'\" et
.TH FSCANF "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
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.
.\"
.SH NAME
fscanf,
scanf,
sscanf
\(em convert formatted input
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
int fscanf(FILE *restrict \fIstream\fP, const char *restrict \fIformat\fP, ...);
int scanf(const char *restrict \fIformat\fP, ...);
int sscanf(const char *restrict \fIs\fP, const char *restrict \fIformat\fP, ...);
.fi
.SH 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\(hy2017 defers to the ISO\ C standard.
.P
The
\fIfscanf\fR()
function shall read from the named input
.IR stream .
The
\fIscanf\fR()
function shall read from the standard input stream
.IR stdin .
The
\fIsscanf\fR()
function shall read from the string
.IR 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
.IR format
described below, and a set of
.IR 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.
.P
Conversions can be applied to the
.IR n th
argument after the
.IR format
in the argument list, rather than to the next unused argument. In this
case, the conversion specifier character
.BR %
(see below) is replaced by the sequence \fR"%\fIn\fR$"\fR, where
.IR n
is a decimal integer in the range [1,{NL_ARGMAX}].
This feature provides for the definition of format strings that select
arguments in an order appropriate to specific languages. In format
strings containing the \fR"%\fIn\fR$"\fR form of conversion
specifications,
it is unspecified whether numbered arguments in the argument list can
be referenced from the format string more than once.
.P
The
.IR format
can contain either form of a conversion specification\(emthat is,
.BR %
or \fR"%\fIn\fR$"\fR\(embut the two forms cannot be mixed
within a single
.IR format
string. The only exception to this is that
.BR %%
or
.BR %*
can be mixed with the \fR"%\fIn\fR$"\fR form. When numbered argument
specifications are used, specifying the
.IR N th
argument requires that all the leading arguments, from the first to
the (\c
.IR N \-1)th,
are pointers.
.P
The
\fIfscanf\fR()
function in all its forms shall allow detection of a language-dependent
radix character in the input string. The radix character is defined in
the current locale (category
.IR LC_NUMERIC ).
In the POSIX locale, or in a locale where the radix character is not
defined, the radix character shall default to a
<period>
(\c
.BR '.' ).
.P
The format is a character string, beginning and ending in its initial
shift state, if any, composed of zero or more directives. Each
directive is composed of one of the following:
one or more white-space characters (\c
<space>,
<tab>,
<newline>,
<vertical-tab>,
or
<form-feed>);
an ordinary character (neither
.BR '%' 
nor a white-space character); or a conversion specification. Each
conversion specification is introduced by the character
.BR '%' 
or the character sequence \fR"%\fIn\fR$"\fR,
after which the following appear in sequence:
.IP " *" 4
An optional assignment-suppressing character
.BR '*' .
.IP " *" 4
An optional non-zero decimal integer that specifies the maximum field
width.
.IP " *" 4
An optional assignment-allocation character
.BR 'm' .
.IP " *" 4
An option length modifier that specifies the size of the receiving
object.
.IP " *" 4
A
.IR "conversion specifier"
character that specifies the type of conversion to be applied. The
valid conversion specifiers are described below.
.P
The
\fIfscanf\fR()
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).
.P
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.
.P
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.
.P
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.
.P
Input white-space characters (as specified by
.IR "\fIisspace\fR\^(\|)")
shall be skipped, unless the conversion specification includes a
.BR [ ,
.BR c ,
.BR C ,
or
.BR n
conversion specifier.
.P
An item shall be read from the input, unless the conversion
specification includes an
.BR 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.
.P
Except in the case of a
.BR %
conversion specifier, the input item (or, in the case of a
.BR %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
.BR '*' ,
the result of the conversion shall be placed in the object pointed to
by the first argument following the
.IR format
argument that has not already received a conversion result if the
conversion specification is introduced by
.BR % ,
or in the
.IR n th
argument if introduced by the character sequence \fR"%\fIn\fR$"\fR.
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.
.P
The
.BR %c ,
.BR %s ,
and
.BR %[
conversion specifiers shall accept an optional assignment-allocation
character
.BR '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
\fImalloc\fR()
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
.IR errno
to
.BR [ENOMEM] 
and a conversion error shall result. If the function returns EOF, any
memory successfully allocated for parameters using assignment-allocation
character
.BR 'm' 
by this call shall be freed before the function returns.
