扩展原来分配空间, 并拷入数据

void
alloccopy( unsigned char ** ptr, unsigned char * data, \
                                     int data_len, int * pos )
{
    if ( data_len == 0 || data == NULL )
        return;
    else if ( * ptr == NULL )
    {
        * ptr = (unsigned char *)malloc( data_len );
        memcpy( * ptr, data, data_len );
        * pos = data_len;
    }
    else
    {
        * ptr = (unsigned char *)realloc( * ptr, * pos + data_len );
        memcpy( ( * ptr ) + * pos, data, data_len );
        * pos = * pos + data_len;
    }

    return;
}