What is the difference between LDADD and LIBADD?

Question

I'm trying to setup an automake project that uses a mix of libtool libraries and exectuables, and I'm having a hard time grokking the automake documentation, esp. as relates to telling the compiler to link against.

So can someone explain the differences between LDADD and LIBADD?

Things like:

• when is one used over the other,
• which one takes -lname_of_library style values vs. direct filenames, etc.

Whenever I try to read the relevant documentation, it seems like it assumes that I know things that I don't.

Answer 1

As mentioned in one of the books, LDADD is ADDitional linker(LD) items - i.e. items that are added when performing linking. This would be, for example, when producing programs. LDADD can specify:

• libtool files e.g. lib/libfudge.la
• libraries e.g. -lm, or
• full paths to libraries e.g. /lib/libmagicalwonderland.a
• link flags & libraries e.g. -L/opt/lib -lmagical

They are in order of preference - Using the last two is just asking for trouble as they're pointing at things that may or may not be present.

LIBADD is to specify ADDitional LIBraries to use. This is used when building a library to specify that additional libraries are needed in order to build or make use of the library. You'll see it specified as something like libfred_la_LIBADD =. It can be used to specify libtool libraries, or system libraries and will place these libraries into the resulting libtool .la for the library so when it comes to linking against the library you get all the appropriate libraries brought along.

You should only specify libraries to link, so, for example, my library libfred.la depends on some math routines i.e. it depends on libm. When I'm specifying the additional libraries for the library, I state:

libfred_la_LIBADD = -lm

This dependency is encoded when I build the library, and gets passed on to consumers of the library as well.

The rule of thumb is:

• LIBADD for things that are being added to a library
• LDADD for things that are being added to a program

Answer 2

Use the LIBADD primary for libraries, and LDADD for executables. If you were building a libtool library libfoo.la, that depended on another library libbar.la, you would use:

libfoo_la_LIBADD = libbar.la

If you had other non-libtool libraries, you would also add these with -L and -l options:

libfoo_la_LIBADD = libbar.la -L/opt/local/lib -lpng

Typically, you would use the configure script to find these extra libraries, and use AC_SUBST to pass them with:

libfoo_la_LIBADD = libbar.la $(EXTRA_FOO_LIBS)

For a program, just use LDADD :

myprog_LDADD = libfoo.la # links libfoo, libbar, and libpng to myprog.

Sometimes the boundaries are a bit vague. $(EXTRA_FOO_LIBS) could have been added to myprog_LDADD. Adding dependencies to a libtool (.la) library, and using libtool do all the platform-specific linker magic, is usually the best approach. It keeps all the linker metadata in the one place.