man3/scanf.3

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.\" This code is derived from software contributed to Berkeley by
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.\"     @(#)scanf.3	6.14 (Berkeley) 1/8/93
.\"
.\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
.\" modified to resemble the GNU libio setup used in the Linux libc
.\" used in versions 4.x (x>4) and 5   Helmut.Geyer@iwr.uni-heidelberg.de
.\" Modified, aeb, 970121
.\" 2005-07-14, mtk, added description of %n$ form; various text
.\"	incorporated from the GNU C library documentation ((C) The
.\"	Free Software Foundation); other parts substantially rewritten.
.\"
.\" 2008-06-23, mtk
.\"     Add ERRORS section.
.\"     Document the 'a' and 'm' modifiers for dynamic string allocation.
.\"
.TH SCANF 3  2011-09-28 "GNU" "Linux Programmer's Manual"
.SH NAME
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf \- input format conversion
.SH SYNOPSIS
.nf
.B #include <stdio.h>

.BI "int scanf(const char *" format ", ...);"
.BI "int fscanf(FILE *" stream ", const char *" format ", ...);"
.BI "int sscanf(const char *" str ", const char *" format ", ...);"
.sp
.B #include <stdarg.h>

.BI "int vscanf(const char *" format ", va_list " ap );
.BI "int vsscanf(const char *" str ", const char *" format ", va_list " ap );
.BI "int vfscanf(FILE *" stream ", const char *" format ", va_list " ap );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.ad l
.sp
.BR vscanf (),
.BR vsscanf (),
.BR vfscanf ():
.RS 4
_XOPEN_SOURCE\ >=\ 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE\ >=\ 200112L;
.br
or
.I "cc -std=c99"
.ad
.RE
.SH DESCRIPTION
The
.BR scanf ()
family of functions scans input according to
.I format
as described below.
This format may contain
.IR "conversion specifications" ;
the results from such conversions, if any,
are stored in the locations pointed to by the
.I pointer
arguments that follow
.IR format .
Each
.I pointer
argument must be of a type that is appropriate for the value returned
by the corresponding conversion specification.

If the number of conversion specifications in
.I format
exceeds the number of
.I pointer
arguments, the results are undefined.
If the number of
.I pointer
arguments exceeds the number of conversion specifications, then the excess
.I pointer
arguments are evaluated, but are otherwise ignored.

The
.BR scanf ()
function reads input from the standard input stream
.IR stdin ,
.BR fscanf ()
reads input from the stream pointer
.IR stream ,
and
.BR sscanf ()
reads its input from the character string pointed to by
.IR str .
.PP
The
.BR vfscanf ()
function is analogous to
.BR vfprintf (3)
and reads input from the stream pointer
.I stream
using a variable argument list of pointers (see
.BR stdarg (3).
The
.BR vscanf ()
function scans a variable argument list from the standard input and the
.BR vsscanf ()
function scans it from a string; these are analogous to the
.BR vprintf (3)
and
.BR vsprintf (3)
functions respectively.
.PP
The
.I format
string consists of a sequence of
.I directives
which describe how to process the sequence of input characters.
If processing of a directive fails, no further input is read, and
.BR scanf ()
returns.
A "failure" can be either of the following:
.IR "input failure" ,
meaning that input characters were unavailable, or
.IR "matching failure" ,
meaning that the input was inappropriate (see below).

