Originální popis anglicky:
fscanf, scanf, sscanf - convert formatted input
Návod, kniha: POSIX Programmer's Manual
#include <stdio.h>
int fscanf(FILE *restrict
stream, const char
*restrict format, ... );
int scanf(const char *restrict
format, ... );
int sscanf(const char *restrict
s, const char
*restrict format, ... );
The
fscanf() function shall read from the named input
stream. The
scanf() function shall read from the standard input stream
stdin. The
sscanf() function shall read from the string
s. Each function reads bytes, interprets them according to a format,
and stores the results in its arguments. Each expects, as arguments, a control
string
format described below, and a set of
pointer arguments
indicating where the converted input should be stored. The result is undefined
if there are insufficient arguments for the format. If the format is exhausted
while arguments remain, the excess arguments shall be evaluated but otherwise
ignored.
Conversions can be applied to the
nth argument after the
format in
the argument list, rather than to the next unused argument. In this case, the
conversion specifier character
% (see below) is replaced by the
sequence
"%n$", where
n is a decimal
integer in the range [1,{NL_ARGMAX}]. This feature provides for the definition
of format strings that select arguments in an order appropriate to specific
languages. In format strings containing the
"%n$" form of conversion specifications, it is
unspecified whether numbered arguments in the argument list can be referenced
from the format string more than once.
The
format can contain either form of a conversion specification-that is,
% or
"%n$"-but the two forms cannot be
mixed within a single
format string. The only exception to this is that
%% or
%* can be mixed with the
"%n$" form. When numbered argument
specifications are used, specifying the
Nth argument requires that all
the leading arguments, from the first to the (
N-1)th, are pointers.
The
fscanf() function in all its forms shall allow detection of a
language-dependent radix character in the input string. The radix character is
defined in the program's locale (category
LC_NUMERIC ). In the
POSIX locale, or in a locale where the radix character is not defined, the
radix character shall default to a period (
'.' ).
The format is a character string, beginning and ending in its initial shift
state, if any, composed of zero or more directives. Each directive is composed
of one of the following: one or more white-space characters ( <space>s,
<tab>s, <newline>s, <vertical-tab>s, or <form-feed>s);
an ordinary character (neither
'%' nor a white-space character); or a
conversion specification. Each conversion specification is introduced by the
character
'%' or the character sequence
"%n$", after which the following appear
in sequence:
- *
- An optional assignment-suppressing character '*'
.
- *
- An optional non-zero decimal integer that specifies the
maximum field width.
- *
- An option length modifier that specifies the size of the
receiving object.
- *
- A conversion specifier character that specifies the
type of conversion to be applied. The valid conversion specifiers are
described below.
The
fscanf() functions shall execute each directive of the format in
turn. If a directive fails, as detailed below, the function shall return.
Failures are described as input failures (due to the unavailability of input
bytes) or matching failures (due to inappropriate input).
A directive composed of one or more white-space characters shall be executed by
reading input until no more valid input can be read, or up to the first byte
which is not a white-space character, which remains unread.
A directive that is an ordinary character shall be executed as follows: the next
byte shall be read from the input and compared with the byte that comprises
the directive; if the comparison shows that they are not equivalent, the
directive shall fail, and the differing and subsequent bytes shall remain
unread. Similarly, if end-of-file, an encoding error, or a read error prevents
a character from being read, the directive shall fail.
A directive that is a conversion specification defines a set of matching input
sequences, as described below for each conversion character. A conversion
specification shall be executed in the following steps.
Input white-space characters (as specified by
isspace() ) shall be
skipped, unless the conversion specification includes a
[ ,
c ,
C , or
n conversion specifier.
An item shall be read from the input, unless the conversion specification
includes an
n conversion specifier. An input item shall be defined as
the longest sequence of input bytes (up to any specified maximum field width,
which may be measured in characters or bytes dependent on the conversion
specifier) which is an initial subsequence of a matching sequence. The first
byte, if any, after the input item shall remain unread. If the length of the
input item is 0, the execution of the conversion specification shall fail;
this condition is a matching failure, unless end-of-file, an encoding error,
or a read error prevented input from the stream, in which case it is an input
failure.
Except in the case of a
% conversion specifier, the input item (or, in
the case of a
%n conversion specification, the count of input bytes)
shall be converted to a type appropriate to the conversion character. If the
input item is not a matching sequence, the execution of the conversion
specification fails; this condition is a matching failure. Unless assignment
suppression was indicated by a
'*' , the result of the conversion shall
be placed in the object pointed to by the first argument following the
format argument that has not already received a conversion result if
the conversion specification is introduced by
% , or in the
nth argument if introduced by the character sequence
"%n $". If this object does not have an
appropriate type, or if the result of the conversion cannot be represented in
the space provided, the behavior is undefined.
The length modifiers and their meanings are:
- hh
- Specifies that a following d , i , o ,
u , x , X , or n conversion specifier applies
to an argument with type pointer to signed char or
unsigned char.
- h
- Specifies that a following d , i , o ,
u , x , X , or n conversion specifier applies
to an argument with type pointer to short or unsigned
short.