.br
.P
The length modifiers and their meanings are:
.IP "\fRhh\fR" 8
Specifies that a following
.BR d ,
.BR i ,
.BR o ,
.BR u ,
.BR x ,
.BR X ,
or
.BR n
conversion specifier applies to an argument with type pointer to
.BR "signed char"
or
.BR "unsigned char" .
.IP "\fRh\fR" 8
Specifies that a following
.BR d ,
.BR i ,
.BR o ,
.BR u ,
.BR x ,
.BR X ,
or
.BR n
conversion specifier applies to an argument with type pointer to
.BR "short"
or
.BR "unsigned short" .
.IP "\fRl\fR\ (ell)" 8
Specifies that a following
.BR d ,
.BR i ,
.BR o ,
.BR u ,
.BR x ,
.BR X ,
or
.BR n
conversion specifier applies to an argument with type pointer to
.BR "long"
or
.BR "unsigned long" ;
that a following
.BR a ,
.BR A ,
.BR e ,
.BR E ,
.BR f ,
.BR F ,
.BR g ,
or
.BR G
conversion specifier applies to an argument with type pointer to
.BR double ;
or that a following
.BR c ,
.BR s ,
or
.BR [
conversion specifier applies to an argument with type pointer to
.BR wchar_t .
If the
.BR 'm' 
assignment-allocation character is specified, the conversion applies
to an argument with the type pointer to a pointer to
.BR wchar_t .
.IP "\fRll\fR\ (ell-ell)" 8
.br
Specifies that a following
.BR d ,
.BR i ,
.BR o ,
.BR u ,
.BR x ,
.BR X ,
or
.BR n
conversion specifier applies to an argument with type pointer to
.BR "long long"
or
.BR "unsigned long long" .
.IP "\fRj\fR" 8
Specifies that a following
.BR d ,
.BR i ,
.BR o ,
.BR u ,
.BR x ,
.BR X ,
or
.BR n
conversion specifier applies to an argument with type pointer to
.BR intmax_t
or
.BR uintmax_t .
.IP "\fRz\fR" 8
Specifies that a following
.BR d ,
.BR i ,
.BR o ,
.BR u ,
.BR x ,
.BR X ,
or
.BR n
conversion specifier applies to an argument with type pointer to
.BR size_t
or the corresponding signed integer type.
.IP "\fRt\fR" 8
Specifies that a following
.BR d ,
.BR i ,
.BR o ,
.BR u ,
.BR x ,
.BR X ,
or
.BR n
conversion specifier applies to an argument with type pointer to
.BR ptrdiff_t
or the corresponding
.BR unsigned
type.
.IP "\fRL\fR" 8
Specifies that a following
.BR a ,
.BR A ,
.BR e ,
.BR E ,
.BR f ,
.BR F ,
.BR g ,
or
.BR G
conversion specifier applies to an argument with type pointer to
.BR "long double" .
.P
If a length modifier appears with any conversion specifier other than
as specified above, the behavior is undefined.
.P
The following conversion specifiers are valid:
.IP "\fRd\fP" 8
Matches an optionally signed decimal integer, whose format is the same
as expected for the subject sequence of
\fIstrtol\fR()
with the value 10 for the
.IR base
argument. In the absence of a size modifier, the application shall
ensure that the corresponding argument is a pointer to
.BR int .
.IP "\fRi\fP" 8
Matches an optionally signed integer, whose format is the same as
expected for the subject sequence of
\fIstrtol\fR()
with 0 for the
.IR base
argument. In the absence of a size modifier, the application shall
ensure that the corresponding argument is a pointer to
.BR int .
.IP "\fRo\fP" 8
Matches an optionally signed octal integer, whose format is the same as
expected for the subject sequence of
\fIstrtoul\fR()
with the value 8 for the
.IR base
argument. In the absence of a size modifier, the application shall
ensure that the corresponding argument is a pointer to
.BR unsigned .
.IP "\fRu\fP" 8
Matches an optionally signed decimal integer, whose format is the same
as expected for the subject sequence of
\fIstrtoul\fR()
with the value 10 for the
.IR base
argument. In the absence of a size modifier, the application shall
ensure that the corresponding argument is a pointer to
.BR unsigned .
