Linking a shared library with another shared lib in linux

When trying to create my own shared library that uses Berkeley DB, I found that I have to put the -ldb at the end of the gcc command or else it blew up saying the symbol 'db_create' was not found. This was under Cygwin.

Specifically, this worked:

gcc -shared -o $b/$libfile nt_*.o -ldb

This did not work:

gcc -ldb -shared -o $b/$libfile nt_*.o

Suppose that libabc.so is obtained from posiition independent object code files abc1.pic.o and abc2.pic.o ; then you have built them with e.g.

 gcc -Wall -fPIC -O -g abc1.c -c -o abc1.pic.o
 gcc -Wall -fPIC -O -g abc2.c -c -o abc2.pic.o

and you build libabc.so with

gcc -shared  abc1.pic.o  abc2.pic.o -L/usr/local/lib -l123 -o libabc.so

^{I added -L/usr/local/lib before -l123 because I am assuming you have a /usr/local/lib/lib123.so shared library.}

Read also the Program Library HowTo.

As you see, you may link a shared library lib123.so into your own shared library libabc.so

Then check with ldd libabc.so

You may want to set up some rpath in your libabc.so by adding -Wl,-rpath and -Wl,$RPATHDIR to the linking command.

For much more details, read Drepper's paper How to write shared libraries

PS. Don't use a static library for lib123.a (it should be PIC). If you link non-PIC code into a shared object, you lose most of the advantages of shared objects, and the dynamic linker ld.so has to do zillions of relocations.

Following the same procedure pointed out by Basile Starynkevitch, for example, I have a library which depends on libm.so, so the compilation for the library objects are:

gcc -fPIC -Wall -g -I include -I src -c src/wavegen.c  -o build/arm/wavegen.o                                                                                          
gcc -fPIC -Wall -g -I include -I src -c src/serial.c  -o build/arm/serial.o

To compile the library, however, in some versions of gcc the order where library references are placed, is important, so I suggest, to ensure compatibility, placing those references at the end of the command:

gcc -shared -Wl,-soname,libserial.so.1 -o lib/libserial.so.1.0 build/arm/wavegen.o build/arm/serial.o -lm

I have tested in PC (gcc v.8.3.0) and in ARM (gcc v.4.6.3).