— Function: char * l64a (long int n)

This function encodes a 32-bit input value using characters from the basic character set. It returns a pointer to a 6 character buffer which contains an encoded version of n. To encode a series of bytes the user must copy the returned string to a destination buffer. It returns the empty string if n is zero, which is somewhat bizarre but mandated by the standard.
Warning: Since a static buffer is used this function should not be used in multi-threaded programs. There is no thread-safe alternative to this function in the C library.
Compatibility Note: The XPG standard states that the return value of l64a is undefined if n is negative. In the GNU implementation, l64a treats its argument as unsigned, so it will return a sensible encoding for any nonzero n; however, portable programs should not rely on this.

To encode a large buffer l64a must be called in a loop, once for each 32-bit word of the buffer. For example, one could do something like this:

          char *
          encode (const void *buf, size_t len)
          {
            /* We know in advance how long the buffer has to be. */
            unsigned char *in = (unsigned char *) buf;
            char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
            char *cp = out;
          
            /* Encode the length. */
            /* Using `htonl' is necessary so that the data can be
               decoded even on machines with different byte order. */
          
            cp = mempcpy (cp, l64a (htonl (len)), 6);
          
            while (len > 3)
              {
                unsigned long int n = *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                len -= 4;
                if (n)
                  cp = mempcpy (cp, l64a (htonl (n)), 6);
                else
                      /* `l64a' returns the empty string for n==0, so we 
                         must generate its encoding ("......") by hand. */
                  cp = stpcpy (cp, "......");
              }
            if (len > 0)
              {
                unsigned long int n = *in++;
                if (--len > 0)
                  {
                    n = (n << 8) | *in++;
                    if (--len > 0)
                      n = (n << 8) | *in;
                  }
                memcpy (cp, l64a (htonl (n)), 6);
                cp += 6;
              }
            *cp = '\0';
            return out;
          }
     

It is strange that the library does not provide the complete functionality needed but so be it.

— Function: long int a64l (const char *string)

The parameter string should contain a string which was produced by a call to l64a. The function processes at least 6 characters of this string, and decodes the characters it finds according to the table below. It stops decoding when it finds a character not in the table, rather like atoi; if you have a buffer which has been broken into lines, you must be careful to skip over the end-of-line characters.

The decoded number is returned as a long int value.