Computer Science
SCANF(3) Linux Programmer's Manual SCANF(3)
NAME
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input
format conversion
SYNOPSIS
#include <stdio.h>
int scanf( const char *format, ...);
int fscanf( FILE *stream, const char *format, ...);
int sscanf( const char *str, const char *format, ...);
#include <stdarg.h>
int vscanf( const char *format, va_list ap);
int vsscanf( const char *str, const char *format, va_list ap);
int vfscanf( FILE *stream, const char *format, va_list ap);
DESCRIPTION
The scanf family of functions scans input according to a
format as described below. This format may contain con-
version specifiers; the results from such conversions, if
any, are stored through the pointer arguments. The scanf
function reads input from the standard input stream stdin,
fscanf reads input from the stream pointer stream, and
sscanf reads its input from the character string pointed
to by str.
The vfscanf function is analogous to vfprintf(3) and reads
input from the stream pointer stream using a variable
argument list of pointers (see stdarg(3). The vscanf
function scans a variable argument list from the standard
input and the vsscanf function scans it from a string;
these are analogous to the vprintf and vsprintf functions
respectively.
Each successive pointer argument must correspond properly
with each successive conversion specifier (but see `sup-
pression' below). All conversions are introduced by the %
(percent sign) character. The format string may also con-
tain other characters. White space (such as blanks, tabs,
or newlines) in the 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. Scan-
ning also stops when an input conversion cannot be made
(see below).
CONVERSIONS
Following the % character introducing a conversion there
may be a number of flag characters, as follows:
* Suppresses assignment. The conversion that follows
occurs as usual, but no pointer is used; the result
of the conversion is simply discarded.
a Indicates that the conversion will be s, the needed
memory space for the string will be malloc'ed and
the pointer to it will be assigned to the char
pointer variable, which does not have to be ini-
tialised before. This flag does not exist in ANSI
C.
h Indicates that the conversion will be one of dioux
or n and the next pointer is a pointer to a short
int (rather than int).
l Indicates either that the conversion will be one of
dioux or n and the next pointer is a pointer to a
long int (rather than int), or that the conversion
will be one of efg and the next pointer is a
pointer to double (rather than float). Specifying
two l flags is equivalent to the L flag.
L Indicates that the conversion will be either efg
and the next pointer is a pointer to long double or
the conversion will be dioux and the next pointer
is a pointer to long long. (Note that long long is
not an ANSI C type. Any program using this will not
be portable to all architectures).
q equivalent to L. This flag does not exist in ANSI
C.
In addition to these flags, there may be an optional maxi-
mum field width, expressed as a decimal integer, between
the % and the conversion. If no width is given, a default
of `infinity' is used (with one exception, below); other-
wise at most this many characters are scanned in process-
ing the conversion. Before conversion begins, most con-
versions skip white space; this white space is not counted
against the field width.
The following conversions are available:
% Matches a literal `%'. That is, `%%' in the format
string matches a single input `%' character. No
conversion is done, and assignment does not occur.
d Matches an optionally signed decimal integer; the
next pointer must be a pointer to int.
D Equivalent to 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.)
i Matches an optionally signed integer; the next
pointer must be a pointer to 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 other-
wise. Only characters that correspond to the base
are used.
o Matches an unsigned octal integer; the next pointer
must be a pointer to unsigned int.
u Matches an unsigned decimal integer; the next
pointer must be a pointer to unsigned int.
x Matches an unsigned hexadecimal integer; the next
pointer must be a pointer to unsigned int.
X Equivalent to x
f Matches an optionally signed floating-point number;
the next pointer must be a pointer to float.
e Equivalent to f.
g Equivalent to f.
E Equivalent to f
s Matches a sequence of non-white-space characters;
the next pointer must be a pointer to char, and the
array must be large enough to accept all the
sequence and the terminating NUL character. The
input string stops at white space or at the maximum
field width, whichever occurs first.
c Matches a sequence of width count characters
(default 1); the next pointer must be a pointer to
char, and there must be enough room for all the
characters (no terminating NUL is added). The
usual skip of leading white space is suppressed.
To skip white space first, use an explicit space in
the format.
[ Matches a nonempty sequence of characters from the
specified set of accepted characters; the next
pointer must be a pointer to char, and there must
be enough room for all the characters in the
string, plus a terminating 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 [ character and
a close bracket ] character. The set excludes
those characters if the first character after the
open bracket is a circumflex ^. 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 -
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.
p Matches a pointer value (as printed by `%p' in
printf(3); the next pointer must be a pointer to
void.
n Nothing is expected; instead, the number of charac-
ters consumed thus far from the input is stored
through the next pointer, which must be a pointer
to int. This is not a conversion, although it can
be suppressed with the * flag. The C standard
says: `Execution of a %n directive does not incre-
ment the assignment count returned at the comple-
tion 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.
RETURN VALUES
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. 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 EOF is returned if an input failure occurs before
any conversion such as an end-of-file occurs. If an error
or end-of-file occurs after conversion has begun, the num-
ber of conversions which were successfully completed is
returned.
SEE ALSO
strtol(3), strtoul(3), strtod(3), getc(3), printf(3)
STANDARDS
The functions fscanf, scanf, and sscanf conform to ANSI
C3.159-1989 (``ANSI C'').
The q flag is the BSD 4.4 notation for long long, while ll
or the usage of L in integer conversions is the GNU nota-
tion.
The Linux version of these functions is based on the GNU
libio library. Take a look at the info documentation of
GNU libc (glibc-1.08) for a more concise description.
BUGS
All functions are fully ANSI C3.159-1989 conformant, but
provide the additional flags q and a as well as an addi-
tional behaviour of the L and l flags. The latter may be
considered to be a bug, as it changes the behaviour of
flags defined in ANSI C3.159-1989.
Some combinations of flags defined by ANSI C are not mak-
ing sense in ANSI C (e.g. %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 ANSI C at all, i.e. use q
instead of L in combination with diouxX conversions or ll.
The usage of q is not the same as on BSD 4.4, as it may be
used in float conversions equivalently to L.
LINUX MANPAGE 1 November 1995 1
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