SCANF(3) Linux Programmer’s Manual SCANF(3)
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conver‐
sion
#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);
The scanf() family of functions scans input according to format as
described below. This format may contain conversion 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 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.
The format string consists of a sequence of directives which describe
how to process the sequence of input characters. 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 conversion fails
— this is a matching failure.
Each conversion specification in format begins with either the charac‐
ter ’%’ or the character sequence "%n$" (see below for the distinction)
followed by:
· An optional ’*’ assignment-suppression character: scanf() reads
input as directed by the conversion specification, but discards
the input. No corresponding pointer argument is required, and
this specification is not included in the count of successful
assignments returned by scanf().
· An optional ’a’ character. This is used with string conver‐
sions, and relieves the caller of the need to allocate a corre‐
sponding buffer to hold the input: instead, scanf() allocates a
buffer of sufficient size, and assigns the address of this
buffer to the corresponding pointer argument, which should be a
pointer to a char * variable (this variable does not need to be
initialised before the call). The caller should subsequently
free(3) this buffer when it is no longer required. This is a
GNU extension; C99 employs the ’a’ character as a conversion
specifier (and it can also be used as such in the GNU implemen‐
tation).
· An optional decimal integer which specifies the maximum field
width. Reading of characters stops either when this maximum is
reached or when a non-matching character is found, whichever
happens first. Most conversions discard initial whitespace
characters (the exceptions are noted below), and these discarded
characters don’t count towards the maximum field width. String
input conversions store a null terminator (’\0’) to mark the end
of the input; the maximum field width does not include this ter‐
minator.
· An optional type modifier character. For example, the l type
modifier is used with integer conversions such as %d to specify
that the corresponding pointer argument refers to a long int
rather than a pointer to an int.
· A conversion specifier that specifies the type of input conver‐
sion to be performed.
The conversion specifications in format are of two forms, either begin‐
ning with ’%’ or beginning with "%n$". The two forms should not be
mixed in the same format string, except that a string containing "%n$"
specifications can include %% and %*. If format contains ’%’ specifi‐
cations then these correspond in order with successive pointer argu‐
ments. In the "%n$" 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 argu‐
ment following format.
The following type modifier characters can appear in a conversion spec‐
ification:
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 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.
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 char‐
acters (no terminating 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 terminating 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 charac‐
ters if the first character after the open bracket is a circum‐
flex (^). 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 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 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.
These functions return the number of input items successfully matched
and assigned, which can be fewer than provided 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.
getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
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.
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 archi‐
tectures. 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.
LINUX MANPAGE 1995-11-01 SCANF(3)