source: openssl/vendor/current/demos/eay/base64.c@ 150

/* This is a simple example of using the base64 BIO to a memory BIO and then
 * getting the data.
 */
#include <stdio.h>
#include <openssl/bio.h>
#include <openssl/evp.h>

main()
        {
        int i;
        BIO *mbio,*b64bio,*bio;
        char buf[512];
        char *p;

        mbio=BIO_new(BIO_s_mem());
        b64bio=BIO_new(BIO_f_base64());

        bio=BIO_push(b64bio,mbio);
        /* We now have bio pointing at b64->mem, the base64 bio encodes on
         * write and decodes on read */

        for (;;)
                {
                i=fread(buf,1,512,stdin);
                if (i <= 0) break;
                BIO_write(bio,buf,i);
                }
        /* We need to 'flush' things to push out the encoding of the
         * last few bytes.  There is special encoding if it is not a
         * multiple of 3
         */
        BIO_flush(bio);

        printf("We have %d bytes available\n",BIO_pending(mbio));

        /* We will now get a pointer to the data and the number of elements. */
        /* hmm... this one was not defined by a macro in bio.h, it will be for
         * 0.9.1.  The other option is too just read from the memory bio.
         */
        i=(int)BIO_ctrl(mbio,BIO_CTRL_INFO,0,(char *)&p);

        printf("%d\n",i);
        fwrite("---\n",1,4,stdout);
        fwrite(p,1,i,stdout);
        fwrite("---\n",1,4,stdout);

        /* This call will walk the chain freeing all the BIOs */
        BIO_free_all(bio);
        }