Call function in C¶

2010

Windows¶

Function call with dynamic library¶

Every function is contained either in a static or dynamic library. When the library is loaded dynamically, the program loader uses the following primitives:

LoadLibrary
GetProcAddress
FreeLibrary

this primitives are include the library windows.h.

Example with function call MessageBox:

#include <windows.h>

typedef int (WINAPI* _MessageBoxA)(HWND, LPSTR, LPSTR, UINT);

int main() {
    HMODULE hUser32 = NULL;
    _MessageBoxA __MessageBoxA = NULL;

    hUser32 = LoadLibraryA("user32.dll");
    __MessageBoxA = (_MessageBoxA) GetProcAddress(hUser32, "MessageBoxA");

    __MessageBoxA(NULL, "Hello world", "Information", 64);

    FreeLibrary(hUser32);
    return 0;
}

Example with calling the function with a printf :

#include <windows.h>

typedef int (WINAPI *_printf)();

int main (int argc, char argv) {
    HMODULE hMsvcrt = NULL;
    _printf __printf = NULL;

    hMsvcrt = LoadLibraryA("msvcrt.dll");

    __printf = (_printf) GetProcAddress(hMsvcrt, "printf");

    __printf("L\'adresse de printf est : %X",__printf);

    FreeLibrary(hMsvcrt);

    return 0;
}

Function call with static library¶

Example with an add function defined:

#include <stdio.h>

typedef int (*_add) (int,int);

int add(int nb, int nb1) {
    return nb + nb1;
}

int main() {
    int total = add(5,5);

    _add __add = (int)&add;

    total = __add(5,5);
    printf("%d", total);

    return 0;
}

With a function in a dynamic library

#include <windows.h>

typedef int (*_printf)();

int main() {
    _printf __printf = 0x77C1186A;
    __printf("Hello world !");

    return 0;
}

Linux¶

Function call with dynamic library¶

Any function can be called dynamically using the following primitives:

dlopen
dlsym
dlclose

To use its primitive include the following file dlfcn.h.

Function call with static library¶

With the add function

#include <stdio.h>

typedef int (*_add)(int, int);

int add(int nb, int nb2) {
    return nb + nb2;
}

int main(int argc, char argv) {
    int total;

    _add __add = NULL;
    __add = &add;

    total = __add(5,5);
    printf("%d",total);
    return 0;
}

How compilation¶

Example of library

// file : libprintf.c
#include <stdio.h>

void myPrintf(char *chaine) {
    printf("%s", chaine);
}

Le fichier .h

// file : libprintf.h
void myPrintf(char *chaine);

Compiling the library statically

gcc -c libprintf.c -o libprintf.o
ar rcs libprintf.a libprintf.o

Creating a dynamic library

gcc -c -fPIC libprintf.c -o libprintf.o
gcc -shared -Wl -o libprintf.so.1.0.1  libprintf.o

Using the library

// file : main.c
#include <stdio.h>
#include "libprintf.h"

int main(int argc, char argv) {
    myPrintf("Hello world");
    return 0;
}

Link between static library and user program

gcc -static main.c -L. c_printf.a -o statically_linked

Link between dynamic library and user program

gcc main.c -o dynamically_linked -L -l c_printf.so.1.0.1