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.
scanf family of functions.
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.
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.
malloc()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.
#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.
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.
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;
}
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 will also save some execution time.
#define FILE struct __file #define stdin (__iob[0]) #define stdout (__iob[1]) #define stderr (__iob[2]) #define EOF -1
FILE is the opaque structure that is passed around between the various standard IO functions.
stdin (type is FILE*)
stream argument.
The first stream opened with read intent using
fdevopen()
will be assigned to stdin.
stdout (type is FILE*)
stream argument.
The first stream opened with write intent using fdevopen()
will be assigned to both, stdin, and stderr.
stderr (type is FILE*)
stdout.
If stderr should point to another stream,
the result of another fdevopen()
must be explicitly assigned to it without closing the previous stderr
(since this would also close stdout).
EOF declares the value that is returned by various standard IO functions in case of an error.
Since the AVR platform (currently) doesn't contain an abstraction for actual files,
its origin as “end of file” is somewhat meaningless here.
#define fdev_setup_stream(stream,pt,get,rwflag) #define FDEV_SETUP_STREAM(put,get,rwflag) #define _FDEV_SETUP_READ __SRD #define _FDEV_SETUP_WRITE __SWR #define _FDEV_SETUP_RW (__SRD|__SWR) #define _FDEV_ERR -1 #define _FDEV_EOF -2 #define fdev_set_udata(stream, u) ((stream)->udata = u) #define fdev_get_udata(stream) ((stream)->udata)
This macro takes a user-supplied buffer stream,
and sets it up as a stream that is valid for stdio operations
similar to one that has been obtained dynamically from fdevopen().
The buffer to setup must be of type FILE.
The arguments put and get are identical to those
that need to be passed to fdevopen().
The rwflag argument can take one of the values _FDEV_SETUP_READ,
_FDEV_SETUP_WRITE, or _FDEV_SETUP_RW, for read, write, or read/write intent, respectively.
FDEV_SETUP_STREAM acts similar to fdev_setup_stream(),
but it is to be used as the initializer of a variable of type FILE.
To be used in the get function of fdevopen().
fdev_get_udata retrieves a pointer to user defined data from a FILE stream object.
fdev_set_udata inserts a pointer to user defined data into a FILE stream object.
The user data can be useful for tracking state in the put and get functions supplied to the fdevopen() function.
#include <stdio.h> FILE * fdevopen (int(*put)(char, FILE *), int(*get)(FILE *)); #define fdev_close() int fclose (FILE *stream); int feof (FILE *stream); int ferror (FILE *stream); void clearerr (FILE *stream);
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.
stream.
It should be called if stream is no longer needed,
right before the application is going to destroy the stream object itself.(Currently, this macro evaluates to nothing, but this might change in future versions of the library.)
stream, and disallows and further IO to and from it.When using fdevopen() to setup the stream, a call to fclose() is needed in order to free the internal resources allocated.
If the stream has been set up using fdev_setup_stream() or FDEV_SETUP_STREAM(), use fdev_close() instead.
It currently always returns 0 (for success).
stream.
This flag can only be cleared by a call to
clearerr().
stream.
This flag can only be cleared by a call to clearerr().
stream.
#define getchar() fgetc(stdin) #define getc(stream) fgetc(stream) int fgetc (FILE *stream); int ungetc (int c, FILE *stream); char * fgets (char *buf, int size, FILE *stream); char * gets (char *buf); size_t fread (void *p, size_t s, size_t e, FILE *stream);
getchar reads a character from stdin.
Return values and error handling is identical to fgetc().
getc used to be a “fast” macro
implementation with a functionality identical to fgetc().
For space constraints, in avr-libc, it is just an alias for fgetc.
fgetc reads a character from stream.
It returns the character, or EOF in case end-of-file
was encountered or an error occurred.
The routines feof() or ferror()
must be used to distinguish between both situations.
ungetc() function pushes the character c
(converted to an unsigned char) back onto the input stream
pointed to by stream.
The pushed-back character will be returned by a subsequent read on the stream.Currently, only a single character can be pushed back onto the stream.
The ungetc() function returns the character
pushed back after the conversion, or EOF if the operation fails.
If the value of the argument c character equals EOF,
the operation will fail and the stream will remain unchanged.
size-1 bytes from stream,
until a newline character was encountered,
and store the characters in the buffer pointed to by buf.
Unless an error was encountered while reading,
the string will then be terminated with a NUL character.
If an error was encountered, the function returns NULL
and sets the error flag of stream,
which can be tested using ferror().
Otherwise, a pointer to the string will be returned.
stdin, and the trailing newline
(if any) will not be stored in the string.