A directive is one of the following:
.TP
\(bu
A sequence of white-space characters (space, tab, newline, etc.; see
.BR isspace (3)).
This directive matches any amount of white space,
including none, in the input.
.TP
\(bu
An ordinary character (i.e., one other than white space or \(aq%\(aq).
This character must exactly match the next character of input.
.TP
\(bu
A conversion specification,
which commences with a \(aq%\(aq (percent) character.
A sequence of characters from the input is converted according to
this specification, and the result is placed in the corresponding
.I pointer
argument.
If the next item of input does not match the conversion specification,
the conversion fails\(emthis is a
.IR "matching failure" .
.PP
Each
.I conversion specification
in
.I format
begins with either the character \(aq%\(aq or the character sequence
"\fB%\fP\fIn\fP\fB$\fP"
(see below for the distinction) followed by:
.TP
\(bu
An optional \(aq*\(aq assignment-suppression character:
.BR scanf ()
reads input as directed by the conversion specification,
but discards the input.
No corresponding
.I pointer
argument is required, and this specification is not
included in the count of successful assignments returned by
.BR scanf ().
.TP
\(bu
An optional \(aqa\(aq character.
This is used with string conversions, and relieves the caller of the
need to allocate a corresponding buffer to hold the input: instead,
.BR scanf ()
allocates a buffer of sufficient size,
and assigns the address of this buffer to the corresponding
.I pointer
argument, which should be a pointer to a
.I "char *"
variable (this variable does not need to be initialized before the call).
The caller should subsequently
.BR free (3)
this buffer when it is no longer required.
This is a GNU extension;
C99 employs the \(aqa\(aq character as a conversion specifier (and
it can also be used as such in the GNU implementation).
.TP
\(bu
An optional decimal integer which specifies the
.IR "maximum field width" .
Reading of characters stops either when this maximum is reached or
when a nonmatching character is found, whichever happens first.
Most conversions discard initial white space characters (the exceptions
are noted below),
and these discarded characters don't count toward the maximum field width.
String input conversions store a terminating null byte (\(aq\\0\(aq)
to mark the end of the input;
the maximum field width does not include this terminator.
.TP
\(bu
An optional
.IR "type modifier character" .
For example, the
.B l
type modifier is used with integer conversions such as
.B %d
to specify that the corresponding
.I pointer
argument refers to a
.I "long int"
rather than a pointer to an
.IR int .
.TP
\(bu
A
.I "conversion specifier"
that specifies the type of input conversion to be performed.
.PP
The conversion specifications in
.I format
are of two forms, either beginning with \(aq%\(aq or beginning with
"\fB%\fP\fIn\fP\fB$\fP".
The two forms should not be mixed in the same
.I format
string, except that a string containing
"\fB%\fP\fIn\fP\fB$\fP"
specifications can include
.B %%
and
.BR %* .
If
.I format
contains \(aq%\(aq
specifications then these correspond in order with successive
.I pointer
arguments.
In the
"\fB%\fP\fIn\fP\fB$\fP"
form (which is specified in POSIX.1-2001, but not C99),
.I n
is a decimal integer that specifies that the converted input should
be placed in the location referred to by the
.IR n -th
.I pointer
argument following
.IR format .
.SS Conversions
The following
.I "type modifier characters"
can appear in a conversion specification:
.TP
.B h
Indicates that the conversion will be one of
\fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, \fBX\fP, or \fBn\fP
and the next pointer is a pointer to a
.I short int
or
.I unsigned short int
(rather than
.IR int ).
.TP
.B hh
As for
.BR h ,
but the next pointer is a pointer to a
.I signed char
or
.IR "unsigned char" .
.TP
.B j
As for
.BR h ,
but the next pointer is a pointer to an
.I intmax_t
or a
.IR uintmax_t .
This modifier was introduced in C99.
.TP
.B l
Indicates either that the conversion will be one of
\fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, \fBX\fP, or \fBn\fP
and the next pointer is a pointer to a
.I long int
or
.I unsigned long int
(rather than
.IR int ),
or that the conversion will be one of
\fBe\fP, \fBf\fP, or \fBg\fP
and the next pointer is a pointer to
.I double
(rather than
.IR float ).
Specifying two
.B l
characters is equivalent to
.BR L .
If used with
.B %c
or
.B %s
the corresponding parameter is considered
as a pointer to a wide character or wide-character string respectively.
.\" This use of l was introduced in Amendment 1 to ISO C90.
.TP
.B L
Indicates that the conversion will be either
\fBe\fP, \fBf\fP, or \fBg\fP
and the next pointer is a pointer to
.I "long double"
or the conversion will be
\fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, or \fBx\fP
and the next pointer is a pointer to
.IR "long long" .
.\" MTK, Jul 05: The following is no longer true for modern
.\" ANSI C (i.e., C99):
.\" (Note that long long is not an
.\" ANSI C
.\" type. Any program using this will not be portable to all
.\" architectures).
.TP
.B q
equivalent to
.BR L .
This specifier does not exist in ANSI C.
.TP
.B t
As for
.BR h ,
but the next pointer is a pointer to a
.IR ptrdiff_t .
This modifier was introduced in C99.
.TP
.B z
As for
.BR h ,
but the next pointer is a pointer to a
.IR size_t .
This modifier was introduced in C99.
.PP
The following
.I "conversion specifiers"
are available:
.TP
.B %
Matches a literal \(aq%\(aq.
That is,
.B %\&%
in the format string matches a
single input \(aq%\(aq character.
No conversion is done (but initial white space characters are discarded),
and assignment does not occur.
.TP
.B d
Matches an optionally signed decimal integer;
the next pointer must be a pointer to
.IR int .
.TP
.B D
Equivalent to
.IR ld ;
this exists only for backward compatibility.
(Note: thus only in libc4.
In libc5 and glibc the
.B %D
is silently ignored, causing old programs to fail mysteriously.)
.TP
.B i
Matches an optionally signed integer; the next pointer must be a pointer to
.IR int .
The integer is read in base 16 if it begins with
.I 0x
or
.IR 0X ,
in base 8 if it begins with
.IR 0 ,
and in base 10 otherwise.
Only characters that correspond to the base are used.
.TP
.B o
Matches an unsigned octal integer; the next pointer must be a pointer to
.IR "unsigned int" .
.TP
.B u
Matches an unsigned decimal integer; the next pointer must be a
pointer to
.IR "unsigned int" .
.TP
.B x
Matches an unsigned hexadecimal integer; the next pointer must
be a pointer to
.IR "unsigned int" .
.TP
.B X
Equivalent to
.BR x .
.TP
.B f
Matches an optionally signed floating-point number; the next pointer must
be a pointer to
.IR float .
.TP
.B e
Equivalent to
.BR f .
.TP
.B g
Equivalent to
.BR f .
.TP
.B E
Equivalent to
.BR f .
.TP
.B a
(C99) Equivalent to
.BR f .
.TP
.B s
Matches a sequence of non-white-space characters;
the next pointer must be a pointer to character array that is
long enough to hold the input sequence and
the terminating null byte (\(aq\\0\(aq), which is added automatically.
The input string stops at white space or at the maximum field
width, whichever occurs first.
.TP
.B c
Matches a sequence of characters whose length is specified by the
.I maximum field width
(default 1); the next pointer must be a pointer to
.IR char ,
and there must be enough room for all the characters (no terminating
null byte
is added).
The usual skip of leading white space is suppressed.
To skip white space first, use an explicit space in the format.
.TP
.B \&[
Matches a nonempty sequence of characters from the specified set of
accepted characters; the next pointer must be a pointer to
.IR char ,
and there must be enough room for all the characters in the string, plus a
terminating null byte.
The usual skip of leading white space is suppressed.
The string is to be made up of characters in (or not in) a particular set;
the set is defined by the characters between the open bracket
.B [
character and a close bracket
.B ]
character.
The set
.I excludes
those characters if the first character after the open bracket is a
circumflex
.RB ( ^ ).
To include a close bracket in the set, make it the first character after
the open bracket or the circumflex; any other position will end the set.
The hyphen character
.B \-
is also special; when placed between two other characters, it adds all
intervening characters to the set.
To include a hyphen, make it the last
character before the final close bracket.
For instance,
.B [^]0\-9\-]
means
the set "everything except close bracket, zero through nine, and hyphen".
The string ends with the appearance of a character not in the (or, with a
circumflex, in) set or when the field width runs out.
.TP
.B p
Matches a pointer value (as printed by
.B %p
in
.BR printf (3);
the next pointer must be a pointer to a pointer to
.IR void .
.TP
.B n
Nothing is expected; instead, the number of characters consumed thus far
from the input is stored through the next pointer, which must be a pointer
to
.IR int .
This is
.I not
a conversion, although it can be suppressed with the
.B *
assignment-suppression character.
The C standard says: "Execution of a
.B %n
directive does not increment
the assignment count returned at the completion of execution"
but the Corrigendum seems to contradict this.
Probably it is wise
not to make any assumptions on the effect of
.B %n
conversions on the return value.
.SH "RETURN VALUE"
These functions return the number of input items
successfully matched and assigned,
which can be fewer than provided for,
or even zero in the event of an early matching failure.

