Standard IO facilities(3)

NAME
Standard IO facilities -

Detailed Description

 #include <stdio.h>

Warning:
    This implementation of the standard IO facilities is new to avr-libc. It is not yet expected to remain stable, so some aspects of the API might change in a future release.

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.

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.

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).

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).

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 Program Space String Utilities) becomes very handy for declaring these format strings.

Note 1:.RS 4 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:.RS 4 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)
    {
      if (c == ')
        uart_putchar('');
      loop_until_bit_is_set(UCSRA, UDRE);
      UDR = c;
      return 0;
    }

Note 3:.RS 4 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 will also save some execution time.

  Defines
#define FILE struct __file
#define stdin (__iob[0])
#define stdout (__iob[1])
#define stderr (__iob[2])
#define EOF (-1)
#define putc(__c, __stream) fputc(__c, __stream)
#define putchar(__c) fputc(__c, stdout)
#define getc(__stream) fgetc(__stream)
#define getchar() fgetc(stdin)

  Functions
int fclose (FILE *__stream)
int vfprintf (FILE *__stream, const char *__fmt, va_list __ap)
int vfprintf_P (FILE *__stream, const char *__fmt, va_list __ap)
int fputc (int __c, FILE *__stream)
int printf (const char *__fmt,...)
int printf_P (const char *__fmt,...)
int sprintf (char *__s, const char *__fmt,...)
int sprintf_P (char *__s, 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 vsprintf (char *__s, const char *__fmt, va_list ap)
int vsprintf_P (char *__s, 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)
int fprintf (FILE *__stream, const char *__fmt,...)
int fprintf_P (FILE *__stream, const char *__fmt,...)
int fputs (const char *__str, FILE *__stream)
int fputs_P (const char *__str, FILE *__stream)
int puts (const char *__str)
int puts_P (const char *__str)
size_t fwrite (const void *__ptr, size_t __size, size_t __nmemb, FILE *__stream)
int fgetc (FILE *__stream)
int ungetc (int __c, FILE *__stream)
char * fgets (char *__str, int __size, FILE *__stream)
char * gets (char *__str)
size_t fread (void *__ptr, size_t __size, size_t __nmemb, FILE *__stream)
void clearerr (FILE *__stream)
int feof (FILE *__stream)
int ferror (FILE *__stream)
int vfscanf (FILE *__stream, const char *__fmt, va_list __ap)
int vfscanf_P (FILE *__stream, const char *__fmt, va_list __ap)
int fscanf (FILE *__stream, const char *__fmt,...)
int fscanf_P (FILE *__stream, const char *__fmt,...)
int scanf (const char *__fmt,...)
int scanf_P (const char *__fmt,...)
int sscanf (const char *__buf, const char *__fmt,...)
int sscanf_P (const char *__buf, const char *__fmt,...)
FILE * fdevopen (int(*put)(char), int(*get)(void), int opts __attribute__((unused)))

Define Documentation

  #define EOF (-1)

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 FILE struct __file

FILE is the opaque structure that is passed around between the various standard IO functions.

  #define getc(__stream) fgetc(__stream)

The macro 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.

  #define getchar(void) fgetc(stdin)

The macro getchar reads a character from stdin. Return values and error handling is identical to fgetc().

  #define putc(__c, __stream) fputc(__c, __stream)

The macro 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.

  #define putchar(__c) fputc(__c, stdout)

The macro putchar sends character c to stdout.

  #define stderr (__iob[2])

Stream destined for error output. Unless specifically assigned, identical to 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).

  #define stdin (__iob[0])

Stream that will be used as an input stream by the simplified functions that don't take a stream argument.

The first stream opened with read intent using fdevopen() will be assigned to stdin.

  #define stdout (__iob[1])

Stream that will be used as an output stream by the simplified functions that don't take a stream argument.

The first stream opened with write intent using fdevopen() will be assigned to both, stdin, and stderr.

