gcc: Difference between -L and -l option AND how to provide complete path to a library

Question

I am new to makefile stuff; so, please accept my apology if my questions are trivials.

Question 1: What is the difference between -L and -l option.

Question 2: How do you provide complete path to some library? For instance, "libeng" and "libmx", mentioned in following makefile,are located at {MATLABROOT}/bin/glnxa64

# root directory of MATLAB installation
MATLABROOT="/usr/local/MATLAB/R2011b"

all: engdemo

engdemo:
    g++ ${MATLABROOT}/extern/examples/eng_mat/engdemo.cpp -o engdemo \
        -I${MATLABROOT}/extern/include \
        -L${MATLABROOT}/extern/lib -llibeng -llibmx

clean:
    rm -f engdemo *.o

Update: Following makefile works:

# root directory of MATLAB installation
MATLABROOT="/usr/local/MATLAB/R2011b"

all: engdemo

engdemo:
    g++ ${MATLABROOT}/extern/examples/eng_mat/engdemo.cpp -o engdemo \
        -I${MATLABROOT}/extern/include \
        #-L${MATLABROOT}/extern/lib -llibeng -llibmx
    -L${MATLABROOT}/bin/glnxa64 -llibeng \
    -L${MATLABROOT}/bin/glnxa64 -llibmx

clean:
rm -f engdemo *.o

I found following link about linking libraries very useful: http://www.cs.swarthmore.edu/~newhall/unixhelp/howto_C_libraries.html

Answer 1

Your question is refering to gcc linker (or simply ld). Description can be found in in gcc's Options for Linking or ld's Command Line Options.

From the documentation you get

-larchive Add archive file archive to the list of files to link. ld will search its path-list for occurrences of libarchive.a for every archive specified.

-Lsearchdir Add path searchdir to the list of paths that ld will search for archive libraries and ld control scripts.

In your example you need to use -L to define the path where libeng and libmx libraries are located. Then use -l option to instruct ld to use these libraries.

Note that in the documentation is noted that:

ld will search its path-list for occurrences of libarchive.a for every archive specified.