.IP "\fRx\fP" 8
Matches an optionally signed hexadecimal integer, whose format is the
same as expected for the subject sequence of
\fIstrtoul\fR()
with the value 16 for the
.IR base
argument. In the absence of a size modifier, the application shall
ensure that the corresponding argument is a pointer to
.BR unsigned .
.IP "\fRa\fR,\ \fRe\fR,\ \fRf\fR,\ \fRg\fR" 8
.br
Matches an optionally signed floating-point number, infinity, or NaN,
whose format is the same as expected for the subject sequence of
\fIstrtod\fR().
In the absence of a size modifier, the application shall ensure that
the corresponding argument is a pointer to
.BR float .
.RS 8 
.P
If the
\fIfprintf\fR()
family of functions generates character string representations for
infinity and NaN (a symbolic entity encoded in floating-point
format) to support IEEE\ Std\ 754\(hy1985, the
\fIfscanf\fR()
family of functions shall recognize them as input.
.RE
.IP "\fRs\fP" 8
Matches a sequence of bytes that are not white-space characters. If the
.BR '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
.BR char ,
.BR "signed char" ,
or
.BR "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
.BR char .
.RS 8 
.P
If an
.BR 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
\fImbrtowc\fR()
function, with the conversion state described by an
.BR mbstate_t
object initialized to zero before the first character is converted.
If the
.BR 'm' 
assignment-allocation character is not specified, the application shall
ensure that the corresponding argument is a pointer to an array of
.BR 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
.BR wchar_t .
.RE
.IP "\fR[\fR" 8
Matches a non-empty sequence of bytes from a set of expected bytes (the
.IR scanset ).
The normal skip over white-space characters shall be suppressed in this
case. If the
.BR '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
.BR char ,
.BR "signed char" ,
or
.BR "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
.BR char .
.RS 8 
.P
If an
.BR 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
\fImbrtowc\fR()
function, with the conversion state described by an
.BR mbstate_t
object initialized to zero before the first character is converted.
If the
.BR 'm' 
assignment-allocation character is not specified, the application shall
ensure that the corresponding argument is a pointer to an array of
.BR wchar_t
large enough to accept the sequence and the terminating null wide
character, which shall be added automatically.
.br
Otherwise, the application shall ensure that the corresponding
argument is a pointer to a pointer to a
.BR wchar_t .
.P
The conversion specification includes all subsequent bytes in the
.IR format
string up to and including the matching
<right-square-bracket>
(\c
.BR ']' ).
The bytes between the square brackets (the
.IR scanlist )
comprise the scanset, unless the byte after the
<left-square-bracket>
is a
<circumflex>
(\c
.BR '\(ha' ),
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
.BR \(dq[\|]\(dq 
or
.BR \(dq[\(ha]\(dq ,
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
.BR '\-' 
is in the scanlist and is not the first character, nor the second where
the first character is a
.BR '\(ha' ,
nor the last character, the behavior is implementation-defined.
.RE
.IP "\fRc\fP" 8
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
.BR '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
.BR char ,
.BR "signed char" ,
or
.BR "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
.BR char .
.RS 8 
.P
If an
.BR 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
\fImbrtowc\fR()
function, with the conversion state described by an
.BR mbstate_t
object initialized to zero before the first character is converted.
No null wide character is added. If the
.BR 'm' 
assignment-allocation character is not specified, the application shall
ensure that the corresponding argument is a pointer to an array of
.BR 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
.BR wchar_t .
.RE
.IP "\fRp\fP" 8
Matches an implementation-defined set of sequences, which shall be the
same as the set of sequences that is produced by the
.BR %p
conversion specification of the corresponding
\fIfprintf\fR()
functions. The application shall ensure that the corresponding argument
is a pointer to a pointer to
.BR 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
.BR %p
conversion specification is undefined.
.IP "\fRn\fP" 8
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
\fIfscanf\fR()
functions. Execution of a
.BR %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.
.IP "\fRC\fP" 8
Equivalent to
.BR lc .
.IP "\fRS\fP" 8
Equivalent to
.BR ls .
.IP "\fR%\fR" 8
Matches a single
.BR '%' 
character; no conversion or assignment occurs. The complete conversion
specification shall be
.BR %% .
.P
If a conversion specification is invalid, the behavior is undefined.
