Apss: Dynamic loading and un loading of shared libraries in C in Linux

Tuesday, March 10, 2015

Dynamic loading and un loading of shared libraries in C in Linux

This is the other way of using Dynamic libraries by loading/unloading at runtime.

Please refer to my earlier posts for more explanation/understanding

Click here to understand stages of compilation in Linux using GCC
Click here to understand Static & Dynamic libraries in C
Click here to understand creation of static library
Click here to understand creation of Dynamic library

Step1: Implement the library source. Let us assume there are two files namely one.c and two.c each implementing one function as follows

$ vim one.c

#include<stdio.h>
int myadd(int a, int b){
return a+b;
}

$ vim two.c

#include<stdio.h>
int mymult(int a, int b){
return a*b;
}

Step2: Compile the sources to generate relocatable.

Note: ‘.so’ is the extension for dynamic libraries.

$ gcc -c -fpic one.c
( to create a relocatable one.o )

$ gcc -c -fpic two.c
( to create a relocatable two.o )

$ gcc -shared -o libmyown.so one.o two.o

( libmyown.so is the name of the dynamic library to be created )

- shared flag used to create a shared library

Step3: Create the source file to dynamically load our share-library.

here funAddPtr and funMultPtr are the function pointer for myadd and mymult

Please refer to man-pages for dlopen, dlsym, dlclose

$ vim test.c

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
int main()
{
void *module;
int (*funAddPtr)(int,int);
int (*funMultPtr)(int,int);

char *error;

printf("Example : Dynamic loading/un-loading of shared library
");

/* Load dynamically loaded library */
module = dlopen("./libmyown.so", RTLD_LAZY);
if (!module) {
fprintf(stderr, "Couldnt open libmine.so: %s
", dlerror());
exit(1);
}

funAddPtr = dlsym(module, "myadd");
if ((error = dlerror()) != NULL) {
fprintf (stderr, "%s
", error);
exit(1);
}

printf("Sum of 3,4 = %d
", funAddPtr(3,4));

funMultPtr = dlsym(module, "mymult");
if ((error = dlerror()) != NULL) {
fprintf (stderr, "%s
", error);
exit(1);
}

printf("Product of 3,4 = %d
", funMultPtr(3,4));

dlclose(module);

return 0;
}

Step4: Compile the source and run

$ gcc -rdynamic -o test test.c -ldl

$./test

Example : Dynamic loading/un-loading of shared library
Sum of 3,4 = 7
Product of 3,4 = 12

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Tuesday, March 10, 2015

Dynamic loading and un loading of shared libraries in C in Linux

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