Function Documentation

  void clearerr (FILE * __stream)

Clear the error and end-of-file flags of stream.

  int fclose (FILE * __stream)

This function closes stream, and disallows and further IO to and from it.

It currently always returns 0 (for success).

  FILE* fdevopen (int(*)(char) put, int(*)(void) get, int opts __attribute__((unused)))

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 one character to write to the device as argument , 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 no arguments, 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 -1.

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.

The third parameter opts is currently unused, but reserved for future extensions.

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

  int feof (FILE * __stream)

Test the end-of-file flag of stream. This flag can only be cleared by a call to clearerr().

Note:
    Since there is currently no notion for end-of-file on a device, this function will always return a false value.

  int ferror (FILE * __stream)

Test the error flag of stream. This flag can only be cleared by a call to clearerr().

  int fgetc (FILE * __stream)

The function 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.

  char* fgets (char * __str, int __size, FILE * __stream)

Read at most size - 1 bytes from stream, until a newline character was encountered, and store the characters in the buffer pointed to by str. 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.

  int fprintf (FILE * __stream, const char * __fmt, ...)

The function fprintf performs formatted output to stream. See vfprintf() for details.

  int fprintf_P (FILE * __stream, const char * __fmt, ...)

Variant of fprintf() that uses a fmt string that resides in program memory.

  int fputc (int __c, FILE * __stream)

The function fputc sends the character c (though given as type int) to stream. It returns the character, or EOF in case an error occurred.

  int fputs (const char * __str, FILE * __stream)

Write the string pointed to by str to stream stream.

Returns 0 on success and EOF on error.

  int fputs_P (const char * __str, FILE * __stream)

Variant of fputs() where str resides in program memory.

  size_t fread (void * __ptr, size_t __size, size_t __nmemb, FILE * __stream)

Read nmemb objects, size bytes each, from stream, to the buffer pointed to by ptr.

Returns the number of objects successfully read, i. e. nmemb unless an input error occured or end-of-file was encountered. feof() and ferror() must be used to distinguish between these two conditions.

  int fscanf (FILE * __stream, const char * __fmt, ...)

The function fscanf performs formatted input, reading the input data from stream.

See vfscanf() for details.

  int fscanf_P (FILE * __stream, const char * __fmt, ...)

Variant of fscanf() using a fmt string in program memory.

  size_t fwrite (const void * __ptr, size_t __size, size_t __nmemb, FILE * __stream)

Write nmemb objects, size bytes each, to stream. The first byte of the first object is referenced by ptr.

Returns the number of objects successfully written, i. e. nmemb unless an output error occured.

  char* gets (char * __str)

Similar to fgets() except that it will operate on stream 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.

  int printf (const char * __fmt, ...)

The function printf performs formatted output to stream stderr. See vfprintf() for details.

  int printf_P (const char * __fmt, ...)

Variant of printf() that uses a fmt string that resides in program memory.

  int puts (const char * __str)

Write the string pointed to by str, and a trailing newline character, to stdout.

  int puts_P (const char * __str)

Variant of puts() where str resides in program memory.

  int scanf (const char * __fmt, ...)

The function scanf performs formatted input from stream stdin.

See vfscanf() for details.

  int scanf_P (const char * __fmt, ...)

Variant of scanf() where fmt resides in program memory.

  int snprintf (char * __s, size_t __n, const char * __fmt, ...)

Like 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.

  int snprintf_P (char * __s, size_t __n, const char * __fmt, ...)

Variant of snprintf() that uses a fmt string that resides in program memory.

  int sprintf (char * __s, const char * __fmt, ...)

Variant of printf() that sends the formatted characters to string s.

  int sprintf_P (char * __s, const char * __fmt, ...)

Variant of sprintf() that uses a fmt string that resides in program memory.

  int sscanf (const char * __buf, const char * __fmt, ...)

The function sscanf performs formatted input, reading the input data from the buffer pointed to by buf.

