path: root/man3p/scanf.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "FSCANF" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" fscanf 
.SH NAME
fscanf, scanf, sscanf \- convert formatted input
.SH SYNOPSIS
.LP
\fB#include <stdio.h>
.br
.sp
int fscanf(FILE *restrict\fP \fIstream\fP\fB, const char *restrict\fP
\fIformat\fP\fB, ... );
.br
int scanf(const char *restrict\fP \fIformat\fP\fB, ... );
.br
int sscanf(const char *restrict\fP \fIs\fP\fB, const char *restrict\fP
\fIformat\fP\fB, ... );
.br
\fP
.SH DESCRIPTION
.LP
The \fIfscanf\fP() function shall read from the named input \fIstream\fP.
The \fIscanf\fP() function shall read from the
standard input stream \fIstdin\fP. The \fIsscanf\fP() function shall
read from the string \fIs\fP. Each function reads bytes,
interprets them according to a format, and stores the results in its
arguments. Each expects, as arguments, a control string
\fIformat\fP described below, and a set of \fIpointer\fP 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.
.LP
Conversions can be applied to the \fIn\fPth argument after the \fIformat\fP
in the argument list, rather than to the next unused
argument. In this case, the conversion specifier character \fB%\fP
(see below) is replaced by the sequence
\fB"%\fP\fIn\fP\fB$"\fP, where \fIn\fP is a decimal integer in the
range [1,{NL_ARGMAX}]. This feature provides for the
definition of format strings that select arguments in an order appropriate
to specific languages. In format strings containing the
\fB"%\fP\fIn\fP\fB$"\fP form of conversion specifications, it is unspecified
whether numbered arguments in the argument list
can be referenced from the format string more than once.
.LP
The \fIformat\fP can contain either form of a conversion specification-that
is, \fB%\fP or
\fB"%\fP\fIn\fP\fB$"\fP-but the two forms cannot be mixed within a
single \fIformat\fP string. The only exception to this
is that \fB%%\fP or \fB%*\fP can be mixed with the \fB"%\fP\fIn\fP\fB$"\fP
form. When numbered argument specifications
are used, specifying the \fIN\fPth argument requires that all the
leading arguments, from the first to the ( \fIN\fP-1)th, are
pointers. 
.LP
The
\fIfscanf\fP() 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 program's locale (category \fILC_NUMERIC
).\fP In the POSIX locale, or in a locale where the
radix character is not defined, the radix character shall default
to a period ( \fB'.'\fP ). 
.LP
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>s, <tab>s,
<newline>s, <vertical-tab>s, or <form-feed>s); an ordinary character
(neither \fB'%'\fP nor a white-space
character); or a conversion specification. Each conversion specification
is introduced by the character \fB'%'\fP   \ or the
character sequence \fB"%\fP\fIn\fP\fB$"\fP,  \ after
which the following appear in sequence:
.IP " *" 3
An optional assignment-suppressing character \fB'*'\fP .
.LP
.IP " *" 3
An optional non-zero decimal integer that specifies the maximum field
width.
.LP
.IP " *" 3
An option length modifier that specifies the size of the receiving
object.
.LP
.IP " *" 3
A \fIconversion specifier\fP character that specifies the type of
conversion to be applied. The valid conversion specifiers are
described below.
.LP
.LP
The \fIfscanf\fP() 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).
.LP
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.
.LP
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.
.LP
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.
.LP
Input white-space characters (as specified by \fIisspace\fP() ) shall
be skipped, unless the
conversion specification includes a \fB[\fP , \fBc\fP , \fBC\fP ,
or \fBn\fP conversion specifier.
.LP
An item shall be read from the input, unless the conversion specification
includes an \fBn\fP 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.
.LP
Except in the case of a \fB%\fP conversion specifier, the input item
(or, in the case of a \fB%n\fP 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 \fB'*'\fP , the result of
the conversion shall be placed in the object pointed to by
the first argument following the \fIformat\fP argument that has not
already received a conversion result if the conversion
specification is introduced by \fB%\fP ,   \ or in the \fIn\fPth argument
if introduced by the character
sequence \fB"%\fP\fIn\fP\fB$"\fP.  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.