The value
.B EOF
is returned if the end of input is reached before either the first
successful conversion or a matching failure occurs.
.B EOF
is also returned if a read error occurs,
in which case the error indicator for the stream (see
.BR ferror (3))
is set, and
.I errno
is set indicate the error.
.SH ERRORS
.TP
.B EAGAIN
The file descriptor underlying
.I stream
is marked nonblocking, and the read operation would block.
.TP
.B EBADF
The file descriptor underlying
.I stream
is invalid, or not open for reading.
.TP
.B EILSEQ
Input byte sequence does not form a valid character.
.TP
.B EINTR
The read operation was interrupted by a signal; see
.BR signal (7).
.TP
.B EINVAL
Not enough arguments; or
.I format
is NULL.
.TP
.B ENOMEM
Out of memory.
.TP
.B ERANGE
The result of an integer conversion would exceed the size
that can be stored in the corresponding integer type.
.SH "CONFORMING TO"
The functions
.BR fscanf (),
.BR scanf (),
and
.BR sscanf ()
conform to C89 and C99 and POSIX.1-2001.
These standards do not specify the
.B ERANGE
error.
.PP
The
.B q
specifier is the 4.4BSD notation for
.IR "long long" ,
while
.B ll
or the usage of
.B L
in integer conversions is the GNU notation.
.PP
The Linux version of these functions is based on the
.I GNU
.I libio
library.
Take a look at the
.I info
documentation of
.I GNU
.I libc (glibc-1.08)
for a more concise description.
.SH NOTES
The GNU C library supports a nonstandard extension that causes
the library to dynamically allocate a string of sufficient size
for input strings for the
.B %s
and
\fB%a[\fP\fIrange\fP\fB]\fP
conversion specifiers.
.\" This feature seems to be present at least as far back as glibc 2.0.
To make use of this feature, specify
.B a
as a length modifier (thus
.B %as
or
\fB%a[\fP\fIrange\fP\fB]\fP).
The caller must
.BR free (3)
the returned string, as in the following example:
.in +4n
.nf

