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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FWSCANF(3P) POSIX Programmer's Manual FWSCANF(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
fwscanf, swscanf, wscanf — convert formatted wide-character input
#include <stdio.h>
#include <wchar.h>
int fwscanf(FILE *restrict stream, const wchar_t *restrict format, ...);
int swscanf(const wchar_t *restrict ws,
const wchar_t *restrict format, ...);
int wscanf(const wchar_t *restrict format, ...);
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
The fwscanf() function shall read from the named input stream.
The wscanf() function shall read from the standard input stream
stdin. The swscanf() function shall read from the wide-character
string ws. Each function reads wide characters, interprets them
according to a format, and stores the results in its arguments.
Each expects, as arguments, a control wide-character 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 are
evaluated but are 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 wide character % (see below)
is replaced by the sequence "%n$", where n is a decimal integer in
the range [1,{NL_ARGMAX}]. This feature provides for the
definition of format wide-character strings that select arguments
in an order appropriate to specific languages. In format wide-
character strings containing the "%n$" form of conversion
specifications, it is unspecified whether numbered arguments in
the argument list can be referenced from the format wide-character
string more than once.
The format can contain either form of a conversion specification—
that is, % or "%n$"— but the two forms cannot normally be mixed
within a single format wide-character string. The only exception
to this is that %% or %* can be mixed with the "%n$" 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 fwscanf() function in all its forms allows for detection of a
language-dependent radix character in the input string, encoded as
a wide-character value. The radix character is defined in the
current 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 wide-character string composed of zero or more
directives. Each directive is composed of one of the following:
one or more white-space wide characters (<space>, <tab>,
<newline>, <vertical-tab>, or <form-feed>); an ordinary wide
character (neither '%' nor a white-space character); or a
conversion specification. It is unspecified whether an encoding
error occurs if the format string contains wchar_t values that do
not correspond to members of the character set of the current
locale and the specified semantics do not require that value to be
processed by wcrtomb().
Each conversion specification is introduced by the '%' or by 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 optional assignment-allocation character 'm'.
* An optional length modifier that specifies the size of the
receiving object.
* A conversion specifier wide character that specifies the type
of conversion to be applied. The valid conversion specifiers
are described below.
The fwscanf() 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 wide characters is
executed by reading input until no more valid input can be read,
or up to the first wide character which is not a white-space wide
character, which remains unread.
A directive that is an ordinary wide character shall be executed
as follows. The next wide character is read from the input and
compared with the wide character that comprises the directive; if
the comparison shows that they are not equivalent, the directive
shall fail, and the differing and subsequent wide characters
remain unread. Similarly, if end-of-file, an encoding error, or a
read error prevents a wide 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
wide character. A conversion specification is executed in the
following steps.
Input white-space wide characters (as specified by iswspace(3p))
shall be skipped, unless the conversion specification includes a
[, c, or n conversion specifier.
An item shall be read from the input, unless the conversion
specification includes an n conversion specifier wide character.
An input item is defined as the longest sequence of input wide
characters, not exceeding any specified field width, which is an
initial subsequence of a matching sequence. The first wide
character, if any, after the input item shall remain unread. If
the length of the input item is zero, 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 wide characters) shall be converted to a type appropriate to
the conversion wide character. If the input item is not a matching
sequence, the execution of the conversion specification shall
fail; 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 wide-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 %c, %s, and %[ conversion specifiers shall accept an optional
assignment-allocation character 'm', which shall cause a memory
buffer to be allocated to hold the wide-character string converted
including a terminating null wide character. In such a case, the
argument corresponding to the conversion specifier should be a
reference to a pointer value that will receive a pointer to the
allocated buffer. The system shall allocate a buffer as if
malloc() had been called. The application shall be responsible for
freeing the memory after usage. If there is insufficient memory to
allocate a buffer, the function shall set errno to [ENOMEM] and a
conversion error shall result. If the function returns EOF, any
memory successfully allocated for parameters using assignment-
allocation character 'm' by this call shall be freed before the
function returns.
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. If the 'm' assignment-
allocation character is specified, the conversion applies
to an argument with the type pointer to a 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 specifier wide characters are valid:
d Matches an optionally signed decimal integer, whose format
is the same as expected for the subject sequence of
wcstol() 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 wcstol() 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
wcstoul() 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
wcstoul() 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
wcstoul() 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 wcstod(). In the absence of a
size modifier, the application shall ensure that the
corresponding argument is a pointer to float.
If the fwprintf() 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 fwscanf() family of functions shall
recognize them as input.
s Matches a sequence of non-white-space wide characters. If
no l (ell) qualifier is present, characters from the input
field shall be converted as if by repeated calls to the
wcrtomb() function, with the conversion state described by
an mbstate_t object initialized to zero before the first
wide character is converted. If the 'm' assignment-
allocation character is not specified, the application
shall ensure that the corresponding argument is a pointer
to a character array large enough to accept the sequence
and the terminating null character, which shall be added
automatically. Otherwise, the application shall ensure
that the corresponding argument is a pointer to a pointer
to a wchar_t.
