Provided by: avr-libc_2.0.0+Atmel3.7.0-1_all 
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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Version 2.0.0 Fri Nov 24 2023 23:59:10 avr_stdio(3avr)