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NAME
fscanf, scanf, sscanf — convert formatted input
SYNOPSIS
#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, ...);
DESCRIPTION
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()) 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().
RETURN VALUE
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.
ERRORS
For the conditions under which the fscanf() functions fail and may fail, refer to fgetc()
or fgetwc().
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.
EXAMPLES
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);
APPLICATION USAGE
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.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.5, Standard I/O Streams, fprintf(), getc(), setlocale(), strtod(), strtol(),
strtoul(), wcrtomb()
The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale, <inttypes.h>,
<langinfo.h>, <stdio.h>, <wchar.h>
COPYRIGHT
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 .