If the l (ell) qualifier is present and the 'm'
assignment-allocation character is not specified, 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. If the l (ell)
qualifier is present and the 'm' assignment-allocation
character is present, the application shall ensure that
the corresponding argument is a pointer to a pointer to a
wchar_t.
[ Matches a non-empty sequence of wide characters from a set
of expected wide characters (the scanset). If no l (ell)
qualifier is present, wide characters from the input field
shall be converted as if by repeated calls to the
wcrtomb() function, with the conversion state described by
an mbstate_t object initialized to zero before the first
wide character is converted. If the 'm' assignment-
allocation character is not specified, the application
shall ensure that the corresponding argument is a pointer
to a character array large enough to accept the sequence
and the terminating null character, which shall be added
automatically. Otherwise, the application shall ensure
that the corresponding argument is a pointer to a pointer
to a wchar_t.
If an l (ell) qualifier is present and the 'm' assignment-
allocation character is not specified, 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. If an l (ell)
qualifier is present and the 'm' assignment-allocation
character is specified, the application shall ensure that
the corresponding argument is a pointer to a pointer to a
wchar_t.
The conversion specification includes all subsequent wide
characters in the format string up to and including the
matching <right-square-bracket> (']'). The wide
characters between the square brackets (the scanlist)
comprise the scanset, unless the wide character after the
<left-square-bracket> is a <circumflex> ('^'), in which
case the scanset contains all wide characters 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 wide character, nor the second where the first wide
character is a '^', nor the last wide character, the
behavior is implementation-defined.
c Matches a sequence of wide characters of exactly the
number specified by the field width (1 if no field width
is present in the conversion specification).
If no l (ell) length modifier is present, characters from
the input field shall be converted as if by repeated calls
to the wcrtomb() function, with the conversion state
described by an mbstate_t object initialized to zero
before the first wide character is converted. No null
character is added. If the 'm' assignment-allocation
character is not specified, the application shall ensure
that the corresponding argument is a pointer to the
initial element of a character array large enough to
accept the sequence. Otherwise, the application shall
ensure that the corresponding argument is a pointer to a
pointer to a char.
No null wide character is added. If an l (ell) length
modifier is present and the 'm' assignment-allocation
character is not specified, the application shall ensure
that the corresponding argument shall be a pointer to the
initial element of an array of wchar_t large enough to
accept the sequence. If an l (ell) qualifier is present
and the 'm' assignment-allocation character is specified,
the application shall ensure that the corresponding
argument is a pointer to a pointer to a wchar_t.
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
fwprintf() 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 is undefined.
n No input is consumed. The application shall ensure that
the corresponding argument is a pointer to the integer
into which is to be written the number of wide characters
read from the input so far by this call to the fwscanf()
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 wide
character or a field width, the behavior is undefined.
C Equivalent to lc.
S Equivalent to ls.
% Matches a single '%' wide character; no conversion or
assignment shall occur. 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, respectively, a, e, f, g, and x.
If end-of-file is encountered during input, conversion is
terminated. If end-of-file occurs before any wide characters
matching the current conversion specification (except for %n) have
been read (other than leading white-space, 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 swscanf() shall be equivalent to
encountering end-of-file for fwscanf().
If conversion terminates on a conflicting input, the offending
input shall be left unread in the input. Any trailing white space
(including <newline>) 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 fwscanf() and wscanf() functions may mark the last data access
timestamp of the file associated with stream for update. The last
data access timestamp shall be marked for update by the first
successful execution of fgetwc(), fgetws(), fwscanf(), getwc(),
getwchar(), vfwscanf(), vwscanf(), or wscanf() using stream that
returns data not supplied by a prior call to ungetwc().
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 conversion (if any) has completed,
and without a matching failure having occurred, EOF shall be
returned. If an error occurs before the first conversion (if any)
has completed, and without a matching failure having occurred, EOF
shall be returned and errno shall be set to indicate the error.
If a read error occurs, the error indicator for the stream shall
be set.
For the conditions under which the fwscanf() functions shall fail
and may fail, refer to fgetwc(3p).
In addition, the fwscanf() function shall fail if:
EILSEQ Input byte sequence does not form a valid character.
ENOMEM Insufficient storage space is available.
In addition, the fwscanf() function may fail if:
EINVAL There are insufficient arguments.
The following sections are informative.
The call:
int i, n; float x; char name[50];
n = wscanf(L"%d%f%s", &i, &x, name);
with the input line:
25 54.32E-1 Hamster
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) wscanf(L"%2d%f%*d %[0123456789]", &i, &x, name);
with input:
56789 0123 56a72
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'.
In format strings containing the '%' form of conversion
specifications, each argument in the argument list is used exactly
once.
For functions that allocate memory as if by malloc(), the
application should release such memory when it is no longer
required by a call to free(). For fwscanf(), this is memory
allocated via use of the 'm' assignment-allocation character.
None.
None.
Section 2.5, Standard I/O Streams, getwc(3p), fwprintf(3p),
setlocale(3p), wcstod(3p), wcstol(3p), wcstoul(3p), wcrtomb(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale,
inttypes.h(0p), stdio.h(0p), wchar.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 FWSCANF(3P)
Pages that refer to this page: wchar.h(0p), fwprintf(3p), swscanf(3p), vfwscanf(3p), wscanf(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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