aes_encrypt() function hooking - aes

Question

I want to know the pid of the process which is performing the AES encryption. I have written the following function hooking code:
#include <stdio.h>
#include <unistd.h>
#include <dlfcn.h>
#include <openssl/aes.h>
void AES_encrypt(const unsigned char *in_var, unsigned char *out_var,
const AES_KEY *key_var)
{
void (*new_aes_encrypt)(const unsigned char *in_var, unsigned char *out_var,
const AES_KEY *key_var);
new_aes_encrypt = dlsym(RTLD_NEXT, "AES_encrypt");
FILE *logfile = fopen("logfile", "a+");
fprintf(logfile, "Process %d:nn%snnn", getpid(), (char *)in_var);
fclose(logfile);
new_aes_encrypt(in_var, out_var, key_var);
}
Then at terminal I did the followings:
#gcc aes_hook.c -o aes_hook.so -fPIC -shared -lssl -D_GNU_SOURCE
#export LD_PRELOAD="/<directory location>/aes_hook.so"
However, when I started the AES encryption (by a dummy process), I could not get it's pid in logfile. Why is this hooking not working?
*P.S.: Following is the declaration of AES_encrypt (in aes.h of OpenSSL) function that is called to perform the AES encryption.
void AES_encrypt(const unsigned char *in, unsigned char *out,
const AES_KEY *key)