How do I list the symbols in a .so file

Question

How do I list the symbols being exported from a .so file? If possible, I'd also like to know their source (e.g. if they are pulled in from a static library).

I'm using gcc 4.0.2, if that makes a difference.

Solution 1

The standard tool for listing symbols is nm, you can use it simply like this:

nm -gD yourLib.so

If you want to see symbols of a C++ library, add the "-C" option which demangle the symbols (it's far more readable demangled).

nm -gDC yourLib.so

If your .so file is in elf format, you have two options:

Either objdump (-C is also useful for demangling C++):

$ objdump -TC libz.so

libz.so:     file format elf64-x86-64

DYNAMIC SYMBOL TABLE:
0000000000002010 l    d  .init  0000000000000000              .init
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 free
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __errno_location
0000000000000000  w   D  *UND*  0000000000000000              _ITM_deregisterTMCloneTable

Or use readelf:

$ readelf -Ws libz.so
Symbol table '.dynsym' contains 112 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000002010     0 SECTION LOCAL  DEFAULT   10
     2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND free@GLIBC_2.2.5 (14)
     3: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __errno_location@GLIBC_2.2.5 (14)
     4: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTable

Solution 2

If your .so file is in elf format, you can use readelf program to extract symbol information from the binary. This command will give you the symbol table:

readelf -Ws /usr/lib/libexample.so

You only should extract those that are defined in this .so file, not in the libraries referenced by it. Seventh column should contain a number in this case. You can extract it by using a simple regex:

readelf -Ws /usr/lib/libstdc++.so.6 | grep '^\([[:space:]]\+[^[:space:]]\+\)\{6\}[[:space:]]\+[[:digit:]]\+'

or, as proposed by Caspin,:

readelf -Ws /usr/lib/libstdc++.so.6 | awk '{print $8}';

Solution 3

objdump -TC /usr/lib/libexample.so

Solution 4

For shared libraries libNAME.so the -D switch was necessary to see symbols in my Linux

nm -D libNAME.so

and for static library as reported by others

nm -g libNAME.a

Solution 5

I kept wondering why -fvisibility=hidden and #pragma GCC visibility did not seem to have any influence, as all the symbols were always visible with nm - until I found this post that pointed me to readelf and objdump, which made me realize that there seem to actually be two symbol tables:

• The one you can list with nm
• The one you can list with readelf and objdump

I think the former contains debugging symbols that can be stripped with strip or the -s switch that you can give to the linker or the install command. And even if nm does not list anything anymore, your exported symbols are still exported because they are in the ELF "dynamic symbol table", which is the latter.

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