Xavier de Gaye added the comment:
With the following module named dlsym.py, the command 'python dlsym.py log10'
produces the same successful output on linux and on Android API 21 (when
_decimal is not explicitly linked with libm.so, i.e.without changeset
b60b46ad8751):
'The symbol "log10" is resolved.'
---------------------- dlsym.py ---------------------------
import sys, os
from ctypes import *
if len(sys.argv) != 2:
sys.exit('Error: the symbol name is required.')
symbol = sys.argv[1]
libdl = CDLL('libdl.so')
libdl.dlopen.restype = c_void_p
libdl.dlsym.restype = c_void_p
hdle = libdl.dlopen(None, os.RTLD_NOW | RTLD_GLOBAL)
if hdle is None:
print('Cannot get a handle on the global symbol object.')
else:
v = libdl.dlsym(c_void_p(hdle), symbol.encode())
_not = 'not ' if v is None else ''
print('The symbol "%s" is %sresolved.' % (symbol, _not))
-------------------------------------------------------------------
This is as expected since the python executable is linked with the math
library. However on Android API 21 importing _decimal fails with the error
message reported earlier:
'dlopen failed: cannot locate symbol "log10"'.
So this seems to be a bug with Android dlopen() at API 21 since the 'log10'
symbol does exist in the process image relocation tables as shown by dlsym.py.
This is confirmed by the fact that the import does not fail anymore at API 24
(still without changeset b60b46ad8751). This change of behavior may be a side
effect of the changes reported in this bionic (Android libc) changelog entry:
https://android.googlesource.com/platform/bionic/+show/refs/heads/master/android-changes-for-ndk-developers.md#36
For completness, my (possibly wrong) interpretation of why changeset
b60b46ad8751 fixes the problem at API 21:
Changeset b60b46ad8751 adds the '-lm' command line option to the linker and
this adds [1] a new DT_NEEDED entry to the .dynamic section of the shared
library ELF file as shown by the command 'readelf -d
build/lib.linux-x86_64-3.7-pydebug/_decimal.cpython-37dm-x86_64-linux-gnu.so':
Dynamic section at offset 0x54d30 contains 26 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libm.so.6]
0x0000000000000001 (NEEDED) Shared library: [libpthread.so.0]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
...
The Android loader lookup for 'log10' is now successful at API 21 and the
symbol is found in the libm.so.6 library listed in the DT_NEEDED lists of
needed libraries.
[1] See option '--as-needed' in the 'ld' man pages or see the ELF
specification. Note that the Android toolchains use the clang compiler but
continue to use the GNU binutils tools including the linker 'ld'.
----------
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue29439>
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