How do I make the linker and loader choose a shared library at a specific path?

Question

I wrote a code for which I have a Makefile like this:

CC=gcc
CXX=g++
DEBUG=-g
COMPILER=${CXX}
#INCLUDE= -I Re2/ -I Re2/re2/
#WARN=-Wall
spambin: main.cpp Mail.o trie.o Spambin.o config.o getdns.o
        ${COMPILER}  ${DEBUG} ${WARN} -o spambin main.cpp Mail.o trie.o Spambin.o config.o getdns.o   /usr/lib/libre2.so

trie.o: trie.cpp
        ${COMPILER}  ${DEBUG} ${WARN} -c trie.cpp ${INCLUDE}

Mail.o: Mail.cpp
        ${COMPILER} ${DEBUG} ${WARN} -c Mail.cpp ${INCLUDE}

config.o: config.cpp
        ${COMPILER} ${DEBUG} ${WARN} -c config.cpp ${INCLUDE}

Spambin.o: Spambin.cpp
        ${COMPILER} ${DEBUG} ${WARN} -c Spambin.cpp ${INCLUDE}

getdns.o: getdns.c
        ${CC} ${DEBUG} ${WARN} -c getdns.c ${INCLUDE}
clean: 
        rm -f *.o

The issue I'm facing is that I want my code to directly pick the /usr/lib/libre2.so. Doing ldd on the final output binary gives:

linux-gate.so.1 =>  (0x00693000)
libre2.so.0 => /usr/lib/libre2.so.0 (0x00159000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x004f4000)
libm.so.6 => /lib/libm.so.6 (0x00ce8000)
libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x002b8000)
libc.so.6 => /lib/libc.so.6 (0x00b83000)
libpthread.so.0 => /lib/libpthread.so.0 (0x00d13000)
/lib/ld-linux.so.2 (0x00b64000)

But when I move this file to live servers and do ldd on the binary, the result is:

linux-gate.so.1 =>  (0x0018b000)
libre2.so.0 => /usr/local/lib/libre2.so.0 (0x00b89000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x0040f000)
libm.so.6 => /lib/libm.so.6 (0x00ad2000)
libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x00c5e000)
libc.so.6 => /lib/libc.so.6 (0x0096c000)
libpthread.so.0 => /lib/libpthread.so.0 (0x00ab9000)
/lib/ld-linux.so.2 (0x00949000)

The binary path is /usr/local/lib/libre2.so.0.

Is there any way we can force compiler to pick the .so file from our preferred path?

sirgeorge sirgeorge · Accepted Answer · 2012-03-06T04:47:20

Yes, there is a way: you can specify full path to the shared library when linking,e.g.:

Let my library is: libtest2.so (it is in /projects/tests/test_so)
Let my main program cpp file is: main.cpp Then:

g++ main.cpp -o test /projects/tests/test_so/libtest2.so

Produces binary test which has embedded absolute path /projects/tests/test_so in it. No matter where you move the test binary it will always look for /projects/tests/test_so/libtest2.so

Alternatively you may look at -rpath switch that you can use with gcc (actually it is a linker switch),e.g:

gcc -Wl,-rpath,'<path-to-lib-directory>'

But this approach may cause your program to look also for other shared libraries in <path-to-lib-directory> which may not be exactly what you want.

How do I make the linker and loader choose a shared library at a specific path?

1 Answers