.LP
The length modifiers and their meanings are:
.TP 7
\fBhh\fP
Specifies that a following \fBd\fP , \fBi\fP , \fBo\fP , \fBu\fP ,
\fBx\fP , \fBX\fP , or \fBn\fP
conversion specifier applies to an argument with type pointer to \fBsigned
char\fP or \fBunsigned char\fP.
.TP 7
\fBh\fP
Specifies that a following \fBd\fP , \fBi\fP , \fBo\fP , \fBu\fP ,
\fBx\fP , \fBX\fP , or \fBn\fP
conversion specifier applies to an argument with type pointer to \fBshort\fP
or \fBunsigned short\fP.
.TP 7
\fBl\fP\ (ell)
Specifies that a following \fBd\fP , \fBi\fP , \fBo\fP , \fBu\fP ,
\fBx\fP , \fBX\fP , or \fBn\fP
conversion specifier applies to an argument with type pointer to \fBlong\fP
or \fBunsigned long\fP; that a following \fBa\fP ,
\fBA\fP , \fBe\fP , \fBE\fP , \fBf\fP , \fBF\fP , \fBg\fP , or \fBG\fP
conversion specifier applies to an
argument with type pointer to \fBdouble\fP; or that a following \fBc\fP
, \fBs\fP , or \fB[\fP conversion specifier
applies to an argument with type pointer to \fBwchar_t\fP.
.TP 7
\fBll\fP\ (ell-ell)
.sp
Specifies that a following \fBd\fP , \fBi\fP , \fBo\fP , \fBu\fP ,
\fBx\fP , \fBX\fP , or \fBn\fP conversion
specifier applies to an argument with type pointer to \fBlong long\fP
or \fBunsigned long long\fP.
.TP 7
\fBj\fP
Specifies that a following \fBd\fP , \fBi\fP , \fBo\fP , \fBu\fP ,
\fBx\fP , \fBX\fP , or \fBn\fP
conversion specifier applies to an argument with type pointer to \fBintmax_t\fP
or \fBuintmax_t\fP.
.TP 7
\fBz\fP
Specifies that a following \fBd\fP , \fBi\fP , \fBo\fP , \fBu\fP ,
\fBx\fP , \fBX\fP , or \fBn\fP
conversion specifier applies to an argument with type pointer to \fBsize_t\fP
or the corresponding signed integer type.
.TP 7
\fBt\fP
Specifies that a following \fBd\fP , \fBi\fP , \fBo\fP , \fBu\fP ,
\fBx\fP , \fBX\fP , or \fBn\fP
conversion specifier applies to an argument with type pointer to \fBptrdiff_t\fP
or the corresponding \fBunsigned\fP type.
.TP 7
\fBL\fP
Specifies that a following \fBa\fP , \fBA\fP , \fBe\fP , \fBE\fP ,
\fBf\fP , \fBF\fP , \fBg\fP , or
\fBG\fP conversion specifier applies to an argument with type pointer
to \fBlong double\fP.
.sp
.LP
If a length modifier appears with any conversion specifier other than
as specified above, the behavior is undefined.
.LP
The following conversion specifiers are valid:
.TP 7
\fBd\fP
Matches an optionally signed decimal integer, whose format is the
same as expected for the subject sequence of \fIstrtol\fP() with the
value 10 for the \fIbase\fP argument. In the absence of a size modifier,
the application shall ensure that the corresponding argument is a
pointer to \fBint\fP.
.TP 7
\fBi\fP
Matches an optionally signed integer, whose format is the same as
expected for the subject sequence of \fIstrtol\fP() with 0 for the
\fIbase\fP argument. In the absence of a size modifier, the
application shall ensure that the corresponding argument is a pointer
to \fBint\fP.
.TP 7
\fBo\fP
Matches an optionally signed octal integer, whose format is the same
as expected for the subject sequence of \fIstrtoul\fP() with the value
8 for the \fIbase\fP argument. In the absence of a size modifier,
the application shall ensure that the corresponding argument is a
pointer to \fBunsigned\fP.
