Writing and using dynamic libraries in linux

Writing and using dynamic libraries in linux

Example 1

Compile header files

Mylib. h

# Include <stdio. h>

Compile library files

Mylib. c

Void Print ();
# Include "mylib. h"

Void Print ()
{
Printf ("This is in my first dynamic lib call ");
}

Write test code

Main. c

# Include "mylib. h"

Int main ()
{
Print ();
}

Write README

Gcc-shared-fPIC mylib. c-o libcac. so
Gcc-fpic-shared mylib. c-o mylib. so
-FPIC: indicates that the compiled code is in a separate position. If this option is not used, the compiled code is location-related. Therefore, during dynamic loading, the code is copied to meet the needs of different processes, but cannot achieve the purpose of truly sharing code segments.

-Shared: indicates that the database is compiled into a dynamic library.

Gcc-o main. c-L./mylib. so
Gcc-o main test. c-L././mylib. so

Example 2

Compile header files

# Ifndef _ GETMAXLEN_H _

# Define _ GETMAXLEN_H _

Int getMaxLen (int * sel, int N );

# Endif

Compile library files

# Include "getmaxlen. h"

Int getMaxLen (int * sel, int N)

{

Int n1 = 1, n2 = 1, I = 1;

For (I; I <N; I ++)

{

If (sel [I]> sel [I-1])

{

N2 ++;

If (n2> n1)

{

N1 = n2;

}

}

Else

{

N2 = 1;

}

}

Return n1;

}

Write test code

# Include "getmaxlen. h"
# Include <stdio. h>
Int main ()

{

Int Sel [] = {2, 3, 6, 3, 2, 1, 2, 3, 4, 5, 7, 6, 5 };

Int m;

M = getMaxLen (Sel, 15 );

Printf ("% d", m );

Return 0;

}

Dynamic library file loading

# Include "getmaxlen. h"
# Include <stdio. h>
# Include <dlfcn. h>

Int main ()

{

Void * handle = NULL;

Int (* getMaxLen) (int * sel, int N );

Int sel [] = };

Handle = dlopen ("./mylib. so", RTLD_LAZY );

If (handle = NULL)

{

Printf ("dll loading error. \ n ");

Return 0;

}

GetMaxLen = (int (*) (int *, int) dlsym (handle, "getMaxLen ");

If (dlerror ()! = NULL)

{

Printf ("fun load error. \ n ");

Return 0;

}

Printf ("% d \ n", getMaxLen (sel, 15 ));

}

Write README

Gcc-shared-fPIC mylib. c-o libcac. so
Gcc-fpic-shared mylib. c-o mylib. so
-FPIC: indicates that the compiled code is in a separate position. If this option is not used, the compiled code is location-related. Therefore, during dynamic loading, the code is copied to meet the needs of different processes, but cannot achieve the purpose of truly sharing code segments.

-Shared: indicates that the database is compiled into a dynamic library.

Gcc-o main. c-L./mylib. so
Gcc-o main test. c-L././mylib. so

Gcc-fpic-shared getmaxlen. c-o mylib. so (used to dynamically load library files)

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