dlopen:
void * dlopen(const char *filename, int flag);
Flag:
RTLD_LAZY --> ``resolve undefined symbols as code from the dynamic library is
executed''
RTLD_NOW --> ``resolve all undefined symbols before dlopen() returns and
fail if this cannot be done''
opening the library take slightly longer (but it speeds up
lookups later);
RTLD_GLOBAL --> Optional -> external symbols defined in the library will be made
available to subsequently loaded libraries
- If the libraries depend on each other (e.g., X depends on Y), then you need to load the dependees first (in this example, load Y first, and then X).
Return:
- ``handle'' that should be considered an opaque value to be used by the other DL library routines.
- Return NULL On error.
dlerror:
- which returns a string describing the error from the last call to dlopen(), dlsym(), or dlclose().
- Return null if there is no error.
- Call dlerror() first (to clear any error condition that may have existed), then call dlsym() to request a symbol, then call dlerror() again to see if an error occurred.
dlsym:
void * dlsym(void *handle, char *symbol);
handle -> value returned from dlopen
symbol -> NIL-terminated string.
Note:
For Run time patch(Dynamic loading the .so file): We must close the old handler & re open the library again,if there is any change in library. Otherwise changes won't be loaded. Hence for Run time patch, we must close & open the library again.
Return:
- Return a NULL result if the symbol wasn't found.
dlclose:
- Closes a DL library.
- The dl library maintains link counts for dynamic file handles, so a dynamic library is not actually deallocated until dlclose has been called on it as many times as dlopen has succeeded on it.
- It's not a problem for the same program to load the same library multiple times.
Return:
- Returns 0 on success, and non-zero on error
Program:
Program 1: create shared library:
/* Dynamic loading lib(run time load or patch update)
* Check : http://velrajcoding.blogspot.in
*/
#include <stdio.h>
void run_load()
{
printf("Velraj Run time loading 1st \n");
return;
}
Program 2: Dynamic loader program:
/* Dynamic loading lib(run time load or patch update)
* Check : http://velrajcoding.blogspot.in
*/
#include <stdlib.h>
#include <stdio.h>
#include <dlfcn.h>
#define LIB_USR_DEF "~/velrajk/sample/learn/librunlib.so"
enum {
DL_LOAD_LIB = 1,
DL_CALL_FUN,
DL_EXIT,
};
void hline(char ch)
{
int i = 0;
for ( ; i < 80; printf("%c", ch), i++);
printf("\n");
return;
}
int main(int argc, char **argv)
{
void *handle = NULL;
void (*fun_symbol)();
char *ret = NULL;
int ch = 0;
while (1) {
printf("\n\n");
hline('*');
printf("Find the DL library options: \n\t 1. Load the library\n\t 2.Call the function\n\t 3.Exit\n Enter your choice: ");
scanf("%d", &ch);
switch (ch) {
case DL_LOAD_LIB:
/*
* For Run time path:
* We must close the old handler & re open the library again, if there is any change in library.
* Otherwise changes won't be loaded.
* Hence for Run time patch, we must close & open the library again.
*/
if (handle) {
dlclose(handle);
}
handle = dlopen(LIB_USR_DEF, RTLD_LAZY);
if (!handle) {
fputs (dlerror(), stderr);
continue;
}
printf("Successfully opened an library \n");
break;
case DL_CALL_FUN:
/* clear error code */
dlerror();
fun_symbol = dlsym(handle, "run_load");
if ((ret = dlerror()) != NULL) {
/* handle error, the symbol wasn't found */
printf("dlerror return error msg as:%s \n", ret);
continue;
}
/* symbol found, calling the symbol */
(*fun_symbol)();
break;
case DL_EXIT:
exit(1);
break;
default:
printf("Choose the correct choice \n");
break;
}
}
dlclose(handle);
return 0;
}
Output:
Program 1: Create Library
:~/velrajk/sample/learn$ gcc -fPIC -shared -o librunlib.so dl_run_lib.c
Program 2: Dynamic loading
:~/velrajk/sample/learn$ gcc dlopen.c -ldl
:~/velrajk/sample/learn$ ./a.out
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice: 1
Successfully opened an library
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice: 2
Velraj Run time loading 1st
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice:
<!-- Here itself changed the library content as 2nd but without dlclose & dlopen, new value won't effect -->
2
Velraj Run time loading 1st
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice: 2
Velraj Run time loading 1st
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice: 2
Velraj Run time loading 1st
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice: 1
Successfully opened an library
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice: 2
Velraj Run time loading 2nd
********************************************************************************
Find the DL library options:
1. Load the library
2.Call the function
3.Exit
Enter your choice: 3
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