.TP 7
\fBu\fP
Matches an optionally signed decimal integer, whose format is the
same as expected for the subject sequence of \fIstrtoul\fP() with
the value 10 for the \fIbase\fP argument. In the absence of a size
modifier,
the application shall ensure that the corresponding argument is a
pointer to \fBunsigned\fP.
.TP 7
\fBx\fP
Matches an optionally signed hexadecimal integer, whose format is
the same as expected for the subject sequence of \fIstrtoul\fP() with
the value 16 for the \fIbase\fP argument. In the absence of a size
modifier,
the application shall ensure that the corresponding argument is a
pointer to \fBunsigned\fP.
.TP 7
\fBa\fP,\ \fBe\fP,\ \fBf\fP,\ \fBg\fP
.sp
Matches an optionally signed floating-point number, infinity, or NaN,
whose format is the same as expected for the subject sequence
of \fIstrtod\fP(). In the absence of a size modifier, the application
shall ensure that the
corresponding argument is a pointer to \fBfloat\fP. 
.LP
If the \fIfprintf\fP() 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
\fIfscanf\fP() family of functions shall recognize them as input.
.TP 7
\fBs\fP
Matches a sequence of bytes that are not white-space characters. The
application shall ensure that the corresponding argument
is a pointer to the initial byte of an array of \fBchar\fP, \fBsigned
char\fP, or \fBunsigned char\fP large enough to accept the
sequence and a terminating null character code, which shall be added
automatically. 
.LP
If an \fBl\fP (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\fP()
function, with the conversion state described by an \fBmbstate_t\fP
object initialized to zero before the first character is
converted. The application shall ensure that the corresponding argument
is a pointer to an array of \fBwchar_t\fP large enough to
accept the sequence and the terminating null wide character, which
shall be added automatically.
.TP 7
\fB[\fP
Matches a non-empty sequence of bytes from a set of expected bytes
(the \fIscanset\fP). The normal skip over white-space
characters shall be suppressed in this case. The application shall
ensure that the corresponding argument is a pointer to the
initial byte of an array of \fBchar\fP, \fBsigned char\fP, or \fBunsigned
char\fP large enough to accept the sequence and a
terminating null byte, which shall be added automatically. 
.LP
If an \fBl\fP (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\fP() function, with the conversion
state described by an \fBmbstate_t\fP object
initialized to zero before the first character is converted. The application
shall ensure that the corresponding argument is a
pointer to an array of \fBwchar_t\fP large enough to accept the sequence
and the terminating null wide character, which shall be
added automatically.
.LP
The conversion specification includes all subsequent bytes in the
\fIformat\fP string up to and including the matching right
square bracket ( \fB']'\fP ). The bytes between the square brackets
(the \fIscanlist\fP) comprise the scanset, unless the byte
after the left square bracket is a circumflex ( \fB'^'\fP ), 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 \fB"[]"\fP or
\fB"[^]"\fP , 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 \fB'-'\fP is in the scanlist and is
not the first character, nor the second where the first
character is a \fB'^'\fP , nor the last character, the behavior is
implementation-defined.
.TP 7
\fBc\fP
Matches a sequence of bytes of the number specified by the field width
(1 if no field width is present in the conversion
specification). The application shall ensure that the corresponding
argument is a pointer to the initial byte of an array of
\fBchar\fP, \fBsigned char\fP, or \fBunsigned char\fP large enough
to accept the sequence. No null byte is added. The normal
skip over white-space characters shall be suppressed in this case.
.LP
If an \fBl\fP (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\fP() function, with the conversion
state described by an \fBmbstate_t\fP object
initialized to zero before the first character is converted. The application
shall ensure that the corresponding argument is a
pointer to an array of \fBwchar_t\fP large enough to accept the resulting
sequence of wide characters. No null wide character is
added.
.TP 7
\fBp\fP
Matches an implementation-defined set of sequences, which shall be
the same as the set of sequences that is produced by the
\fB%p\fP conversion specification of the corresponding \fIfprintf\fP()
functions. The
application shall ensure that the corresponding argument is a pointer
to a pointer to \fBvoid\fP. 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 \fB%p\fP conversion specification is undefined.
