fscanf
SCANF(3) Linux Programmer’s Manual SCANF(3)
NAME
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conver-
sion
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 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 ‘suppression’ below). All
conversions are introduced by the % (percent sign) character. The
format string may also contain 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. Scanning also stops when an input conversion
cannot be made (see below).
CONVERSIONS
Following the % character introducing a conversion there may be a num-
ber 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 conver-
sion. 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 conver-
sions 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 char-
acter. 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 [ charac-
ter 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 circum-
flex, 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 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 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 conversion
such as an end-of-file occurs. If an error or end-of-file occurs after
conversion has begun, the number of conversions which were success-
fully 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 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 1995-11-01 SCANF(3)
