How to use pkg-config to link a library statically

The pkg-config --static option relies on proper tagging in the .pc files. If providing the --static option does not return correct information necessary to link against the libpng archive, then you cannot use pkg-config for that purpose.

I suspect libpng (along with a majority of other packages) dropped support for static linking some time after libpng 1.2. They may still provide a library archive, but the libpng pkg-config file is no longer properly tagged to support a static link. You will have to manually tell ld to use the static lib.

Try:

-L/usr/local/Cellar/libpng/1.6.15/lib -l:libpng16.a -lz

Using -l with a : character allows you to specify the filename extension.

The -l: option is documented in the GNU ld 2.24 manual:

-l namespec

--library=namespec

Add the archive or object file specified by namespec to the list of files to link. This option may be used any number of times. If namespec is of the form :filename, ld will search the library path for a file called filename, otherwise it will search the library path for a file called libnamespec.a.

On systems which support shared libraries, ld may also search for files other than libnamespec.a. Specifically, on ELF and SunOS systems, ld will search a directory for a library called libnamespec.so before searching for one called libnamespec.a. (By convention, a .so extension indicates a shared library.) Note that this behavior does not apply to :filename, which always specifies a file called filename.

You could edit the .pc file to make it support static linking, especially if you are in a position to be compiling, patching, and installing software yourself instead of relying on some Linux distribution.

Here is an example of a .pc file that supports both dynamic and static linking. This is taken from /usr/lib/x86_64-linux-gnu/pkgconfig/xcb.pc on my Ubuntu system:

prefix=/usr
exec_prefix=${prefix}
libdir=${prefix}/lib/x86_64-linux-gnu
includedir=${prefix}/include
xcbproto_version=1.11

Name: XCB
Description: X-protocol C Binding
Version: 1.11.1
Requires.private: pthread-stubs xau >= 0.99.2 xdmcp
Libs: -L${libdir} -lxcb
Libs.private: 
Cflags: -I${includedir}

If you run pkg-config --libs xcb, it assumes you want the dynamic version and it gives you just -lxcb. The xcb.so dynamically shared object will know how to load all of its own dependencies so you don't have to specify them when linking against it.

If you run pkg-config --libs xcb --static, then the .private fields come into play, and you get -lxcb -lXau -lXdmcp.

I have not encountered many build systems that know to pass the --static argument to pkg-config. So if your .pc file is only intended to support static linking, it's probably best to not use .private fields, and just provide all the dependencies people will need unconditionally. That way people can link against the library successfully even if they don't know it's static or don't know to pass --static to pkg-config.