so.6 with Python 3.8, 3.9 and 3.10 on Fedora. I cannot 
reproduce the issue on Fedora 33 (x86-64).

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nosy: +vstinner

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[issue42580] ctypes.util.find_library("libc") fails

[RedEyed]

RedEyed  added the comment:

can't reproduce

(py39) ➜  ~ ipython 
Python 3.9.1 (default, Dec 11 2020, 14:32:07) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.20.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import ctypes.util
In [2]: a = ctypes.util.find_library("libc")
In [3]: print(a)
None

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nosy: +RedEyed

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[issue42580] ctypes.util.find_library("libc") fails

[Alex Shpilkin]

Change by Alex Shpilkin :


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nosy: +Alex Shpilkin

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[issue42580] ctypes.util.find_library("libc") fails

[Matthias Klose]

Matthias Klose  added the comment:

ctypes.util.find_library("c") works in both 3.8 and 3.9

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[issue42580] ctypes.util.find_library("libc") fails

[Matthias Klose]

New submission from Matthias Klose :

regression compared to 3.8:

$ python3.9
Python 3.9.1rc1 (default, Nov 27 2020, 19:38:39) 
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> ctypes.util.find_library("libc")
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'ctypes' is not defined
>>> import ctypes.util
>>> ctypes.util.find_library("libc")
Traceback (most recent call last):
  File "", line 1, in 
  File "/usr/lib/python3.9/ctypes/util.py", line 341, in find_library
_get_soname(_findLib_gcc(name)) or _get_soname(_findLib_ld(name))
  File "/usr/lib/python3.9/ctypes/util.py", line 147, in _findLib_gcc
if not _is_elf(file):
  File "/usr/lib/python3.9/ctypes/util.py", line 99, in _is_elf
with open(filename, 'br') as thefile:
FileNotFoundError: [Errno 2] No such file or directory: b'liblibc.a'

also note that the shared library is installed.

--
components: ctypes
keywords: 3.9regression
messages: 382592
nosy: doko
priority: normal
severity: normal
status: open
title: ctypes.util.find_library("libc") fails
versions: Python 3.9

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