.P
The conversion specifiers
.BR A ,
.BR E ,
.BR F ,
.BR G ,
and
.BR X
are also valid and shall be equivalent to
.BR a ,
.BR e ,
.BR f ,
.BR g ,
and
.BR x ,
respectively.
.P
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
.BR %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.
.P
Reaching the end of the string in
\fIsscanf\fR()
shall be equivalent to encountering end-of-file for
\fIfscanf\fR().
.P
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
.BR %n
conversion specification.
.P
The
\fIfscanf\fR()
and
\fIscanf\fR()
functions may mark the last data access timestamp of the file
associated with
.IR stream
for update. The last data access timestamp shall be
marked for update by the first successful execution of
\fIfgetc\fR(),
\fIfgets\fR(),
\fIfread\fR(),
\fIgetc\fR(),
\fIgetchar\fR(),
\fIgetdelim\fR(),
\fIgetline\fR(),
\fIgets\fR(),
\fIfscanf\fR(),
or
\fIscanf\fR()
using
.IR stream
that returns data not supplied by a prior call to
\fIungetc\fR().
.SH "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
.IR errno
shall be set to indicate the error.
If a read error occurs, the error indicator for the stream shall be set.
.SH ERRORS
For the conditions under which the
\fIfscanf\fR()
functions fail and may fail, refer to
.IR "\fIfgetc\fR\^(\|)"
or
.IR "\fIfgetwc\fR\^(\|)".
.P
In addition, the
\fIfscanf\fR()
function shall fail if:
.TP
.BR EILSEQ
Input byte sequence does not form a valid character.
.TP
.BR ENOMEM
Insufficient storage space is available.
.P
In addition, the
\fIfscanf\fR()
function may fail if:
.TP
.BR EINVAL
There are insufficient arguments.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
The call:
.sp
.RS 4
.nf

int i, n; float x; char name[50];
n = scanf("%d%f%s", &i, &x, name);
.fi
.P
.RE
.P
with the input line:
.sp
.RS 4
.nf

25 54.32E-1 Hamster
.fi
.P
.RE
.P
assigns to
.IR n
the value 3, to
.IR i
the value 25, to
.IR x
the value 5.432, and
.IR name
contains the string
.BR \(dqHamster\(dq .
.P
The call:
.sp
.RS 4
.nf

int i; float x; char name[50];
(void) scanf("%2d%f%*d %[0123456789]", &i, &x, name);
.fi
.P
.RE
.P
with input:
.sp
.RS 4
.nf

56789 0123 56a72
.fi
.P
.RE
.P
assigns 56 to
.IR i ,
789.0 to
.IR x ,
skips 0123, and places the string
.BR \(dq56\e0\(dq 
in
.IR name .
The next call to
\fIgetchar\fR()
shall return the character
.BR 'a' .
.SS "Reading Data into an Array"
.P
The following call uses
\fIfscanf\fR()
to read three floating-point numbers from standard input into the
.IR input
array.
.sp
.RS 4
.nf

float input[3]; fscanf (stdin, "%f %f %f", input, input+1, input+2);
.fi
.P
.RE
.SH "APPLICATION USAGE"
If the application calling
\fIfscanf\fR()
has any objects of type
.BR wint_t
or
.BR wchar_t ,
it must also include the
.IR <wchar.h> 
header to have these objects defined.
.P
For functions that allocate memory as if by
\fImalloc\fR(),
the application should release such memory when it is no longer
required by a call to
\fIfree\fR().
For
\fIfscanf\fR(),
this is memory allocated via use of the
.BR 'm' 
assignment-allocation character.
.SH 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.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.5" ", " "Standard I/O Streams",
.IR "\fIfprintf\fR\^(\|)",
.IR "\fIgetc\fR\^(\|)",
.IR "\fIsetlocale\fR\^(\|)",
.IR "\fIstrtod\fR\^(\|)",
.IR "\fIstrtol\fR\^(\|)",
.IR "\fIstrtoul\fR\^(\|)",
.IR "\fIwcrtomb\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 7" ", " "Locale",
.IR "\fB<inttypes.h>\fP",
.IR "\fB<langinfo.h>\fP",
.IR "\fB<stdio.h>\fP",
.IR "\fB<wchar.h>\fP"
.\"
.SH 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 .
.PP
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 .