- l (ell)
- Specifies that a following d , i , o ,
u , x , X , or n conversion specifier applies
to an argument with type pointer to long or unsigned long;
that a following a , A , e , E , f ,
F , g , or G conversion specifier applies to an
argument with type pointer to double; or that a following c
, s , or [ conversion specifier applies to an argument with
type pointer to wchar_t.
- ll (ell-ell)
-
Specifies that a following d , i , o , u ,
x , X , or n conversion specifier applies to an
argument with type pointer to long long or unsigned long
long.
- j
- Specifies that a following d , i , o ,
u , x , X , or n conversion specifier applies
to an argument with type pointer to intmax_t or
uintmax_t.
- z
- Specifies that a following d , i , o ,
u , x , X , or n conversion specifier applies
to an argument with type pointer to size_t or the corresponding
signed integer type.
- t
- Specifies that a following d , i , o ,
u , x , X , or n conversion specifier applies
to an argument with type pointer to ptrdiff_t or the corresponding
unsigned type.
- L
- Specifies that a following a , A , e ,
E , f , F , g , or G conversion
specifier applies to an argument with type pointer to long double.
If a length modifier appears with any conversion specifier other than as
specified above, the behavior is undefined.
The following conversion specifiers are valid:
- d
- Matches an optionally signed decimal integer, whose format
is the same as expected for the subject sequence of strtol() with
the value 10 for the base argument. In the absence of a size
modifier, the application shall ensure that the corresponding argument is
a pointer to int.
- i
- Matches an optionally signed integer, whose format is the
same as expected for the subject sequence of strtol() with 0 for
the base argument. In the absence of a size modifier, the
application shall ensure that the corresponding argument is a pointer to
int.
- o
- Matches an optionally signed octal integer, whose format is
the same as expected for the subject sequence of strtoul() with the
value 8 for the base argument. In the absence of a size modifier,
the application shall ensure that the corresponding argument is a pointer
to unsigned.
- u
- Matches an optionally signed decimal integer, whose format
is the same as expected for the subject sequence of strtoul() with
the value 10 for the base argument. In the absence of a size
modifier, the application shall ensure that the corresponding argument is
a pointer to unsigned.
- x
- Matches an optionally signed hexadecimal integer, whose
format is the same as expected for the subject sequence of
strtoul() with the value 16 for the base argument. In the
absence of a size modifier, the application shall ensure that the
corresponding argument is a pointer to unsigned.
- a, e, f, g
-
Matches an optionally signed floating-point number, infinity, or NaN, whose
format is the same as expected for the subject sequence of
strtod(). In the absence of a size modifier, the application shall
ensure that the corresponding argument is a pointer to float.
If the
fprintf() family of functions generates character string
representations for infinity and NaN (a symbolic entity encoded in
floating-point format) to support IEEE Std 754-1985, the
fscanf() family of functions shall recognize them as input.
- s
- Matches a sequence of bytes that are not white-space
characters. The application shall ensure that the corresponding argument
is a pointer to the initial byte of an array of char, signed
char, or unsigned char large enough to accept the sequence
and a terminating null character code, which shall be added
automatically.
If an
l (ell) qualifier is present, the input is a sequence of characters
that begins in the initial shift state. Each character shall be converted to a
wide character as if by a call to the
mbrtowc() function, with the
conversion state described by an
mbstate_t object initialized to zero
before the first character is converted. The application shall ensure that the
corresponding argument is a pointer to an array of
wchar_t large enough
to accept the sequence and the terminating null wide character, which shall be
added automatically.
- [
- Matches a non-empty sequence of bytes from a set of
expected bytes (the scanset). The normal skip over white-space
characters shall be suppressed in this case. The application shall ensure
that the corresponding argument is a pointer to the initial byte of an
array of char, signed char, or unsigned char
large enough to accept the sequence and a terminating null byte, which
shall be added automatically.
If an
l (ell) qualifier is present, the input is a sequence of characters
that begins in the initial shift state. Each character in the sequence shall
be converted to a wide character as if by a call to the
mbrtowc()
function, with the conversion state described by an
mbstate_t object
initialized to zero before the first character is converted. The application
shall ensure that the corresponding argument is a pointer to an array of
wchar_t large enough to accept the sequence and the terminating null
wide character, which shall be added automatically.
The conversion specification includes all subsequent bytes in the
format
string up to and including the matching right square bracket (
']' ).
The bytes between the square brackets (the
scanlist) comprise the
scanset, unless the byte after the left square bracket is a circumflex (
'^' ), in which case the scanset contains all bytes that do not appear
in the scanlist between the circumflex and the right square bracket. If the
conversion specification begins with
"[]" or
"[^]" , the right square bracket is included in the scanlist
and the next right square bracket is the matching right square bracket that
ends the conversion specification; otherwise, the first right square bracket
is the one that ends the conversion specification. If a
'-' is in the
scanlist and is not the first character, nor the second where the first
character is a
'^' , nor the last character, the behavior is
implementation-defined.
