Pureholic

    <h3>Introduction to the Standard IO facilities</h3>

    This file declares the standard IO facilities that are implemented
    in \c avr-libc.  Due to the nature of the underlying hardware,
    only a limited subset of standard IO is implemented.  There is no
    actual file implementation available, so only device IO can be
    performed.  Since there's no operating system, the application
    needs to provide enough details about their devices in order to
    make them usable by the standard IO facilities.

    Due to space constraints, some functionality has not been
    implemented at all (like some of the \c printf conversions that
    have been left out).  Nevertheless, potential users of this
    implementation should be warned: the \c printf and \c scanf families of functions, although
    usually associated with presumably simple things like the
    famous "Hello, world!" program, are actually fairly complex
    which causes their inclusion to eat up a fair amount of code space.
    Also, they are not fast due to the nature of interpreting the
    format string at run-time.  Whenever possible, resorting to the
    (sometimes non-standard) predetermined conversion facilities that are
    offered by avr-libc will usually cost much less in terms of speed
    and code size.

    <h3>Tunable options for code size vs. feature set</h3>

    In order to allow programmers a code size vs. functionality tradeoff,
    the function vfprintf() which is the heart of the printf family can be
    selected in different flavours using linker options.  See the
    documentation of vfprintf() for a detailed description.  The same
    applies to vfscanf() and the \c scanf family of functions.

    <h3>Outline of the chosen API</h3>

    The standard streams \c stdin, \c stdout, and \c stderr are
    provided, but contrary to the C standard, since avr-libc has no
    knowledge about applicable devices, these streams are not already
    pre-initialized at application startup.  Also, since there is no
    notion of "file" whatsoever to avr-libc, there is no function
    \c fopen() that could be used to associate a stream to some device.
    (See \ref stdio_note1 "note 1".)  Instead, the function \c fdevopen()
    is provided to associate a stream to a device, where the device
    needs to provide a function to send a character, to receive a
    character, or both.  There is no differentiation between "text" and
    "binary" streams inside avr-libc.  Character \c \\n is sent
    literally down to the device's \c put() function.  If the device
    requires a carriage return (\c \\r) character to be sent before
    the linefeed, its \c put() routine must implement this (see
    \ref stdio_note2 "note 2").

    As an alternative method to fdevopen(), the macro
    fdev_setup_stream() might be used to setup a user-supplied FILE
    structure.

    It should be noted that the automatic conversion of a newline
    character into a carriage return - newline sequence breaks binary
    transfers.  If binary transfers are desired, no automatic
    conversion should be performed, but instead any string that aims
    to issue a CR-LF sequence must use <tt>"\r\n"</tt> explicitly.

    For convenience, the first call to \c fdevopen() that opens a
    stream for reading will cause the resulting stream to be aliased
    to \c stdin.  Likewise, the first call to \c fdevopen() that opens
    a stream for writing will cause the resulting stream to be aliased
    to both, \c stdout, and \c stderr.  Thus, if the open was done
    with both, read and write intent, all three standard streams will
    be identical.  Note that these aliases are indistinguishable from
    each other, thus calling \c fclose() on such a stream will also
    effectively close all of its aliases (\ref stdio_note3 "note 3").

    It is possible to tie additional user data to a stream, using
    fdev_set_udata().  The backend put and get functions can then
    extract this user data using fdev_get_udata(), and act
    appropriately.  For example, a single put function could be used
    to talk to two different UARTs that way, or the put and get
    functions could keep internal state between calls there.

    <h3>Format strings in flash ROM</h3>

    All the \c printf and \c scanf family functions come in two flavours: the
    standard name, where the format string is expected to be in
    SRAM, as well as a version with the suffix "_P" where the format
    string is expected to reside in the flash ROM.  The macro
    \c PSTR (explained in \ref avr_pgmspace) becomes very handy
    for declaring these format strings.

    \anchor stdio_without_malloc
    <h3>Running stdio without malloc()</h3>

    By default, fdevopen() requires malloc().  As this is often
    not desired in the limited environment of a microcontroller, an
    alternative option is provided to run completely without malloc().

    The macro fdev_setup_stream() is provided to prepare a
    user-supplied FILE buffer for operation with stdio.

    <h4>Example</h4>

    \code
    #include <stdio.h>

    static int uart_putchar(char c, FILE *stream);

    static FILE mystdout = FDEV_SETUP_STREAM(uart_putchar, NULL,
                                             _FDEV_SETUP_WRITE);

    static int
    uart_putchar(char c, FILE *stream)
    {

      if (c == '\n')
        uart_putchar('\r', stream);
      loop_until_bit_is_set(UCSRA, UDRE);
      UDR = c;
      return 0;
    }

    int
    main(void)
    {
      init_uart();
      stdout = &mystdout;
      printf("Hello, world!\n");

      return 0;
    }
    \endcode

    This example uses the initializer form FDEV_SETUP_STREAM() rather
    than the function-like fdev_setup_stream(), so all data
    initialization happens during C start-up.

    If streams initialized that way are no longer needed, they can be
    destroyed by first calling the macro fdev_close(), and then
    destroying the object itself.  No call to fclose() should be
    issued for these streams.  While calling fclose() itself is
    harmless, it will cause an undefined reference to free() and thus
    cause the linker to link the malloc module into the application.

    <h3>Notes</h3>

    \anchor stdio_note1 \par Note 1:
    It might have been possible to implement a device abstraction that
    is compatible with \c fopen() but since this would have required
    to parse a string, and to take all the information needed either
    out of this string, or out of an additional table that would need to be
    provided by the application, this approach was not taken.

    \anchor stdio_note2 \par Note 2:
    This basically follows the Unix approach: if a device such as a
    terminal needs special handling, it is in the domain of the
    terminal device driver to provide this functionality.  Thus, a
    simple function suitable as \c put() for \c fdevopen() that talks
    to a UART interface might look like this:

    \code
    int
    uart_putchar(char c, FILE *stream)
    {

      if (c == '\n')
        uart_putchar('\r');
      loop_until_bit_is_set(UCSRA, UDRE);
      UDR = c;
      return 0;
    }
    \endcode

    \anchor stdio_note3 \par Note 3:
    This implementation has been chosen because the cost of maintaining
    an alias is considerably smaller than the cost of maintaining full
    copies of each stream.  Yet, providing an implementation that offers
    the complete set of standard streams was deemed to be useful.  Not
    only that writing \c printf() instead of <tt>fprintf(mystream, ...)</tt>
    saves typing work, but since avr-gcc needs to resort to pass all
    arguments of variadic functions on the stack (as opposed to passing
    them in registers for functions that take a fixed number of
    parameters), the ability to pass one parameter less by implying
    \c stdin will also save some execution time.
*/