Retrieving Global Variable Values from Command Line - c

Question

In one particular project, we're trying to embed version information into shared object files. We'd like to be able to use some standard linux tool to parse the shared object to determine the version for automated testing.
Currently I have "const int plugin_version = 14;". I can use 'nm' and 'objdump' and verify that it's there:
00000000000dcfbc r plugin_version
I can't, however, seem to be able to get the value of that variable easily from command line. I figured there'd be a POSIX tool for showing the initialized values for globals. I have contemplated using a format for the variable as the information itself, ie, plugin_version_14, but that seems like a huge hack. Embedding the information in the filename unfortunately is NOT an option. Any other suggestions welcome.

Answer 1

You could embed it as a string
"MAGIC MARKER STRING VERSION: 4.56 END OF MAGIC" then just look for "MAGIC MARKER STRING" in the file and extract the version information that comes after it.
if you make it a standard, you could easily make command line tool to find these embeded strings on all your software.
if you require it also to be an int, a little macro magic will construct both the int and magic string to make sure they are never out of synch.

Answer 2

There's a couple of options I think.
My first instinct is to make sure the version information lives in its own section in the ELF file. You can use objdump -s -j name of section /bin/whatever.
This rather relies on objdump being available of course.
Alternatively you can do what Keith suggested, and just use 'strings', along with a magical marker string. This feels a little hackish, but should work quite well.
Finally, why don't you just add a --version command line option? You can then store the version information however you like, and trivially retrieve it using the one tool which is certain to be installed on any system which has your software.

Answer 3

A terrible hack that I've used in the past is to embed the version information in a variable name, so nm will show:
00000000000dcfbc r plugin_version_14

Answer 4

Why not writing your own tool to get that version in C/C++ ? You could Use dlopen, then dlsym to get the symbol and print its value to standard output. This way you also verify if the symbol is already there. It looks like 20 ~ 30 lines of code to me and about 20 minutes of your life :)
I know that the question is about command line, but writing such a tool yourself should be easy (especially if such a command line tool does not exist).

Answer 5

If the binary is not stripped, you could use gdb to print the variable. (I just tried to script gdb, but it seems to refuse work if stdin is not a tty, maybe expect will do the job ? )

Answer 6

If you can accept using python, this might help:
import struct
import sys
import subprocess
if __name__ == '__main__':
so = sys.argv[1]
sym = sys.argv[2]
addr = subprocess.check_output('nm %s | grep %s' % (so, sym), shell=True)
addr = int(addr.split()[0], 16)
so_file = open(so)
so_file.seek(addr)
data = so_file.read(4)
print struct.unpack('#i', data)[0]
Disclaimer: This script doesn't do any error checking (if you like it I'm sure you can come up with some ;)). It also assumes you're reading a 4-byte native int value.
$ cat global.c
const int plugin_version = 14;
$ python readsym.py global.so plugin_version
14