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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 conversion 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 vss-
canf 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 con-
version specifier (but see ‘suppression’ below). All conversions are introduced by
the % (percent sign) character. The format string may also contain other charac-
ters. 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. Scanning 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 (glibc) 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 initialized before. This
flag does not exist in ANSI C (C89) and has a different meaning in C99.
a (C99) Equivalent to f.
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 maximum 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); 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.
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 otherwise. 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 charac-
ters (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 for-
mat.
[ 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 charac-
ter 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 circum-
flex; 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 cir-
cumflex, 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 characters 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 increment
the assignment count returned at the completion of execution’ but the Corri-
gendum seems to contradict this. Probably it is wise not to make any assump-
tions on the effect of %n conversions on the return value.
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. 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 con-
version such as an end-of-file occurs. If an error or end-of-file occurs after con-
version has begun, the number of conversions which were successfully completed is
returned.
SEE ALSO
strtol(3), strtoul(3), strtod(3), getc(3), printf(3)
CONFORMING TO
The functions fscanf, scanf, and sscanf conform to ANSI X3.159-1989 (‘‘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 notation.
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 X3.159-1989 conformant, but provide the additional
flags q and a as well as an additional 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
X3.159-1989.
Some combinations of flags defined by ANSI C are not making 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 con-
versions or ll.
The usage of q is not the same as on BSD 4.4, as it may be used in float conver-
sions equivalently to L.
LINUX MANPAGE 1995-11-01 SCANF(3)
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