G++: in What Order Should Static and Dynamic Libraries Be Linked

When to use dynamic vs. static libraries

Static libraries increase the size of the code in your binary. They're always loaded and whatever version of the code you compiled with is the version of the code that will run.

Dynamic libraries are stored and versioned separately. It's possible for a version of the dynamic library to be loaded that wasn't the original one that shipped with your code if the update is considered binary compatible with the original version.

Additionally dynamic libraries aren't necessarily loaded -- they're usually loaded when first called -- and can be shared among components that use the same library (multiple data loads, one code load).

Dynamic libraries were considered to be the better approach most of the time, but originally they had a major flaw (google DLL hell), which has all but been eliminated by more recent Windows OSes (Windows XP in particular).

Use both static and dynamically linked libraries in gcc

Statically linking against any system library, and especially against libc, on modern UNIX or Linux systems makes the binary significantly less portable. Just don't do it.

Instead, use backward compatibility (binaries linked on an older system continue to run on all newer ones) to your advantage, either by linking your binary on an old system (I use RedHat 6.2, and I have not seen a Linux system where my binary will not run in the last 8 years), or by using something like autopackage (which has been deleted after this answer was written).

To answer your original question:

gcc main.o -Wl,-Bstatic -lfoo -Wl,-Bdynamic

will cause linker to use archive version of libfoo. [It is important to have the trailing -Wl,-Bdynamic precisely so you don't force static libc.]

GCC static linking order

You may be confused (as it seems many people are) by taking the shortcut of
compiling and linking in one command.

g++ -lIrrlicht -lGL -lXxf86vm -lXext -lX11 -lXcursor main.cpp

is a shortcut functionally equivalent to:

# g++ invokes the C++ compiler (cc1plus). The linkage options are ignored.
g++ -c -o deleteme.o main.cpp

# g++ invokes the system linker (ld). The linkage options are passed.
g++ -lIrrlicht -lGL -lXxf86vm -lXext -lX11 -lXcursor -o a.out deleteme.o
rm deleteme.o

If you executed both steps explicitly, you would do, e.g.

# Compile step
g++ -c -o main.o main.cpp
# Link step.
g++ -lIrrlicht -lGL -lXxf86vm -lXext -lX11 -lXcursor -o prog main.o

In the link step, g++ covertly adds the default C++ linkage options for the host
system to the commandline, then hands it off to the linker. The C++ compiler is
not involved.

By default, the linker will examine a library at most once, when it is encountered
in the commandline linkage sequence, and it will examine the library only to see
if the library can resolve any hitherto unresolved symbols referenced earlier in the
linkage sequence. At the end of the linkage sequence, if all referenced symbols are
resolved, the linkage succeeds and otherwise fails.

The linkage:

g++ -lIrrlicht -lGL -lXxf86vm -lXext -lX11 -lXcursor -o a.out deleteme.o

will fail because all the libraries appear before any object file. When each library
is reached, the linker has discovered no unresolved symbols yet so the library is ignored.
When the linker finally reaches the trailing object file and discovers some unresolved
symbols, they remain unresolved.


g++ main.cpp -lIrrlicht -lGL -lXxf86vm -lXext -lX11 -lXcursor

is equivalent to:

g++ -c -o deleteme.o main.cpp    
g++ -o a.out deleteme.o -lIrrlicht -lGL -lXxf86vm -lXext -lX11 -lXcursor
rm deleteme.o

in which the linkage order is correct, with the symbols that require definitions
being discovered before the libraries that provide them.

Same symbols in different libraries and linking order

This absolutely violates the one definition rule in cases 1&2. In case 3, since you explicitly specify which version of the function to execute it may or may not. Violating the ODR is undefined behavior, no diagnostic required.


Every program shall contain exactly one definition of every non-inline
function or variable that is odr-used in that program; no diagnostic

gcc ld: method to determine link order of static libraries

You want a topological sort.

The tsort program will do that, but you'll need to do more work to use it [be prepared to write a perl/python script]. Also, there is another way as well. And, I will get to the "howto" below as I've done this sort of thing before.

The short answer: Use --start-group liblist --end-group and be done with it.

For a few reasons:

An ld group is smart. It doesn't just loop on the files. It makes an initial pass through the group, but remembers the symbols. So, on subsequent passes it uses the cached symbol table information, so it's very fast.

For complex interactions, you may not be able to get rid of all the cycles with a toposort, so you'll still need a group even if liblist has been topo sorted.

Just how much time are we talking about? And, how much time do you think will be saved? How will you measure things to prove you really need this.

Go for the gold

Instead of using ld, consider using ld.gold. It has been rewritten from scratch to not use libbfd [which is slow] and operates on ELF files directly. The primary motivation for creating it was simplicity and speed.

How to topologically sort a library list

If we do info coreutils, the tsort section will give an example of how to toposort a symbol table.

But, before we can get to that, we'll need to get the symbols. For a .a file, nm can provide the list: nm -go <liblist>.

