Creating a Shared Library

1
Creating a Shared Library

• How you can create and use your own object-code
  libraries within the Linux environment

2
Building an executable file
source text
linker
compiler
object code
executable file
Translates programming language statements
into cpus machine-language
instructions
Adjusts any memory references to fit the
Operating Sytems memory model
3
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
• .globl main

4
Compiling the source text

• gcc c hello.s compile
• ld hello.o o hello link

5
Normally need a runtime library
source text
object file
compiler
linker
executable file
object code library
A previously compiled collection of standard
program functions
6
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

7
Static versus Dynamic Linking

• gcc c hello.s
• ld static hello.o o hello
• or
• ld shared hello.o o hello

8
Smaller is more efficient
objject file
object file
static linking
function library
function library
executable files
object file
object file
dynamic linking
pointer
pointer
shared function library
9
Building a Shared Library

• Functions must be reentrant
• This implies No global variables
• Code must be position-independent
• Cant jump or call to fixed addresses
• Cant access data at fixed locations

10
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,.

11
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

12
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