Why 2 static libs can't call to each other's function

Question

I have 2 static libs: liba.a, libb.a and a executable that link both. liba.a is calling to function foo which is defined in libb.a both libs compile successfully BUT my exe seems to have linker errors:

undefined reference to `foo'

need help...

using ubuntu 14.04. gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)

You need help with writing problem descriptions. You also don't seem to be able to spell "static"...? — Lightness Races in Orbit, Jan 29 '15 at 17:22
How are you compiling the executable? Are you making sure to link both libs? — sjdowling, Jan 29 '15 at 17:23
You need to show the code that defines `foo`. There's not enough information here to answer your question. — Mark Ransom, Jan 29 '15 at 17:24
@yehudahs: What prevented you from simply writing that out in full, so as to be unambiguous? — Lightness Races in Orbit, Jan 29 '15 at 17:26
your question should look like this: this is liba (header and src with one line defining foo), this is libb (header and src with one line defining a function (if any). And this is how I compile: 3 compile commands. This is the error I get — bolov, Jan 29 '15 at 17:26

score 4 · Accepted Answer · edited May 23 '17 at 12:33

4

When you build:

g++ liba.a libb.a myCode.o -o myExe

This is wrong, because the order of the arguments matters. If myCode uses symbols from liba and libb, those libraries must be specified after it on the command line:

g++ myCode.o liba.a libb.a -o myExe

Alternatively, you can request that the linker treat all three as a "group"; if you do so, dependencies will be resolved for you within that group without needing to worry about order:

g++ "-Wl,--start-group" liba.a libb.a myCode.o "-Wl,--end-group" -o myExe

edited May 23 '17 at 12:33

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answered Jan 29 '15 at 17:23

Lightness Races in Orbit

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you are right !!! I always thought that the linker searches ALL the libs and .o file for function names. Didn't know that it searches only forwards... :( it's a pity that I had to spend some hours of my life to understand it. – yehudahs Jan 29 '15 at 17:30
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@yehudahs: No, it's a pity you didn't read [the FAQ](http://stackoverflow.com/a/24675715/560648)! – Lightness Races in Orbit Jan 29 '15 at 17:32

Why 2 static libs can't call to each other's function

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