gcc - A static library with undefined symbols?

Question

I'm trying to build a project using a static library, so that the binary can be used even if the library isn't installed. However, I get lots of errors about undefined symbols when I try to do so.

Looking at the library, I see it has tons of undefined symbols, even though it's a .a static lib:

nm - u /usr/local/lib/libthis.a
....
U EVP_DigestFinal_ex
U EVP_DigestInit_ex
U EVP_DigestUpdate
U EVP_MD_CTX_cleanup
U EVP_MD_CTX_init

Those seem to be from openssl; others seem to be from libbzip2; etc.

Questions: 1. Why does the static (.a) lib have dependencies on shared objects (e.g. libopenssl) that aren't statically compiled? 2. How do I solve this? Trying to manually add -lssl doesn't seem to work. How do I get the binary to compile and not have external dependcies?

Static libs are supposed to have plenty of undefined symbols (they are after all just a bunch of .o files with nothing extra). Those are not dependencies on shared objects, they are just plain undefined symbols. It is when you build the final executable that you will have to link with a gazillion libraries to resolve all symbols. — Marc Glisse, Nov 22 '15 at 07:51

score 28 · Accepted Answer · edited May 23 '17 at 12:16

Why does the static (.a) lib have dependencies on shared objects (e.g. libopenssl) that aren't statically compiled?

Just about every static library that you can build will have unresolved symbols, e.g.

int my_open_for_read(const char *filename)
{
  return open(filename, O_RDONLY);  // unresolved reference to open
}

As Marc Glisse pointed out, this a plain unresolved symbol, not a dependency on libc.so.

How do I solve this?

There is no problem to solve here. When you link your binary, you get to decide which libraries to link statically, and which to link dynamically.

Trying to manually add -lssl doesn't seem to work.

This should work:

gcc main.o -lthis -lssl

Possibly you did something like

gcc main.o -lssl -lthis

which is wrong: the order of libraries on the link line matters.

How do I get the binary to compile and not have external dependcies?

Most OSes support using fully-static binaries. Generally this should not be your goal: it makes for less portable binaries, and their use is strongly discouraged.

If you really do want to produce a fully-static binary, link it with -static flag.

Why do you say full static is less portable?

Because they are.

if the user doesn't have the exact same build of the lib, the binary won't be portable with shared libs, but will be portable with static.

This is incorrect: most shared libraries support backward compatibility, e.g. libc.so.6 version 2.22 will happily run executables linked against version 2.3.6 from 10 years ago.

If you do ldd firefox

You need to pay attention to what you are doing:

file -L `which /usr/bin/firefox`
/usr/bin/firefox: POSIX shell script, ASCII text executable

If you look inside the shell script, you'll discover that it invokes /usr/lib/firefox/firefox, and that binary is dynamically linked:

ldd /usr/lib/firefox/firefox
    linux-vdso.so.1 =>  (0x00007ffca278d000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f511731b000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f5117117000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f5116e13000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f5116b0d000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f51168f7000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f5116532000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f5117757000)

Thanks. Why do you say full static is _less_ portable? Since Linux uses .so and not DLLs, if the user doesn't have the exact same build of the lib, the binary won't be portable with shared libs, but will be portable with static. If you do `ldd firefox`, for example, you'll see it's fully static - how else can you have a portable binary on Linux? — SRobertJames, Nov 23 '15 at 09:38
Nice update, especially catching the firefox script! But the libs ldd shows are all the system libs, which I know a) can't be be static and b) are usually backwards compatible. My points still stand for non-system libs (eg zlib ssl and dozens more) ; notice that firefox uses those static, which I believe is the only portable way to do so — SRobertJames, Nov 23 '15 at 23:00
@SRobertJames I only argued against fully-static binaries. Linking in non-system libraries statically is fine, so long as you continue to use system libraries dynamically. A *literal* reading of your question: "How to ... not have external dependcies" implies fully-static linking. — Employed Russian, Nov 24 '15 at 00:52
@SRobertJames Your point regarding using non-static system libraries is a very good one. — qweruiop, Dec 15 '16 at 20:57

gcc - A static library with undefined symbols?

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