See vfscanf() for details.

  int sscanf_P (const char * __buf, const char * __fmt, ...)

Variant of sscanf() using a fmt string in program memory.

  int ungetc (int __c, FILE * __stream)

The 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.

  int vfprintf (FILE * __stream, const char * __fmt, va_list __ap)

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:

•   Zero or more of the following flags:
•   # 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.
•   An optional decimal digit string specifying a minimum field width. If the converted value has fewer characters than the field width, it will be padded with spaces on the left (or right, if the left-adjust­ ment flag has been given) to fill out the field width.
•   An optional precision, in the form of a period . followed by an optional digit string. If the digit string is omitted, the precision is taken as zero. This gives the minimum number of digits to appear for d, i, o, u, x, and X conversions, or the maximum number of characters to be printed from a string for s conversions.
•   An optional l length modifier, that specifies that the argument for the d, i, o, u, x, or X conversion is a 'long int' rather than int.
•   A character that specifies the type of conversion to be applied.
The conversion specifiers and their meanings are:
•   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.dddeħdd' 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.
In no case does a non-existent or small field width cause truncation of a numeric field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.
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 none of the additional options that 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_min
If the full functionality including the floating point conversions is required, the following options should be used:
   -Wl,-u,vfprintf -lprintf_flt -lm
Limitations:.RS 4
•   The specified width and precision can be at most 127.
•   For floating-point conversions, trailing digits will be lost if a number close to DBL_MAX is converted with a precision > 0.

  int vfprintf_P (FILE * __stream, const char * __fmt, va_list __ap)
Variant of vfprintf() that uses a fmt string that resides in program memory.

  int vfscanf (FILE * __stream, const char * __fmt, va_list __ap)
Formatted input. This function is the heart of the scanf family of functions.
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 :
•   a * indicating that the conversion should be performed but the conversion result is to be discarded; no parameters will be processed from ap,
•   the character h indicating that the argument is a pointer to short int (rather than int),
•   the character 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).
In addition, a maximal field width may be specified as a nonzero positive decimal integer, which will restrict the conversion to at most this many characters from the input stream. This field width is limited to at most 127 characters which is also the default value (except for the 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, 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.
•   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.
These functions return the number of input items assigned, which can be fewer than provided for, or even zero, in the event of a matching failure. Zero indicates that, while there was input available, no conversions were assigned; typically this is due to an invalid input character, such as an alphabetic character for a 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.
By default, all the conversions described above are available except the floating-point conversions, and the [ conversion. These conversions will be available in the extended version provided by the library libscanf_flt.a. Note that either of these conversions requires the availability of a buffer that needs to be obtained at run-time using malloc(). If this buffer cannot be obtained, the operation is aborted, returning the value EOF. To link a program against the extended version, use the following compiler flags in the link stage:
     -Wl,-u,vfscanf -lscanf_flt -lm
A third version is available for environments that are tight on space. 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 -lm
In addition to the restrictions of the standard version, this version implements no field width specification, no conversion assignment suppression flag (*), no n specification, and no general format character matching at all. All characters in fmt that do not comprise a conversion specification will simply be ignored, including white space (that is normally used to consume any amount of white space in the input stream). However, the usual skip of initial white space in the formats that support it is implemented.

  int vfscanf_P (FILE * __stream, const char * __fmt, va_list __ap)
Variant of vfscanf() using a fmt string in program memory.

  int vsnprintf (char * __s, size_t __n, const char * __fmt, va_list ap)
Like 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.

  int vsnprintf_P (char * __s, size_t __n, const char * __fmt, va_list ap)
Variant of vsnprintf() that uses a fmt string that resides in program memory.

  int vsprintf (char * __s, const char * __fmt, va_list ap)
Like sprintf() but takes a variable argument list for the arguments.

  int vsprintf_P (char * __s, const char * __fmt, va_list ap)
Variant of vsprintf() that uses a fmt string that resides in program memory.