It is the caller's responsibility to provide
enough storage to hold the characters read.
gets_s, and is no more C standard.
e objects, s bytes each, from stream,
to the buffer pointed to by p.
Returns the number of objects successfully read, i. e. e
unless an input error occured or end-of-file was encountered.
feof() or ferror()
must be used to distinguish between these two conditions.
#include <stdio.h> int scanf ( const char *fmt, ...); int scanf_P( const char *fmt, ...); int fscanf (FILE *stream, const char *fmt, ...); int fscanf_P(FILE *stream, const char *fmt, ...); int sscanf (const char*s, const char *fmt, ...); int sscanf_P(const char*s, const char *fmt, ...); int vscanf ( const char *fmt, va_list ap); int vfscanf (FILE *stream, const char *fmt, va_list ap); int vfscanf_P(FILE *stream, const char *fmt, va_list ap);
scanf performs formatted input from stream stdin.See vfscanf() for details.
scanf_P is a variant of scanf()
where fmt resides in program memory.
fscanf performs formatted input,
reading the input data from stream.See vfscanf() for details.
fscanf_P is a variant of fscanf()
using a fmt string in program memory.
sscanf performs formatted input,
reading the input data from the buffer pointed to by buf.See vfscanf() for details.
sscanf_P is a variant of sscanf()
using a fmt string in program memory.
vscanf performs formatted input from stream stdin,
taking a variable argument list as in vfscanf().See vfscanf() for details.
Characters are read from stream and processed in a way described by fmt. Conversion results will be assigned to the parameters passed via ap.
The format string fmt is scanned for conversion specifications. Anything that doesn't comprise a conversion specification is taken as text that is matched literally against the input. White space in the format string will match any white space in the data (including none), all other characters match only itself. Processing is aborted as soon as the data and format string no longer match, or there is an error or end-of-file condition on stream.
Most conversions skip leading white space before starting the actual conversion.
Conversions are introduced with the character %. Possible options can follow the %:
* indicating that the conversion should be performed
but the conversion result is to be discarded;
no parameters will be processed from ap,
h indicating that the argument
is a pointer to short int
(rather than int),
hh
indicating that the argument is a pointer to char
(rather than int).
l indicating that the argument is a pointer
to long int (rather than int, for integer type conversions),
or a pointer to double (for floating point conversions),
c conversion that defaults to 1).The following conversion flags are supported:
% Matches a literal % character.
This is not a conversion.
d Matches an optionally signed decimal integer;
the next pointer must be a pointer to int.
i Matches an optionally signed integer;
the next pointer must be a pointer to int.
The integer is read in base 16 if it begins with 0x or 0X,
in base 8 if it begins with 0, and in base 10 otherwise.
Only characters that correspond to the base are used.
o Matches an octal integer;
the next pointer must be a pointer to unsigned int.
u Matches an optionally signed decimal integer;
the next pointer must be a pointer to unsigned int.
x Matches an optionally signed hexadecimal integer;
the next pointer must be a pointer to unsigned int.
f Matches an optionally signed floating-point number;
the next pointer must be a pointer to float.
e, g, F, E, G Equivalent to f.
s Matches a sequence of non-white-space characters;
the next pointer must be a pointer to char,
and the array must be large enough to accept all the sequence
and the terminating NUL character.
The input string stops at white space or at the maximum field width,
whichever occurs first.
c Matches a sequence of width count characters (default 1);
the next pointer must be a pointer to char,
and there must be enough room for all the characters
(no terminating NUL is added).
The usual skip of leading white space is suppressed.
To skip white space first, use an explicit space in the format.
[ Matches a nonempty sequence of characters
from the specified set of accepted characters;
the next pointer must be a pointer to char,
and there must be enough room for all the characters in the string,
plus a terminating NUL character.
The usual skip of leading white space is suppressed.
The string is to be made up of characters in (or not in) a particular set;
the set is defined by the characters between the open bracket
[ character and a close bracket ] character.
The set excludes those characters if the first character
after the open bracket is a circumflex ^.
To include a close bracket in the set,
make it the first character after the open bracket or the circumflex;
any other position will end the set.
The hyphen character - is also special;
when placed between two other characters,
it adds all intervening characters to the set.
To include a hyphen, make it the last character before the final close bracket.
For instance, [^]0-9-] means the set of everything except close bracket,
zero through nine, and hyphen.
The string ends with the appearance of a character not in the
(or, with a circumflex, in) set or when the field width runs out.
Note that usage of this conversion enlarges the stack expense.
p Matches a pointer value (as printed by p in printf());
the next pointer must be a pointer to void.
n Nothing is expected;
instead, the number of characters consumed thus far from the input
is stored through the next pointer,
which must be a pointer to int.
