Linux Libraries

Linux Libraries comes in two forms: Static and shared. Static libraries are linked at compile time while shared libraries are linked at runtime. The following three sourcefiles file1.c, file2.c and file3.c will be used to build a static and a shared library.

Source files

The following four sourcefiles are used:

• file1.c is quite small
• file2.c contains a const array of one megabyte
• file3.c contains a const array of two megabyte
• hi.c contains main and uses functions from file1.c and file2.c but not file3.c

File1.c

//Author: Henrik Thomsen <heth@mercantec.dk>
//Name: file1.c
#include <stdio.h>
#define MB 1024*1024
void p1( void ) {
        // Initialize 1MB array first and last member
        // so we are sure the array is not truncated.
        const char txt[MB];// = {[0]=0x0ff, [MB-1]=0xff };
        printf("I am file1\n");
        printf("txt is %d bytes\n", sizeof(txt) );
}

File2.c

//Author: Henrik Thomsen <heth@mercantec.dk>
//Name: file2.c
#include <stdio.h>
void p2( void ) {
        printf("I am file2\n");
}

File3.c

//Author: Henrik Thomsen <heth@mercantec.dk>
//Name: file3.c
#include <stdio.h>
#define MB 1024*1024
void p3( void ) {
        // Initialize 2MB array first and last member
        // so we are sure the array is not truncated.
        const char txt[MB*2] = {[0]=0x0ff, [sizeof(txt)-1]=0xff };
        printf("I am file3\n");
        printf("txt is %d bytes\n", sizeof(txt) );
}

Sourcefile containing main

This sourcefile only use functions from file1.c and file2.c - note that file3.c is not used.

//Author: Henrik Thomsen <heth@mercantec.dk>
//Name: hi.c
#include <stdio.h>

extern void p1( void );
extern void p2( void );
extern void p3( void );

int main( void ) {
        printf("Starting main\n=============\n\n");
        p1();
        p2();
        p3();
        return(0);
}

Static Libraries

Static libraries link needed code from the object (.o) files or an archive of object files into the binary file at compile time. See example below.

Creating libfilearchive.a

Notice the size of the object files from and the size of the .a archivefile. It contains all three .o files.

[heth@localhost ]$ gcc -c file1.c
[heth@localhost ]$ gcc -c file2.c
[heth@localhost ]$ gcc -c file3.c
[heth@localhost ]$ ar rc libfilearchive.a file1.o file2.o file3.o
[heth@localhost ]$ ls -l
-rw-rw-r--. 1 heth heth     334 Dec 23 10:32 file1.c
-rw-rw-r--. 1 heth heth 1050248 Dec 23 10:32 file1.o
-rw-rw-r--. 1 heth heth     125 Dec 23 10:26 file2.c
-rw-rw-r--. 1 heth heth    1496 Dec 23 10:31 file2.o
-rw-rw-r--. 1 heth heth     345 Dec 23 10:39 file3.c
-rw-rw-r--. 1 heth heth 2098824 Dec 23 10:40 file3.o
-rw-rw-r--. 1 heth heth 3150842 Dec 23 10:40 libfilearchive.a

Using the archive

Compiling and linking

When using the archive link it in - and in this case add current directory to the linkers searchpath -L."

[heth@localhost ]$ gcc hi.c -lfilearchive -L. -o hi
[heth@localhost ]$ ./hi
Starting main
=============

I am file1
txt is 1048576 bytes
I am file2

Note: The size of the binary file hi proves that file3 is not linked in.

[heth@localhost libleg]$ ls -l
total 15432
-rw-rw-r--. 1 heth heth     334 Dec 23 10:32 file1.c
-rw-rw-r--. 1 heth heth 1050248 Dec 23 10:32 file1.o
-rw-rw-r--. 1 heth heth     125 Dec 23 10:26 file2.c
-rw-rw-r--. 1 heth heth    1496 Dec 23 10:31 file2.o
-rw-rw-r--. 1 heth heth     345 Dec 23 10:39 file3.c
-rw-rw-r--. 1 heth heth 2098824 Dec 23 10:40 file3.o
-rwxrwxr-x. 1 heth heth 1067104 Dec 23 11:25 hi
-rw-rw-r--. 1 heth heth     219 Dec 23 11:24 hi.c

Shared example

Shared libraries are linked at compile time - meaning that all binaries that use a shared library can be updated by updating the shared library only. See example below.

Creating libfilearchive.a

Notice the size of the object files from and the size of the .so shared library file. It contains all three .o files.

[heth@localhost ]$ gcc -fPIC -c file1.c
[heth@localhost ]$ gcc -fPIC -c file2.c
[heth@localhost ]$ gcc -fPIC -c file3.c
[heth@localhost ]$ gcc -shared -o libshared.so file1.o file2.o file3.o 
[heth@localhost ]$ ls -l
-rw-rw-r--. 1 heth heth     334 Dec 23 10:32 file1.c
-rw-rw-r--. 1 heth heth 1050288 Dec 23 10:32 file1.o
-rw-rw-r--. 1 heth heth     125 Dec 23 10:26 file2.c
-rw-rw-r--. 1 heth heth    1536 Dec 23 10:31 file2.o
-rw-rw-r--. 1 heth heth     345 Dec 23 10:39 file3.c
-rw-rw-r--. 1 heth heth 2098864 Dec 23 10:40 file3.o
-rw-rw-r--. 1 heth heth 3162256 Dec 23 13:17 libshared.so

Using the archive

Shared library files are normally kept in a /usr/lib subdirectory and called .so (Shared Obkect). Actual path can be seen in /etc/ld.so.conf. In our example the the we add the path to our .so file in the working directory.

Compiling and linking

[heth@localhost libleg]$ gcc hi.c -lshared -L. -o hi
[heth@localhost libleg]$ LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH ./hi
Starting main
=============

I am file1
txt is 1048576 bytes
I am file2
[heth@localhost libleg]$ ls -l
total 18524
-rw-rw-r--. 1 heth heth     334 Dec 23 10:32 file1.c
-rw-rw-r--. 1 heth heth 1050288 Dec 23 11:42 file1.o
-rw-rw-r--. 1 heth heth     125 Dec 23 10:26 file2.c
-rw-rw-r--. 1 heth heth    1536 Dec 23 11:42 file2.o
-rw-rw-r--. 1 heth heth     345 Dec 23 10:39 file3.c
-rw-rw-r--. 1 heth heth 2098864 Dec 23 11:42 file3.o
-rwxrwxr-x. 1 heth heth   18400 Dec 23 13:24 hi
-rw-rw-r--. 1 heth heth     219 Dec 23 11:24 hi.c
-rwxrwxr-x. 1 heth heth 3162256 Dec 23 13:17 libshared.so