What is the difference between -I and -L in makefile?

Question

What is the usage of the -I and -L flags in a makefile?

Answer 1

These are typically part of the linker command line, and are either supplied directly in a target action, or more commonly assigned to a make variable that will be expanded to form link command. In that case:

-L is the path to the directories containing the libraries. A search path for libraries.

-l is the name of the library you want to link to.

For instance, if you want to link to the library ~/libs/libabc.a you'd add:

-L$(HOME)/libs -labc

To take advantage of the default implicit rule for linking, add these flags to the variable LDFLAGS, as in

LDFLAGS+=-L$(HOME)/libs -labc

---

It's a good habit to separate LDFLAGS and LIBS, for example

# LDFLAGS contains flags passed to the compiler for use during linking
LDFLAGS = -Wl,--hash-style=both
# LIBS contains libraries to link with
LIBS = -L$(HOME)/libs -labc
program: a.o b.o c.o
        $(CC) $(LDFLAGS) $^ $(LIBS) -o $@
        # or if you really want to call ld directly,
        # $(LD) $(LDFLAGS:-Wl,%=%) $^ $(LIBS) -o $@

Even if it may work otherwise, the -l... directives are supposed to go after the objects that reference those symbols. Some optimizations (-Wl,--as-needed is the most obvious) will fail if linking is done in the wrong order.