This is not a conversion, although it can be suppressed with the * flag.
d conversion.
The value EOF is returned if an input failure occurs
before any conversion such as an end-of-file occurs.
If an error or end-of-file occurs after conversion has begun,
the number of conversions which were successfully completed is returned.
Discarded conversion results (with * option) are not(?) counted.
By default, all the conversions described above are available
except the floating-point conversions and the width is limited to 255 characters.
The float-point conversion will be available in the extended version
provided by the library libscanf_flt.a.
Also in this case the width is not limited (exactly, it is limited to 65535 characters).
To link a program against the extended version,
use the following compiler flags in the link stage:
-Wl,-u,vfscanf -lscanf_flt -lmA third version is available for environments that are tight on space. In addition to the restrictions of the standard one, this version implements no
%[ specification.
This version is provided in the library libscanf_min.a,
and can be requested using the following options in the link stage:
-Wl,-u,vfscanf -lscanf_min
vfscanf_P is a variant of vfscanf()
using a fmt string in program memory.
#include <stdio.h> #define putchar(c) fputc(c, stdout) #define putc(c, stream) fputc(c,stream) int puts (const char *s); int puts_P(const char *s); int fputc (int c, FILE *stream); int fputs (const char *s, FILE *stream); int fputs_P(const char *s, FILE *stream); size_t fwrite (const void *p, size_t s, size_t e, FILE *stream); int fflush (FILE *stream);
putchar sends character c to stdout.
putc used to be a “fast” macro implementation
with a functionality identical to fputc().
For space constraints, in avr-libc,
it is just an alias for fputc.
s,
and a trailing newline character, to stdout.
puts_P is a variant of puts() where s resides in program memory.
fputc sends the character c
(though given as type int) to stream.
It returns the character, or EOF in case an error occurred.
s to stream stream.Returns 0 on success and EOF on error.
fputs_P is a variant of fputs()
where s resides in program memory.
e objects, s bytes each, to stream.
The first byte of the first object is referenced by p.
Returns the number of objects successfully written,
i. e. e unless an output error occured.
stream.This is a null operation provided for source-code compatibility only, as the standard IO implementation currently does not perform any buffering.
#include <stdio.h> int printf ( const char *fmt, ...); int printf_P( const char *fmt, ...); int fprintf (FILE *stream, const char *fmt, ...); int fprintf_P(FILE *stream, const char *fmt, ...); int sprintf (char *buf, const char *fmt, ...); int sprintf_P(char *buf, const char *fmt, ...); int snprintf (char*s,size_t n,const char *fmt, ...); int snprintf_P(char*s,size_t n,const char *fmt, ...); int vprintf ( const char *fmt, va_list ap); int vfprintf (FILE *stream, const char *fmt, va_list ap); int vfprintf_P(FILE *stream, const char *fmt, va_list ap); int vsprintf (char *buf, const char *fmt, va_list ap); int vsprintf_P(char *buf, const char *fmt, va_list ap); int vsnprintf (char*s,size_t n,const char *fmt, va_list ap); int vsnprintf_P(char*s,size_t n,const char *fmt, va_list ap);
printf performs formatted output to
stream stdout.
See vfprintf() for details.
printf_P is a variant of printf()
that uses a fmt string that resides in program memory.
fprintf performs formatted output to stream.
See vfprintf() for details.
fprintf_P is a variant of fprintf()
that uses a fmt string that resides in program memory.
printf()
that sends the formatted characters to string buf.
sprintf_P is a variant of sprintf()
that uses a fmt string that resides in program memory.
sprintf(),
but instead of assuming s to be of infinite size,
no more than n characters (including the trailing NUL character)
will be converted to s.
Returns the number of characters that would have been written to
s if there were enough space.
snprintf_P is a variant of snprintf()
that uses a fmt string that resides in program memory.
vprintf performs formatted output to stream stdout,
taking a variable argument list as in vfprintf().See vfprintf() for details.
vfprintf is the central facility of the printf family of functions.
It outputs values to stream under control of a format string passed in fmt.
The actual values to print are passed as a variable argument list ap.
vfprintf returns the number of characters written to stream,
or EOF in case of an error.
Currently, this will only happen if stream has not been opened with write intent.
The format string is composed of zero or more directives: ordinary characters (not %),
which are copied unchanged to the output stream; and conversion specifications,
each of which results in fetching zero or more subsequent arguments.
Each conversion specification is introduced by the % character.
The arguments must properly correspond (after type promotion) with the conversion specifier.
After the %, the following appear in sequence:
# The value should be converted to an "alternate form".
For c, d, i, s, and u conversions, this option has no effect.