The output will look like:

libbfd.a:archive.o:0000000000000790 T _bfd_add_bfd_to_archive_cache
libbfd.a:archive.o: U bfd_alloc
libbfd.a:archive.o:0000000000000c20 T _bfd_append_relative_path
libbfd.a:archive.o: U bfd_assert
libbfd.a:archive.o: U bfd_bread
libbfd.a:archive.o:00000000000021b0 T _bfd_bsd44_write_ar_hdr
libbfd.a:archive.o: U strcpy
libbfd.a:archive.o: U strlen
libbfd.a:archive.o: U strncmp
libbfd.a:archive.o: U strncpy
libbfd.a:archive.o: U strtol
libbfd.a:archive.o: U xstrdup
libbfd.a:bfd.o: U __asprintf_chk
libbfd.a:bfd.o:00000000000002b0 T _bfd_abort
libbfd.a:bfd.o:0000000000000e40 T bfd_alt_mach_code
libbfd.a:bfd.o: U bfd_arch_bits_per_address
libbfd.a:bfd.o:0000000000000260 T bfd_assert
libbfd.a:bfd.o:0000000000000000 D _bfd_assert_handler
libbfd.a:bfd.o:0000000000000450 T bfd_canonicalize_reloc
libbfd.a:bfd.o: U bfd_coff_get_comdat_section
libbfd.a:bfd.o:0000000000000510 T _bfd_default_error_handler
libbfd.a:bfd.o:0000000000000fd0 T bfd_demangle
libbfd.a:bfd.o: U memcpy
libbfd.a:bfd.o: U strchr
libbfd.a:bfd.o: U strlen
libbfd.a:opncls.o:0000000000000a50 T bfd_openr
libbfd.a:opncls.o:0000000000001100 T bfd_openr_iovec
libbfd.a:opncls.o:0000000000000b10 T bfd_openstreamr
libbfd.a:opncls.o:0000000000000bb0 T bfd_openw
libbfd.a:opncls.o:0000000000001240 T bfd_release
libbfd.a:opncls.o: U bfd_set_section_contents
libbfd.a:opncls.o: U bfd_set_section_size
libbfd.a:opncls.o:0000000000000000 B bfd_use_reserved_id
libbfd.a:opncls.o:00000000000010d0 T bfd_zalloc
libbfd.a:opncls.o:00000000000011d0 T bfd_zalloc2

libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000100 T g_allocator_free
libglib-2.0.a:libglib_2_0_la-gallocator.o:00000000000000f0 T g_allocator_new
libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000150 T g_blow_chunks
libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000160 T g_list_push_allocator
libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000060 T g_mem_chunk_alloc
libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000090 T g_mem_chunk_alloc0
libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000110 T g_mem_chunk_clean
libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000120 T g_mem_chunk_reset
libglib-2.0.a:libglib_2_0_la-gallocator.o:00000000000001b0 T g_node_pop_allocator
libglib-2.0.a:libglib_2_0_la-gallocator.o:00000000000001a0 T g_node_push_allocator
libglib-2.0.a:libglib_2_0_la-gallocator.o: U g_return_if_fail_warning
libglib-2.0.a:libglib_2_0_la-gallocator.o: U g_slice_alloc
libglib-2.0.a:libglib_2_0_la-gallocator.o: U g_slice_alloc0
libglib-2.0.a:libglib_2_0_la-gallocator.o: U g_slice_free1
libglib-2.0.a:libglib_2_0_la-gallocator.o:0000000000000190 T g_slist_pop_allocator
libglib-2.0.a:libglib_2_0_la-gslice.o: U g_private_get
libglib-2.0.a:libglib_2_0_la-gslice.o: U g_private_set
libglib-2.0.a:libglib_2_0_la-gslice.o: U g_return_if_fail_warning
libglib-2.0.a:libglib_2_0_la-gslice.o:00000000000010d0 T g_slice_alloc
libglib-2.0.a:libglib_2_0_la-gslice.o:0000000000001770 T g_slice_alloc0
libglib-2.0.a:libglib_2_0_la-gslice.o:00000000000017a0 T g_slice_copy
libglib-2.0.a:libglib_2_0_la-gslice.o:00000000000017e0 T g_slice_free1
libglib-2.0.a:libglib_2_0_la-gslice.o:0000000000001ae0 T g_slice_free_chain_with_offset

So, the syntax will be:

<libname.a>:<objname.o>:<address> [TDB] <symbol>
<libname.a>:<objname.o>: U <symbol>

and we'll need to extract libname.a, symbol type (e.g. T, D, B, U), and the symbol.

We create a list of files. In each file struct, we remember all symbols and their types. Any type that is not U [undefined symbol] will define the symbol.

Note that as we build the symbol table, a library may have multiple U's [in various .o's] that refer to a symbol defined by another .o within it. So, we only record the symbol once and if we see a non-U type, we "promote" it (e.g. if we saw U foo and later saw T foo we change the type of foo to T [likewise for D and B].

Now we traverse the file list (e.g. curfile). For each symbol in the file's symbol table, if it's of type U [undefined], we scan all files looking for a non-U symbol definition. If we find one (in symfile (e.g.)), we can output a dependency line for tsort: <curfile> <symfile>. We repeat this for all files and symbols.

Note that this is bit wasteful because we could output many file dependency lines that are identical because the above will generate a line for each symbol. So, we should keep track of the lines output and only output a dependency line for unique file pairs. Also, note, it is possible to have both foo bar and bar foo. That is, actually, a cycle. While we just want one copy of foo bar and/or bar foo, they should not exclude one another.

Okay, so now feed the output of the above to tsort and it will give us the topologically sorted version of liblist that we want.

As should be obvious, the script parsing can take some time, so the tsort output should be cached in a file, and rebuilt in a makefile, based upon a dependency list of liblist

Convert some .a files to .o files

If a given library uses all [or most] of the its .o files, instead of doing ar rv libname.a ..., consider doing ld -r libname.o ....

This is similar in approach to creating a shared library .so file, but the "big" .o can still be statically linked.

Now, you have a single .o that will link faster than the .a because the intra-library links have already been resolved. Also, it will help with dependency cycles a bit.

A slight extension to the topo script could tell you which libraries are good candidates for this.

Even if the normal build makefiles can't be changed, the "final" top level could take a .a, either extract it into .o's, or use an ld force load option with -r to get the "big" .o

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