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FSCANF(3P) POSIX Programmer's Manual FSCANF(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.
fscanf, scanf, sscanf — convert formatted input
#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 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.1‐2008 defers to the ISO C standard.
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 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 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>, <tab>, <newline>, <vertical-tab>, or
<form-feed>); 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 optional assignment-allocation character 'm'.
* 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(3p)) 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 %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 string converted including a
terminating null character. In such a case, the argument
corresponding to the conversion specifier should be a reference to a
pointer variable 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 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. If the 'm' assignment-allocation character is not
specified, 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. Otherwise, the
application shall ensure that the corresponding argument is a
pointer to a pointer to a char.
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. If 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. Otherwise, the application shall ensure that
the corresponding argument is a pointer to a pointer to a
wchar_t.
[ 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. If the 'm'
assignment-allocation character is not specified, 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.
Otherwise, the application shall ensure that the
corresponding argument is a pointer to a pointer to a char.
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. If 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.
Otherwise, the application shall ensure that the
corresponding argument is a pointer to a pointer to a
wchar_t.
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). No null byte is added. The normal skip over
white-space characters shall be suppressed in this case. If
the 'm' assignment-allocation character is not specified, 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.
Otherwise, the application shall ensure that the
corresponding argument is a pointer to a pointer to a char.
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. No null
wide character is added. If 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 resulting sequence of wide
characters. Otherwise, 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
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> characters) 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 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 fgetc(), fgets(), fread(), getc(), getchar(),
getdelim(), getline(), gets(), fscanf(), or scanf() 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 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 fscanf() functions fail and may
fail, refer to fgetc(3p) or fgetwc(3p).
In addition, the fscanf() function shall fail if:
EILSEQ Input byte sequence does not form a valid character.
ENOMEM Insufficient storage space is available.
In addition, the fscanf() 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 = scanf("%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) scanf("%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'.
Reading Data into an Array
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.
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 fscanf(), this is memory allocated via use of the 'm'
assignment-allocation character.
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.
Section 2.5, Standard I/O Streams, fprintf(3p), getc(3p),
setlocale(3p), strtod(3p), strtol(3p), strtoul(3p), wcrtomb(3p)
The Base Definitions volume of POSIX.1‐2008, Chapter 7, Locale,
langinfo.h(0p), stdio.h(0p), wchar.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 FSCANF(3P)
Pages that refer to this page: stdio.h(0p), fgetc(3p), fgets(3p), fprintf(3p), fread(3p), localeconv(3p), scanf(3p), setlocale(3p), sscanf(3p), stdin(3p), strptime(3p), strtod(3p), strtol(3p), strtoul(3p), vfscanf(3p), wcstod(3p), wcstol(3p), wcstoul(3p)