03. Static and Shared Libraries

Static and Shared Libraries

Simple application

The 'hello.c' listing:

#include <stdio.h>
#include <stdlib.h>

int main()
{
  printf("Hello World\n");
  exit(0);
}

To compile, execute this:

gcc hello.c -o hello

To run, execute this:

./hello

Static and shared libraries

• Static - .a extension
• Shared - .so extension

Names of libraries start with the 'lib' prefix.

To see the full list of libs in the system, execute the command:

ls /usr/lib | less

To list of libraries loaded by application, use the ldd command. For example:

ldd "$(which ls)"

Static libraries

Use mcedit to create the fred.c and bill.c files.

fred.c:

mcedit fred.c
#include <stdio.h>

void fred(int arg)
{
   printf("fred: you passed %d\n", arg);
}

bill.c:

mcedit
bill.c
#include <stdio.h>

void bill(char *arg)
{
   printf("bill: you passed %s\n", arg);
}

Compile the sources into object files:

gcc -c fred.c bill.c

See the object files:

ls *.o
bill.o
fred.o

Make a static library (archive):

ar crv libfoo.a bill.o fred.o
r - bill.o
r - fred.o

Write the header file for the library:

lib.h:

mcedit lib.h
void bill(char *);
void fred(int);

Write a program that uses the library:

program.c:

mcedit program.c
#include <stdlib.h>
#include "lib.h"

int main()
{
  bill("Hello World!");
  exit(0);
}

Build the program object file:

gcc -c program.c

Build a program from object files:

gcc -o program program.o bill.o

Build a program that used the static library:

gcc -o program program.o -L. -lfoo

Shared libraries

Compile sources into position-independent code (PIC):

gcc -c -Wall -fPIC fred.c bill.c

Build shared library from object files:

gcc -shared -o libfoo.so fred.o bill.o

Build the program that uses the shared library:

gcc -Wall -o program program.c -lfoo -L.

Run the program:

export LD_LIBRARY_PATH=.
./program

See loaded libraries:

ldd ./program

H/W