The mysterious case of the missing Base64 library

Many versions of base 64 encoding and decoding can be found on the Internet, but if you would like to use a system one, here is how to access the FreeBSD one. There isn't a documented library for the Base 64 encoding, but there is a base 64 encoder and decoder in the C library. The library functions are __b64_ntop and __b64_pton. They read and write to user buffers with specified lengths. Here is an example program which encodes and then decodes the command line:

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

#define b64_ntop __b64_ntop
#define b64_pton __b64_pton
/* There are no header files for these functions. */

int b64_ntop(unsigned char const *src, size_t srclength,
             char *target, size_t targsize);
int b64_pton(char const *src, unsigned char *target, size_t targsize);

/* Demonstrate the use of the functions by encoding and decoding the
   command line arguments. */

#define targsize 0x100
int main (int argc, char ** argv)
{
    int i;
    char target[targsize];
    unsigned char revert[targsize];
    for (i = 0; i < argc; i++) {
        int end;
        /* Input buffer, input length, output buffer, max output length */
        b64_ntop ((unsigned char*) argv[i], strlen (argv[i]), target, targsize);
        printf ("%s\n", target);
        /* Input buffer (null terminated), output buffer, max output length. */
        end = b64_pton (target, revert, targsize);
        revert[end] = '\0';
        printf ("%s\n", revert);
    }
    return 0;
}

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The functions aren't documented anywhere, and there doesn't seem to be any header file you can use. There is a header file resolv.h which defines them, but it seems like this include file requires other include files to also be included before it. I don't know if there is a better way to call this function.

The output from the example program looks like this:

$ ./a.out slicey slicey oncey twicey         
Li9hLm91dA==
./a.out
c2xpY2V5
slicey
c2xpY2V5
slicey
b25jZXk=
oncey
dHdpY2V5
twicey