Directory search applies in a special way to libraries used with the linker. This special feature comes into play when you write a prerequisite whose name is of the form ‘-lname’. (You can tell something strange is going on here because the prerequisite is normally the name of a file, and the file name of a library generally looks like libname.a, not like ‘-lname’.)
When a prerequisite's name has the form ‘-lname’,
handles it specially by searching for the file libname.so in
the current directory, in directories specified by matching
search paths and the
VPATH search path, and then in the
directories /lib, /usr/lib, and prefix/lib
(normally /usr/local/lib, but MS-DOS/MS-Windows versions of
make behave as if prefix is defined to be the root of the
DJGPP installation tree).
If that file is not found, then the file libname.a is searched for, in the same directories as above.
For example, if there is a /usr/lib/libcurses.a library on your system (and no /usr/lib/libcurses.so file), then
foo : foo.c -lcurses cc $^ -o $@
would cause the command ‘cc foo.c /usr/lib/libcurses.a -o foo’ to be executed when foo is older than foo.c or than /usr/lib/libcurses.a.
Although the default set of files to be searched for is
libname.so and libname.a, this is customizable
.LIBPATTERNS variable. Each word in the value of this
variable is a pattern string. When a prerequisite like
‘-lname’ is seen,
make will replace the percent in
each pattern in the list with name and perform the above directory
searches using that library filename. If no library is found, the next
word in the list will be used.
The default value for
.LIBPATTERNS is ‘lib%.so lib%.a’,
which provides the default behavior described above.
You can turn off link library expansion completely by setting this variable to an empty value.