- c
- Matches a sequence of bytes of the number specified by the
field width (1 if no field width is present in the conversion
specification). The application shall ensure that the corresponding
argument is a pointer to the initial byte of an array of char,
signed char, or unsigned char large enough to accept the
sequence. No null byte is added. The normal skip over white-space
characters shall be suppressed in this case.
If an
l (ell) qualifier is present, the input shall be a sequence of
characters that begins in the initial shift state. Each character in the
sequence is converted to a wide character as if by a call to the
mbrtowc() function, with the conversion state described by an
mbstate_t object initialized to zero before the first character is
converted. The application shall ensure that the corresponding argument is a
pointer to an array of
wchar_t large enough to accept the resulting
sequence of wide characters. No null wide character is added.
- p
- Matches an implementation-defined set of sequences, which
shall be the same as the set of sequences that is produced by the
%p conversion specification of the corresponding fprintf()
functions. The application shall ensure that the corresponding argument is
a pointer to a pointer to void. The interpretation of the input
item is implementation-defined. If the input item is a value converted
earlier during the same program execution, the pointer that results shall
compare equal to that value; otherwise, the behavior of the %p
conversion specification is undefined.
- n
- No input is consumed. The application shall ensure that the
corresponding argument is a pointer to the integer into which shall be
written the number of bytes read from the input so far by this call to the
fscanf() functions. Execution of a %n conversion
specification shall not increment the assignment count returned at the
completion of execution of the function. No argument shall be converted,
but one shall be consumed. If the conversion specification includes an
assignment-suppressing character or a field width, the behavior is
undefined.
- C
- Equivalent to lc .
- S
- Equivalent to ls .
- %
- Matches a single '%' character; no conversion or
assignment occurs. The complete conversion specification shall be
%% .
If a conversion specification is invalid, the behavior is undefined.
The conversion specifiers
A ,
E ,
F ,
G , and
X are also valid and shall be equivalent to
a ,
e ,
f ,
g , and
x , respectively.
If end-of-file is encountered during input, conversion shall be terminated. If
end-of-file occurs before any bytes matching the current conversion
specification (except for
%n ) have been read (other than leading
white-space characters, where permitted), execution of the current conversion
specification shall terminate with an input failure. Otherwise, unless
execution of the current conversion specification is terminated with a
matching failure, execution of the following conversion specification (if any)
shall be terminated with an input failure.
Reaching the end of the string in
sscanf() shall be equivalent to
encountering end-of-file for
fscanf().
If conversion terminates on a conflicting input, the offending input is left
unread in the input. Any trailing white space (including <newline>s)
shall be left unread unless matched by a conversion specification. The success
of literal matches and suppressed assignments is only directly determinable
via the
%n conversion specification.
The
fscanf() and
scanf() functions may mark the
st_atime
field of the file associated with
stream for update. The
st_atime field shall be marked for update by the first successful
execution of
fgetc(),
fgets(),
fread(),
getc(),
getchar(),
gets(),
fscanf(), or
fscanf() using
stream that returns data not supplied by a prior call to
ungetc().
Upon successful completion, these functions shall return the number of
successfully matched and assigned input items; this number can be zero in the
event of an early matching failure. If the input ends before the first
matching failure or conversion, EOF shall be returned. If a read error occurs,
the error indicator for the stream is set, EOF shall be returned, and
errno shall be set to indicate the error.
For the conditions under which the
fscanf() functions fail and may fail,
refer to
fgetc() or
fgetwc() .
In addition,
fscanf() may fail if:
- EILSEQ
- Input byte sequence does not form a valid character.
- EINVAL
- There are insufficient arguments.
The following sections are informative.
The call:
int i, n; float x; char name[50];
n = scanf("%d%f%s", &i, &x, name);
with the input line:
assigns to
n the value 3, to
i the value 25, to
x the value
5.432, and
name contains the string
"Hamster" .
The call:
int i; float x; char name[50];
(void) scanf("%2d%f%*d %[0123456789]", &i, &x, name);
with input:
assigns 56 to
i, 789.0 to
x, skips 0123, and places the string
"56\0" in
name. The next call to
getchar()
shall return the character
'a' .
The following call uses
fscanf() to read three floating-point numbers
from standard input into the
input array.
float input[3]; fscanf (stdin, "%f %f %f", input, input+1, input+2);
If the application calling
fscanf() has any objects of type
wint_t
or
wchar_t, it must also include the
<wchar.h> header to
have these objects defined.
This function is aligned with the ISO/IEC 9899:1999 standard, and in
doing so a few "obvious" things were not included. Specifically, the
set of characters allowed in a scanset is limited to single-byte characters.
In other similar places, multi-byte characters have been permitted, but for
alignment with the ISO/IEC 9899:1999 standard, it has not been done
here. Applications needing this could use the corresponding wide-character
functions to achieve the desired results.
None.
getc() ,
printf() ,
setlocale() ,
strtod() ,
strtol() ,
strtoul() ,
wcrtomb() , the Base Definitions
volume of IEEE Std 1003.1-2001, Chapter 7, Locale,
<langinfo.h>,
<stdio.h>,
<wchar.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
