man-pages/man3/scanf.3

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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
.\"
.TH SCANF 3  1995-11-01 "LINUX MANPAGE" "Linux Programmer's Manual"
.SH NAME
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf \- input format conversion
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.na
.BI "int scanf(const char *" format ", ..." );
.br
.BI "int fscanf(FILE *" stream ", const char *" format ", ..." );
.br
.BI "int sscanf(const char *" str ", const char *" format ", ..." );
.sp
.B #include <stdarg.h>
.BI "int vscanf(const char *" format ", va_list " ap );
.br
.BI "int vsscanf(const char *" str ", const char *" format ", va_list " ap );
.br
.BI "int vfscanf(FILE *" stream ", const char *" format ", va_list " ap );
.ad
.SH DESCRIPTION
The
.B scanf
family of functions scans input according to a
.I format
as described below.  This format may contain
.IR "conversion specifiers" ;
the results from such conversions, if any, are stored through the
.I pointer
arguments.  The
.B scanf
function reads input from the standard input stream
.IR stdin ,
.B fscanf
reads input from the stream pointer
.IR stream ,
and
.B sscanf
reads its input from the character string pointed to by
.IR str .
.PP
The
.B 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
.B vscanf
function scans a variable argument list from the standard input and the
.B vsscanf
function scans it from a string; these are analogous to the
.B vprintf
and
.B vsprintf
functions respectively.
.PP
Each successive
.I pointer
argument must correspond properly with each successive conversion specifier
(but see `suppression' below).  All conversions are introduced by the
.B %
(percent sign) character.  The
.I format
string may also contain other characters.  White space (such as blanks,
tabs, or newlines) in the
.I format
string match any amount of white space, including none, in the input.
Everything else matches only itself.  Scanning stops when an input
character does not match such a format character.  Scanning also stops when
an input conversion cannot be made (see below).
.SH CONVERSIONS
Following the
.B %
character introducing a conversion there may be a number of
.I flag
characters, as follows:
.TP
.B *
Suppresses assignment.  The conversion that follows occurs as usual, but no
pointer is used; the result of the conversion is simply discarded.
.TP
.B a 
(glibc) Indicates that the conversion will be 
.BR s ,
the needed memory space for the string will be malloc'ed  and
the pointer to it will be assigned to the  
.I  char
pointer variable, which does not have to be initialized before.
This flag does not exist in 
.IR "ANSI C"
(C89) and has a different meaning in C99.
.TP
.B a
(C99) Equivalent to
.BR f .
.TP
.B h
Indicates that the conversion will be one of
.B dioux
or
.B n
and the next pointer is a pointer to a
.I short  int
(rather than
.IR int ).
.TP
.B l
Indicates either that the conversion will be one of
.B dioux
or
.B n
and the next pointer is a pointer to a
.I long  int
(rather than
.IR int ),
or that the conversion will be one of
.B efg
and the next pointer is a pointer to
.I double
(rather than
.IR float ).
Specifying two 
.B l
flags is equivalent to the 
.B L
flag.
.TP
.B L
Indicates that the conversion will be either
.B efg
and the next pointer is a pointer to
.IR "long double" 
or the conversion will be 
.B dioux
and the next pointer is a pointer to
.IR "long long" .
(Note that long long is not an 
.I ANSI C 
type. Any program using this will not be portable to all
architectures).
.TP
.B q
equivalent to L. 
This flag does not exist in 
.IR "ANSI C" .
.PP
In addition to these flags, there may be an optional maximum field width,
expressed as a decimal integer, between the
.B %
and the conversion.  If no width is given, a default of `infinity' is used
(with one exception, below); otherwise at most this many characters are
scanned in processing the conversion.  Before conversion begins, most
conversions skip white space; this white space is not counted against the
field width.
.PP
The following conversions are available:
.TP
.B %
Matches a literal `%'.  That is, `%\&%' in the format string matches a
single input `%' character.  No conversion is done, 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
.BR ld ;
this exists only for backwards compatibility.
(Note: thus only in libc4. In libc5 and glibc the %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 `0x' or `0X', in base 8 if
it begins with `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 s
Matches a sequence of non-white-space characters; the next pointer must be
a pointer to
.IR char ,
and the array must be large enough to accept all the sequence and the
terminating
.B NUL
character.  The input string stops at white space or at the maximum field
width, whichever occurs first.
.TP
.B c
Matches a sequence of
.I width
count characters (default 1); the next pointer must be a pointer to
.IR char ,
and there must be enough room for all the characters (no terminating
.B NUL
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
.B NUL
character.  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
.BR ^ .
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, `[^]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 `%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 *
flag.
The C standard says: `Execution of a %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 %n conversions on
the return value.

.PP
.SH "RETURN VALUE"
These functions return the number of input items assigned, which can be
fewer than provided for, or even zero, in the event of a matching failure.
A return of zero indicates that, while there was input available,
no conversions were assigned; 
typically this is due to an invalid input character, such as an
alphabetic character for a `%d' conversion.  

The value
.B EOF
is returned if the end of input is reached before the first 
successful conversion or matching failure.
.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.

If an error or end-of-file occurs after conversion has
begun, the number of conversions which were successfully completed is
returned.
.SH "SEE ALSO"
.BR getc (3),
.BR printf (3),
.BR strtod (3),
.BR strtol (3),
.BR strtoul (3)
.SH "CONFORMING TO"
The functions
.BR fscanf ,
.BR scanf ,
and
.BR sscanf
conform to ANSI X3.159-1989 (``ANSI C'').
.PP
The 
.B q
flag is the 
.I BSD 4.4
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 BUGS
All functions are fully ANSI X3.159-1989 conformant, but provide the 
additional flags 
.B q
and
.B a
as well as an additional behaviour of the
.B L
and 
.B l 
flags. The latter may be considered to be a bug, as it changes the
behaviour of flags defined in ANSI X3.159-1989.
.PP
Some combinations of flags defined by
.I ANSI C
are not making sense in 
.IR "ANSI C" 
(e.g. 
.BR "%Ld" ).
While they may have a well-defined behaviour on Linux, this need not
to be so on other architectures. Therefore it usually is better to use
flags that are not defined by 
.I ANSI C
at all, i.e. use 
.B q
instead of 
.B L
in combination with 
.B diouxX
conversions or 
.BR ll .
.PP
The usage of
.B q
is not the same as on
.IR "BSD 4.4" ,
as it may be used in float conversions equivalently to 
.BR L .