For o conversions, the precision of the number is increased
to force the first character of the output string to a zero
(except if a zero value is printed with an explicit precision of zero).
For x and X conversions, a non-zero result has the string
"0x" (or "0X" for X conversions) prepended to it.
0 (zero) Zero padding.
For all conversions, the converted value is padded on the left with zeros rather than blanks.
If a precision is given with a numeric conversion (d, i, o, u, i, x, and X), the 0 flag is ignored.
- A negative field width flag;
the converted value is to be left adjusted on the field boundary.
The converted value is padded on the right with blanks,
rather than on the left with blanks or zeros.
A - overrides a 0 if both are given.
+ A sign must always be placed before a number
produced by a signed conversion.
A + overrides a space if both are used.
s conversions.
l or h length modifier,
that specifies that the argument for the d, i, o, u, x, or X conversion
is a "long int" rather than int.
The h is ignored, as "short int" is equivalent to
int.
diouxX The int (or appropriate variant) argument
is converted to signed decimal (d and i), unsigned octal (o),
unsigned decimal (u), or unsigned hexadecimal (x and X) notation.
The letters "abcdef" are used for x conversions;
the letters "ABCDEF" are used for X conversions.
The precision, if any, gives the minimum number of digits that must appear;
if the converted value requires fewer digits,
it is padded on the left with zeros.
p The void * argument is taken
as an unsigned integer, and converted similarly as a %#x
command would do.
c The int argument is converted to an
"unsigned char", and the resulting character is written.
s The "char *" argument is expected
to be a pointer to an array of character type (pointer to a string).
Characters from the array are written up to (but not including)
a terminating NUL character; if a precision is specified,
no more than the number specified are written.
If a precision is given, no null character need be present;
if the precision is not specified, or is greater than the size of the array,
the array must contain a terminating NUL character.
% A % is written.
No argument is converted.
The complete conversion specification is "%%".
eE The double argument is rounded and converted
in the format "[-]d.ddde±dd" where there is one digit
before the decimal-point character and the number of digits
after it is equal to the precision;
if the precision is missing, it is taken as 6; if the precision is zero,
no decimal-point character appears.
An E conversion uses the letter 'E'
(rather than 'e') to introduce the exponent.
The exponent always contains two digits;
if the value is zero, the exponent is 00.
fF The double argument is rounded and converted
to decimal notation in the format "[-]ddd.ddd",
where the number of digits after the decimal-point character
is equal to the precision specification.
If the precision is missing, it is taken as 6;
if the precision is explicitly zero, no decimal-point character appears.
If a decimal point appears, at least one digit appears before it.
gG The double argument is converted in style
f or e
(or F or E for G conversions).
The precision specifies the number of significant digits.
If the precision is missing, 6 digits are given;
if the precision is zero, it is treated as 1.
Style e is used if the exponent from its conversion
is less than -4 or greater than or equal to the precision.
Trailing zeros are removed from the fractional part of the result;
a decimal point appears only if it is followed by at least one digit.
S Similar to the s format,
except the pointer is expected to point to a program-memory
(ROM) string instead of a RAM string.
Since the full implementation of all the mentioned features
becomes fairly large, three different flavours
of vfprintf() can be selected using linker options.
The default vfprintf() implements
all the mentioned functionality except floating point conversions.
A minimized version of vfprintf()
is available that only implements the very basic integer
and string conversion facilities, but only the #
additional option can be specified using conversion flags
(these flags are parsed correctly from the format specification,
but then simply ignored).
This version can be requested using the following
compiler options:
-Wl,-u,vfprintf -lprintf_minIf the full functionality including the floating point conversions is required, the following options should be used:
-Wl,-u,vfprintf -lprintf_flt -lm
.
will be output and double argument will be skiped.
So you output below will not be crashed.
For default version the width field and the "pad to left"
( symbol minus ) option will work in this case.
hh length modifier is ignored
(char argument is promouted to int).
More exactly, this realization does not check the number of
h symbols.
ll length modifier will to abort the output,
as this realization does not operate long long arguments.
* symbol) is not realized
and will to abort the output.
vfprintf_P is a variant of vfprintf()
that uses a fmt string that resides in program memory.
sprintf()
but takes a variable argument list for the arguments.
vsprintf_P is a variant of vsprintf()
that uses a fmt string that resides in program memory.
vsprintf(),
but instead of assuming s to be of infinite size,
no more than n characters (including the trailing NUL character)
will be converted to s.
Returns the number of characters that would have been
written to s if there were enough space.
vsnprintf_P is a variant of vsnprintf()
that uses a fmt string that resides in program memory.