char *p;
int n;

errno = 0;
n = scanf("%a[a-z]", &p);
if (n == 1) {
    printf("read: %s\\n", p);
    free(p);
} else if (errno != 0) {
    perror("scanf");
} else {
    fprintf(stderr, "No matching characters\\n");
}
.fi
.in
.PP
As shown in the above example, it is only necessary to call
.BR free (3)
if the
.BR scanf ()
call successfully read a string.
.PP
The
.B a
modifier is not available if the program is compiled with
.I "gcc -std=c99"
or
.IR "gcc -D_ISOC99_SOURCE"
(unless
.B _GNU_SOURCE
is also specified), in which case the
.B a
is interpreted as a specifier for floating-point numbers (see above).

Since version 2.7, glibc also provides the
.B m
modifier for the same purpose as the
.BR a
modifier.
The
.B m
modifier has the following advantages:
.IP * 2
It may also be applied to
.B %c
conversion specifiers (e.g.,
.BR %3mc ).
.IP *
It avoids ambiguity with respect to the
.B %a
floating-point conversion specifier (and is unaffected by
.IR "gcc -std=c99"
etc.)
.IP *
It is specified in the upcoming revision of the POSIX.1 standard.
.SH BUGS
All functions are fully C89 conformant, but provide the
additional specifiers
.B q
and
.B a
as well as an additional behavior of the
.B L
and
.B l
specifiers.
The latter may be considered to be a bug, as it changes the
behavior of specifiers defined in C89.
.PP
Some combinations of the type modifiers and conversion
specifiers defined by ANSI C do not make sense
(e.g.,
.BR "%Ld" ).
While they may have a well-defined behavior on Linux, this need not
to be so on other architectures.
Therefore it usually is better to use
modifiers that are not defined by ANSI C at all, that is, use
.B q
instead of
.B L
in combination with
\fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, and \fBX\fP
conversions or
.BR ll .
.PP
The usage of
.B q
is not the same as on 4.4BSD,
as it may be used in float conversions equivalently to
.BR L .
.SH "SEE ALSO"
.BR getc (3),
.BR printf (3),
.BR setlocale (3),
.BR strtod (3),
.BR strtol (3),
.BR strtoul (3)