simple test for memory allocation for global variables in dynamically loaded lib

dl_func.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define __USE_GNU
#include <dlfcn.h> // Dl_info, dladdr()

#ifdef linux
  #ifdef PATH_MAX
        #define _MAX_PATH PATH_MAX
  #else
        #define _MAX_PATH 4096
  #endif
#endif

static int *a;

void func(void)
{
        a = (int*)malloc(1*sizeof(int));
        printf("addr_a=%p, a=%d\n", a, *a);
        *a = 111;
        printf("new a=%d\n", *a);
        free(a); // can be commented out
}

__attribute__((constructor))
void on_dlopen(void) {
        char library_path[_MAX_PATH] = {0};
        Dl_info dl_info;
        dladdr(on_dlopen, &dl_info);
        strcpy(library_path, dl_info.dli_fname);
        printf("module %s opened\n", dl_info.dli_fname);
}

__attribute__((destructor))
void on_dlclose(void) {
        char library_path[_MAX_PATH] = {0};
        Dl_info dl_info;
        dladdr(on_dlclose, &dl_info);
        strcpy(library_path, dl_info.dli_fname);
        printf("module %s closed\n", dl_info.dli_fname);
}

Makefile

CC=gcc
CFLAGS=-Wall -g

all: libdl_func.so.1.0.0 test_dl

libdl_func.so.1.0.0: dl_func.o
	rm -f libdl_func.so
	rm -f libdl_func.so.1
	$(CC) -o $@ $^ -shared -Wl,-soname,libdl_func.so.1
	ln -s libdl_func.so.1.0.0 libdl_func.so
	ln -s libdl_func.so.1.0.0 libdl_func.so.1

dl_func.o: dl_func.c
	$(CC) $(CFLAGS) -fPIC -c $<

test_dl: test_dl.o
	$(CC) -o $@ $^ -ldl

test_dl.o: test_dl.c
	$(CC) $(CFLAGS) -c $<

clean:
	rm -f libdl_func.so.1.0.0 libdl_func.so.1 libdl_func.so dl_func.o test_dl.o test_dl

output.txt

module ./libdl_func.so.1.0.0 opened
addr_a=0x86523b0, a=0
new a=111
module ./libdl_func.so.1.0.0 closed
module ./libdl_func.so.1.0.0 opened
addr_a=0x86523b0, a=0
new a=111
module ./libdl_func.so.1.0.0 closed
to continue press ENTER

------------------------------------
or if free(a) is commented out:

module ./libdl_func.so.1.0.0 opened
addr_a=0x8e093b0, a=0
new a=111
module ./libdl_func.so.1.0.0 closed
module ./libdl_func.so.1.0.0 opened
addr_a=0x8e093c0, a=0
new a=111
module ./libdl_func.so.1.0.0 closed

test_dl.c

// run: LD_LIBRARY_PATH=. ./test_dl

#include <stdio.h>
#include <dlfcn.h>

typedef void (*Func)(void);

const char *filename = "libdl_func.so.1.0.0";

int main(int argc, char *argv[], char **env)
{
        void *plugin_handle;
        int i;

        for (i = 0; i < 2; i++) {
                plugin_handle = dlopen(filename, RTLD_LAZY);
                if (!plugin_handle) {
                        printf("failed to load library!\n");
                        return 1;
                }
                Func func = (Func) dlsym(plugin_handle, "func");
                func();
                dlclose(plugin_handle);
        }

        printf("to continue press ENTER\n");
        getchar();

        return 0;
}