From Linux Man Pages
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 format as described below. This format may contain con-
version specifications; the results from such conversions, if any, are stored in the locations pointed to by the
pointer arguments that follow format. Each 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 format exceeds the number of pointer arguments, the results are
undefined. If the number of pointer arguments exceeds the number of conversion specifications, then the excess
pointer arguments are evaluated, but are otherwise ignored.
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 vari-
able 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.
The format string consists of a sequence of directives which describe how to process the sequence of input char-
acters. If processing of a directive fails, no further input is read, and scanf() returns. A "failure" can be
either of the following: input failure, meaning that input characters were unavailable, or matching failure,
meaning that the input was inappropriate (see below).
A directive is one of the following:
� A sequence of white-space characters (space, tab, newline, etc; see isspace(3)). This directive matches
any amount of white space, including none, in the input.
� An ordinary character (i.e., one other than white space or '%'). This character must exactly match the
next character of input.
� A conversion specification, which commences with a '%' (percent) character. A sequence of characters from
the input is converted according to this specification, and the result is placed in the corresponding
pointer argument. If the next item of input does not match the the conversion specification, the conver-
sion fails -- this is a matching failure.
Each conversion specification in format begins with either the character '%' or the character sequence "%n$" (see
below for the distinction) followed by:
� An optional '*' assignment-suppression character: scanf() reads input as directed by the conversion speci-
fication, but discards the input. No corresponding pointer argument is required, and this specification
is not included in the count of successful assignments returned by scanf().
� An optional 'a' character. This is used with string conversions, and relieves the caller of the need to
allocate a corresponding buffer to hold the input: instead, scanf() allocates a buffer of sufficient size,
and assigns the address of this buffer to the corresponding pointer argument, which should be a pointer to
a char * variable (this variable does not need to be initialised before the call). The caller should sub-
sequently free(3) this buffer when it is no longer required. This is a GNU extension; C99 employs the 'a'
character as a conversion specifier (and it can also be used as such in the GNU implementation).
� An optional decimal integer which specifies the maximum field width. Reading of characters stops either
when this maximum is reached or when a non-matching character is found, whichever happens first. Most
conversions discard initial whitespace characters (the exceptions are noted below), and these discarded
characters don't count towards the maximum field width. String input conversions store a null terminator
('\0') to mark the end of the input; the maximum field width does not include this terminator.
� An optional type modifier character. For example, the l type modifier is used with integer conversions
such as %d to specify that the corresponding pointer argument refers to a long int rather than a pointer
to an int.
� A conversion specifier that specifies the type of input conversion to be performed.
The conversion specifications in format are of two forms, either beginning with '%' or beginning with "%n$". The
two forms should not be mixed in the same format string, except that a string containing "%n$" specifications can
include %% and %*. If format contains '%' specifications then these correspond in order with successive pointer
arguments. In the "%n$" form (which is specified in POSIX.1-2001, but not C99), n is a decimal integer that
specifies that the converted input should be placed in the location referred to by the n-th pointer argument fol-
lowing format.
CONVERSIONS
The following type modifier characters can appear in a conversion specification:
h Indicates that the conversion will be one of diouxX or n and the next pointer is a pointer to a short int
or unsigned short int (rather than int).
hh As for h, but the next pointer is a pointer to a signed char or unsigned char.
j As for h, but the next pointer is a pointer to a intmax_t or uintmax_t. This modifier was introduced in
C99.
l Indicates either that the conversion will be one of diouxX or n and the next pointer is a pointer to a
long int or unsigned 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 characters is equivalent to L. If
used with %c or %s the corresponding parameter is considered as a pointer to a wide character or wide
character string respectively.
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.
q equivalent to L. This specifier does not exist in ANSI C.
t As for h, but the next pointer is a pointer to a ptrdiff_t. This modifier was introduced in C99.
z As for h, but the next pointer is a pointer to a size_t. This modifier was introduced in C99.
The following conversion specifiers are available:
% Matches a literal '%'. That is, %% in the format string matches a single input '%' character. No conver-
sion 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 charac-
ters 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.
a (C99) Equivalent to f.
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 character ('\0'), which is added
automatically. The input string stops at white space or at the maximum field width, whichever occurs
first.
c Matches a sequence of characters whose length is specified by the maximum field width (default 1); the
next pointer must be a pointer to char, and there must be enough room for all the characters (no terminat-
ing 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.
[ 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 ter-
minating 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
[ 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 char-
acter - 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 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 * assignment-suppression character. 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 con-
tradict 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 successfully matched and assigned, which can be fewer than pro-
vided for, or even zero in the event of an early matching failure.
The value EOF is returned if the end of input is reached before either the first successful conversion or a
matching failure occurs. EOF is also returned if a read error occurs, in which case the error indicator for the
stream (see ferror(3)) is set, and errno is set indicate the error.
RELATED
getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
CONFORMING TO
The functions fscanf(), scanf(), and sscanf() conform to C89 and C99.
The q specifier is the 4.4BSD 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 C89 conformant, but provide the additional specifiers q and a as well as an additional
behaviour of the L and l specifiers. The latter may be considered to be a bug, as it changes the behaviour of
specifiers defined in C89.
Some combinations of the type modifiers and conversion specifiers defined by ANSI C do not make sense (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 modifiers 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 4.4BSD, as it may be used in float conversions equivalently to L.
CATEGORY