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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017 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.1‐2017, Chapter 7, Locale,
inttypes.h(0p), langinfo.h(0p), stdio.h(0p), wchar.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, 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 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)
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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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