int rle_encode_file (FILE * in, FILE * out)
{
    int block_len, block_char, c;
    block_len = 0;
    block_char = -1;
    
    for(;;)
    {
        c = getc(in);
        if (c == EOF) break;
        
        if (c == block_char && block_len < 255)
        {
            /* same block */
            block_len++;
        }
        else
        {
            if (block_len)
            {   /* write block */
                putc(block_len, out);
                putc(block_char, out);
            }
            /* start new block */
            block_len = 1;
            block_char = c;
        }
    }
    
    if (block_len)
    {
        /* last block */
        putc(block_len, out);
        putc(block_char, out);
    }
    return 1;
}

int rle_decode_file (FILE * in, FILE * out)
{
    int i, len, c;
    
    for(;;)
    {
        len = getc(in); /* block length */
        c = getc(in); /* character */
        
        /* checking */
        if (len == EOF) return 1; /* end of file */
        if (c == EOF) return 0; /* bad format */
        
        /* output */
        for(i = 0; i < len; i++) putc(c, out);
    }
}

http://en.wikipedia.org/wiki/Run-length_encoding

Encode and decode RLE from files

RLE Encoding and decoding

C