Consider example files:

  • ctest1.c
void ctest1(int *i)
{
   *i=5;
}
  • ctest2.c
void ctest2(int *i)
{
   *i=100;
}
  • prog.c
#include <stdio.h>
void ctest1(int *);
void ctest2(int *);

int main()
{
   int x;
   ctest1(&x);
   printf("Valx=%d\n",x);

   return 0;
}

How to generate a dynamically linked library (shared object) libctest.so?

At first, create object code:

$ cc -Wall -fPIC -c ctest1.c ctest2.c

The -fPIC flag means to output Position Independent Code, a characteristic required by shared libraries.

Then, create library:

$ cc -shared -o libctest.so ctest1.o ctest2.o

The -shared flag is required to produce a shared object which can then be linked with other objects to form an executable.

Optionally, Move the shared object to some special directories, such as /usr/local/lib to make linker could find it easily.

$ sudo mv libctest.so /usr/local/lib

The last step, link with shared library to create an executable file:

$ cc -o prog -lctest

When you got an executable file, you can list its shared library dependencies by using ldd command:

$ ldd prog

linux-vdso.so.1 (0x00007ffe9aba5000)
libctest.so => /usr/local/lib/libctest.so (0x00007f87a541f000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f87a522d000)
/lib64/ld-linux-x86-64.so.2 (0x00007f87a5440000)

TODO: make it better, add contents about ldconfig and list /etc/ld.so.conf.d