CODEWITHSUNDEEP
c++clinkerstatic-librariesautomake
Arkady
Arkady

Reputation: 2207

What do link flags mean for static library?

"ar" -- is just tool to create archives.

And all static libraries of form "lib*.a" are in fact just archived compiled objects + additional file with symbol table, added there by "ranlib". No linking is performing during creation of such library.

So why do most of projects use ***_LDFLAGS ***_LIBADD in their Makefile.am during creation of such ("lib*.a" static library) archives?

Does automake ignore those flags (in case when they relate to any "lib*.a" static library), or does it in fact link something there?

Upvotes: 2

Views: 4203

Answers (2)

Novice
Novice

Reputation: 560

Answer to your question Does automake ignore them is: NO

That is completely true: ""ar" -- is just tool to create archives."
But, Automake does not ignore ***_LDFLAGS ***_LIBADD in their Makefile.am at all, otherwise what is point of having such flag if they are not making any sense for the build system!

From documentation (link give below):

  • The ‘library_LIBADD’ variable should be used to list extra libtool objects (.lo files) or libtool libraries (.la) to add to library.
  • The ‘library_LDFLAGS’ variable is the place to list additional libtool linking flags, such as -version-info, -static, and a lot more.

For more details, you should go through this Libtool Documentation for more clarity in these flags.



EDIT: As your question is still generic, let me put it in two different scenario...

  1. Static Lib is stand-alone:

    • In this scenario ***_LIBADD could be not much useful, as mentioned in documentation: "the library_LIBADD variable should be used to list extra libtool objects (.lo files) or libtool libraries (.la) to add to library"
    • Which means, if your static lib does not have dependency then this flag is no of use in it's Makefile.am.

  2. Static Lib is Dependent (not stand-alone)

    • From above quote from documentation, it is now clear that, ***_LIBADD flag is used to mentioned libs names which are required to build your current library.
    • So this flag would be necessary in such requirement.

And about ***_LDFLAGS, as mentioned in documentation, The library_LDFLAGS variable is the place to list additional libtool linking flags for that library.

  • If your lib does not require such flags, then this can be ignored too. It is all about how you want your final result.

Few more additional links for your reference:

I hope this EDIT suffices what you're looking for.

PS: If you would read my answer carefully and went through documentation, you could have get the same data. Njoy. :)

Upvotes: 1

ldav1s
ldav1s

Reputation: 16305

So why do most of projects use ***_LDFLAGS ***_LIBADD in their Makefile.am during creation of such ("lib*.a" static library) archives?

The GNU Build System is capable of creating dynamic and static libs (or both) determined at configure time using the --enable-shared and --enable-static flags. As you guessed, _LDFLAGS and _LIBADD are more oriented to dynamic shared objects or program linkage than to the static linker. The static linker of libtool is essentially another link pass that invokes ar to create the archive (omitting all the flags). For example:

lib_LTLIBRARIES=libfoo.la
libfoo_la_SOURCES=$(SRCS)
libfoo_la_LDFLAGS=-Wl,-t

when both shared and static libs are generated outputs something like:

libtool: link: gcc -shared  -fPIC -DPIC  .libs/foo.o    -g -O2 -Wl,-t   -Wl,-soname -Wl,libfoo.so.0 -o .libs/libfoo.so.0.0.0
...
libtool: link: (cd ".libs" && rm -f "libfoo.so.0" && ln -s "libfoo.so.0.0.0" "libfoo.so.0")
libtool: link: (cd ".libs" && rm -f "libfoo.so" && ln -s "libfoo.so.0.0.0" "libfoo.so")
libtool: link: ar cru .libs/libfoo.a  foo.o
libtool: link: ranlib .libs/libfoo.a
libtool: link: ( cd ".libs" && rm -f "libfoo.la" && ln -s "../libfoo.la" "libfoo.la" )

automake does ignore _LDFLAGS; however the script that performs the linking (libtool) does not. It looks for flags that affect linking there also. For example:

lib_LTLIBRARIES=libfoo.la
libfoo_la_SOURCES=$(SRCS)
libfoo_la_LDFLAGS=-Wl,-t -static

will only generate a static lib, even if configure --disable-static was run to generate the Makefile.

libtool is just a wrapper script over the native compiler/linker tools for portability.

Upvotes: 3

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