Creating a Shared Library How you can create and use your own object-code libraries within the Linux environment

Building an executable file source text compiler object code linker executable file Translates programming language statements into cpu’s machine-language instructions Adjusts any memory references to fit the Operating Sytem’s memory model

Example: source text in assembly.data msg:string“ Hello \n”.text main:movl$4, %eax movl$1, %ebx movl$msg, %ecx movl$8, %edx int$0x80 ret.globlmain

Compiling the source text $gcc –c hello.s; compile $ ld hello.o –o hello; link

Normally need a runtime library source text object file object code library executable file compiler linker A previously compiled collection of standard program functions

How to build/use an archive file $ gcc –c func1.c $ gcc –c func2.c $ ar cr myarchive.a func1.o func.o $ g++ myapp.cpp myarchive.a –o myapp

Static versus Dynamic Linking $ gcc –c hello.s $ ld –static hello.o –o hello or $ ld –shared hello.o –o hello

Smaller is more efficient objject file function library object file pointer static linking object file function library dynamic linking object file pointer shared function library executable files

Building a ‘Shared Library’ Functions must be ‘reentrant’ –This implies: No global variables Code must be ‘position-independent’ –Can’t jump or call to fixed addresses –Can’t access data at fixed locations

Command-formats For building the shared library: $ gcc –c –fPIC func.c $ gcc –shared –fPIC func.o –o libfunc.so For linking application with shared library: $ g++ app.cpp -oapp –L. –lfunc –Wl,-rpath,.

Command-line options You need the -L. option to tell the linker where to search the for your libxxx.so files You need the -l option to tell the linker which shared object files to link with first You need the -Wl option to tell gcc/g++ compilers to pass options on to the linker See the man pages for additional details

Class Exercise Compile the files ‘comb.c’ and ‘other.c’ Build a shared library containing both Compile the ‘uselib.cpp’ demo-program so it will be linked with your shared library Run the ‘uselib’ demo-program and turn in a printout that shows its output