Difference between revisions of "Linux Libraries"
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Latest revision as of 06:51, 25 October 2023
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.
Contents
Source files
The following four sourcefiles are used:
- file1.c contains a const array of one megabyte
- file2.c is quite small
- 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();
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