GNU C : How can I compile a C program with dynamic library option -lmylib but without -L option

Question

I have used several libraries for example pthread as -lpthread and math as -lmath but I need not use the -L option to specify linking path.

But,

1. I created a library mylib by compiling my program mylib.c as gcc -fPIC mylib.c -o libmylib.so
2. placed it in /usr/local/lib/libcustom
3. Added path /usr/local/lib/libcustom into a file /etc/ld.so.conf.d/libcustom.conf
4. run ldconfig
5. run export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib/customlib
6. created a sample.c called the function in mylib

Now when I compile sample.c a gcc -L /usr/local/lib/customlib -o sample.o -lmylib it work fine where as when I try gcc sample.c -o sample.o -lmylib it gives ld error as /usr/bin/ld: cannot find -lmylib

I want to know why is -L flag optional in case of -lpthread and mandatory in case of -lmylib ? How can I skip the use of -L in case of -lmylib ?

Thanks.

Answer 1

You're looking for LIBRARY_PATH .

LD_LIBRARY_PATH is for loading dynamic libraries at runtime , not compile time.

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Side note: when adding on to existing environment variables, make sure to use $LD_LIBRARY_PATH instead of just LD_LIBRARY_PATH . Otherwise, you're discarding the original contents and putting in the literal text LD_LIBRARY_PATH .

So it should change from:

export LD_LIBRARY_PATH=LD_LIBRARY_PATH:/usr/local/lib/customlib

to:

export LIBRARY_PATH=$LIBRARY_PATH:/usr/local/lib/customlib

Answer 2

For this reason you need to have mylib placed in the paths known to the linker. For example somewhere like /usr/lib or /usr/local/lib. In your case it works with pthread because it is already located in the system known paths.