BIO_write() is not writing to the file when using BIO_f_base64() - c++

Question

I am trying to learn OPENSSL and following this tutorial. I have written following code to read from a file and write the encoded form in another file.
#include <iostream>
#include <openssl/ssl.h>
#include <openssl/bio.h>
#define MAX_BUFFER_SIZE 512
int main (int argc, char* argv[])
{
BIO* bio_out = nullptr;
BIO* bio_in = nullptr;
BIO* bio_b64 = nullptr;
int in_byte, out_byte;
char buffer[MAX_BUFFER_SIZE];
std::memset(buffer, '\0', MAX_BUFFER_SIZE);
if (argc != 3)
{
std::cout << "Usage: bio_b64_encode <source-read> <encoded-write>\n";
return 1;
}
bio_in = BIO_new_file(argv[1], "r");
bio_out = BIO_new_file(argv[2], "wb");
bio_b64 = BIO_new(BIO_f_base64());
BIO_push(bio_b64, bio_out);
in_byte = BIO_read(bio_in, buffer, MAX_BUFFER_SIZE);
while (in_byte > 0)
{
std::cout << "Read " << in_byte << " bytes.\n";
out_byte = BIO_write(bio_b64, buffer, in_byte);
std::cout << "Wrote " << out_byte << " bytes.\n";
if (in_byte != out_byte)
{
std::cout << "In bytes: " << in_byte << "and Out bytes: " << out_byte << " are not equal.\n";
BIO_free(bio_in);
BIO_free(bio_out);
return 1;
}
in_byte = BIO_read(bio_in, buffer, MAX_BUFFER_SIZE);
}
BIO_free(bio_in);
BIO_free_all(bio_b64);
return 0;
}
The code reads properly from the input file given in argv[1] and tries to write to argv[2] but when I open the output file, it is empty but it should have the encoded text from the input file.
the output of the above program when I run ./bio_b64_encode in.txt out.txt is
Read 22 bytes.
Wrote 22 bytes.
but the output file is empty. If I remove the encoding constraints and try to simply write whatever I read from input file (the code is below), it works properly.
#include <iostream>
#include <openssl/ssl.h>
#include <openssl/bio.h>
#define MAX_BUFFER_SIZE 512
int main (int argc, char* argv[])
{
BIO* bio_out = nullptr;
BIO* bio_in = nullptr;
BIO* bio_b64 = nullptr;
int in_byte, out_byte;
char buffer[MAX_BUFFER_SIZE];
std::memset(buffer, '\0', MAX_BUFFER_SIZE);
if (argc != 3)
{
std::cout << "Usage: bio_write <file-read> <file-write>\n";
return 1;
}
bio_in = BIO_new_file(argv[1], "r");
bio_out = BIO_new_file(argv[2], "w");
in_byte = BIO_read(bio_in, buffer, MAX_BUFFER_SIZE);
while (in_byte > 0)
{
std::cout << "Read " << in_byte << " bytes.\n";
out_byte = BIO_write(bio_out, buffer, in_byte);
std::cout << "Wrote " << out_byte << " bytes.\n";
if (in_byte != out_byte)
{
std::cout << "In bytes: " << in_byte << "and Out bytes: " << out_byte << " are not equal.\n";
BIO_free(bio_in);
BIO_free(bio_out);
return 1;
}
in_byte = BIO_read(bio_in, buffer, MAX_BUFFER_SIZE);
}
BIO_free(bio_in);
BIO_free_all(bio_out);
return 0;
}
I am not able to figure out what am I missing here.