.TP 7
\fBn\fP
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\fP() functions. Execution of a \fB%n\fP
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.
.TP 7
\fBC\fP
Equivalent to \fBlc\fP . 
.TP 7
\fBS\fP
Equivalent to \fBls\fP . 
.TP 7
\fB%\fP
Matches a single \fB'%'\fP character; no conversion or assignment
occurs. The complete conversion specification shall be
\fB%%\fP .
.sp
.LP
If a conversion specification is invalid, the behavior is undefined.
.LP
The conversion specifiers \fBA\fP , \fBE\fP , \fBF\fP , \fBG\fP ,
and \fBX\fP are also valid and shall be
equivalent to \fBa\fP , \fBe\fP , \fBf\fP , \fBg\fP , and \fBx\fP
, respectively.
.LP
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 \fB%n\fP ) 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.
.LP
Reaching the end of the string in \fIsscanf\fP() shall be equivalent
to encountering end-of-file for \fIfscanf\fP().
.LP
If conversion terminates on a conflicting input, the offending input
is left unread in the input. Any trailing white space
(including <newline>s) shall be left unread unless matched by a conversion
specification. The success of literal matches and
suppressed assignments is only directly determinable via the \fB%n\fP
conversion specification.
.LP
The
\fIfscanf\fP() and \fIscanf\fP() functions may mark the \fIst_atime\fP
field of the file associated with \fIstream\fP for
update. The \fIst_atime\fP field shall be marked for update by the
first successful execution of \fIfgetc\fP(), \fIfgets\fP(), \fIfread\fP(),
\fIgetc\fP(), \fIgetchar\fP(), \fIgets\fP(), \fIfscanf\fP(), or
\fIfscanf\fP() using \fIstream\fP that returns data not supplied by
a prior call to \fIungetc\fP(). 
.SH RETURN VALUE
.LP
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 matching failure or conversion,
EOF shall be returned. If a read error occurs, the error indicator
for the stream is set, EOF shall be returned,   \ and
\fIerrno\fP shall be set to indicate the error. 
.SH ERRORS
.LP
For the conditions under which the \fIfscanf\fP() functions fail and
may fail, refer to \fIfgetc\fP()
or \fIfgetwc\fP() .
.LP
In addition, \fIfscanf\fP() may fail if:
.TP 7
.B EILSEQ
Input byte sequence does not form a valid character. 
.TP 7
.B EINVAL
There are insufficient arguments. 
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
The call:
.sp
.RS
.nf

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

\fB25 54.32E-1 Hamster
\fP
.fi
.RE
.LP
assigns to \fIn\fP the value 3, to \fIi\fP the value 25, to \fIx\fP
the value 5.432, and \fIname\fP contains the string
\fB"Hamster"\fP .
.LP
The call:
.sp
.RS
.nf

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

\fB56789 0123 56a72
\fP
.fi
.RE
.LP
assigns 56 to \fIi\fP, 789.0 to \fIx\fP, skips 0123, and places the
string \fB"56\\0"\fP in \fIname\fP. The next call to \fIgetchar\fP()
shall return the character \fB'a'\fP .
.SS Reading Data into an Array
.LP
The following call uses \fIfscanf\fP() to read three floating-point
numbers from standard input into the \fIinput\fP
array.
.sp
.RS
.nf

\fBfloat input[3]; fscanf (stdin, "%f %f %f", input, input+1, input+2);
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
If the application calling \fIfscanf\fP() has any objects of type
\fBwint_t\fP or \fBwchar_t\fP, it must also include the \fI<wchar.h>\fP
header to have these objects defined.
.SH RATIONALE
.LP
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
.LP
None.
.SH SEE ALSO
.LP
\fIgetc\fP() , \fIprintf\fP() , \fIsetlocale\fP() , \fIstrtod\fP()
, \fIstrtol\fP() ,
\fIstrtoul\fP() , \fIwcrtomb\fP() , the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Chapter 7, Locale, \fI<langinfo.h>\fP, \fI<stdio.h>\fP,
\fI<wchar.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .