Question

Below are the steps of how I generate the executable file using shared library.

1. I have three files:

   File libhello.c

   /* hello.c - demonstrate library use. */
   #include <stdio.h>
   void hello(void)
   { 
       printf("Hello, library world./n");
   }
   

   File libhello.h

   /* hello.h - demonstrate library use. */
   void hello(void);
   

   File main.c

   /* main.c -- demonstrate direct use of the "hello" routine */
   #include "hello.h"
   int main(void)
   {
       hello();
       return 0;
   }
   

2. I use the commands below to generate the shared library.

   gcc -g -Wall -fPIC -c hello.c -o hello.o
   gcc -shared -W,soname,-libhello.so.0 -o libhello.so.0.0.0 hello.o
   

3. Finally, I add the library path to the LD_LIBRARY_PATH variable and try to create the executable file using the shared library.

   export LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH
   ln -s libhello.so.0.0.0 libhello.so.0
   gcc -g -Wall -c main.c -o main.o -I.
   gcc -o main main.o -lhello -L.
   

However, at the last step, there is one error: can’t find -lhello. So, where am I wrong?

Thanks.

Answer 1

gcc looks for libhello.so when linking a new program. libhello.so.0 is used when the dynamic dependencies of an already linked program are searched.

In other terms: gcc -o main main.o -lhello -L. looks for libhello.so, and ./main looks for libhello.so.0. This allows to have multiple versions of a library available for legacy programs while precisely identifying the library that matches the installed headers.

A symlink libhello.so -> libhello.so.0.0.0 should do the trick.