oracular (3) fdevopen.3avr.gz

NAME

       avr_stdio - <stdio.h>: Standard IO facilities

SYNOPSIS

   Functions
       FILE * fdevopen (int(*put)(char, FILE *), int(*get)(FILE *))

Detailed Description

       #include <stdio.h>

   Introduction to the Standard IO facilities
       This file declares the standard IO facilities that are implemented in 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 printf
       conversions that have been left out). Nevertheless, potential users of this implementation should be
       warned: the printf and 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.

   Tunable options for code size vs. feature set
       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 scanf
       family of functions.

   Outline of the chosen API
       The standard streams stdin, stdout, and 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 fopen() that could be used to associate a stream to some device. (See note 1.) Instead, the
       function 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 \n is sent literally down to the device's put() function.
       If the device requires a carriage return (\r) character to be sent before the linefeed, its put() routine
       must implement this (see 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 '\r\n' explicitly.

       For convenience, the first call to fdevopen() that opens a stream for reading will cause the resulting
       stream to be aliased to stdin. Likewise, the first call to fdevopen() that opens a stream for writing
       will cause the resulting stream to be aliased to both, stdout, and 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 fclose() on such a stream will also effectively close
       all of its aliases (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.

   Format strings in flash ROM
       All the printf and 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 PSTR (explained in <avr/pgmspace.h>: Program Space
       Utilities) becomes very handy for declaring these format strings.

   Running stdio without malloc()
       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.

   Example
       #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;
       }

       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.

   Notes
       Note 1:
           It might have been possible to implement a device abstraction that is compatible with 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.

       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 put() for fdevopen() that talks to a UART interface might look like this:

       int
       uart_putchar(char c, FILE *stream)
       {

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

       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 printf() instead
           of fprintf(mystream, ...) 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 stdin or stdout will
           also save some execution time.

Function Documentation

   FILE * fdevopen (int(*)(char, FILE *) put, int(*)(FILE *) get)
       This function is a replacement for fopen().

       It opens a stream for a device where the actual device implementation needs to be provided by the
       application. If successful, a pointer to the structure for the opened stream is returned. Reasons for a
       possible failure currently include that neither the put nor the get argument have been provided, thus
       attempting to open a stream with no IO intent at all, or that insufficient dynamic memory is available to
       establish a new stream.

       If the put function pointer is provided, the stream is opened with write intent. The function passed as
       put shall take two arguments, the first a character to write to the device, and the second a pointer to
       FILE, and shall return 0 if the output was successful, and a nonzero value if the character could not be
       sent to the device.

       If the get function pointer is provided, the stream is opened with read intent. The function passed as
       get shall take a pointer to FILE as its single argument, and return one character from the device, passed
       as an int type. If an error occurs when trying to read from the device, it shall return _FDEV_ERR. If an
       end-of-file condition was reached while reading from the device, _FDEV_EOF shall be returned.

       If both functions are provided, the stream is opened with read and write intent.

       The first stream opened with read intent is assigned to stdin, and the first one opened with write intent
       is assigned to both, stdout and stderr.

       fdevopen() uses calloc() (und thus malloc()) in order to allocate the storage for the new stream.

       Note
           If the macro __STDIO_FDEVOPEN_COMPAT_12 is declared before including <stdio.h>, a function prototype
           for fdevopen() will be chosen that is backwards compatible with avr-libc version 1.2 and before. This
           is solely intented for providing a simple migration path without the need to immediately change all
           source code. Do not use for new code.

Author

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