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SCANF(3) Linux Programmer's Manual SCANF(3)
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format con‐
version
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vscanf(), vsscanf(), vfscanf():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
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(3) and
vsprintf(3) 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 conversion specification, the
conversion 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 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().
· For decimal conversions, an optional quote character (').
This specifies that the input number may include thousands'
separators as defined by the LC_NUMERIC category of the
current locale. (See setlocale(3).) The quote character may
precede or follow the '*' assignment-suppression character.
· An optional 'm' character. This is used with string
conversions (%s, %c, %[), 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 initialized before the call). The caller
should subsequently free(3) this buffer when it is no longer
required.
· An optional decimal integer which specifies the maximum field
width. Reading of characters stops either when this maximum
is reached or when a nonmatching character is found, whichever
happens first. Most conversions discard initial white space
characters (the exceptions are noted below), and these
discarded characters don't count toward the maximum field
width. String input conversions store a terminating null byte
('\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 following format.
Conversions
The following type modifier characters can appear in a conversion
specification:
h Indicates that the conversion will be one of d, i, o, u, x, X,
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 an intmax_t or
a uintmax_t. This modifier was introduced in C99.
l Indicates either that the conversion will be one of d, i, o,
u, x, X, 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 e, f, or g 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 e, f, or g and
the next pointer is a pointer to long double or the conversion
will be d, i, o, u, or x 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
(but initial white space characters are discarded), 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 backward 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 the initial element of a
character array that is long enough to hold the input sequence
and the terminating null byte ('\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 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 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 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 and does not increase the count returned by the
function. The assignment can be suppressed with the *
assignment-suppression character, but the effect on the return
value is undefined. Therefore %*n conversions should not be
used.
On success, these functions return the number of input items
successfully matched and assigned; this 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 to
indicate the error.
EAGAIN The file descriptor underlying stream is marked nonblocking,
and the read operation would block.
EBADF The file descriptor underlying stream is invalid, or not open
for reading.
EILSEQ Input byte sequence does not form a valid character.
EINTR The read operation was interrupted by a signal; see signal(7).
EINVAL Not enough arguments; or format is NULL.
ENOMEM Out of memory.
ERANGE The result of an integer conversion would exceed the size that
can be stored in the corresponding integer type.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────┬───────────────┬────────────────┐
│Interface │ Attribute │ Value │
├─────────────────────┼───────────────┼────────────────┤
│scanf(), fscanf(), │ Thread safety │ MT-Safe locale │
│sscanf(), vscanf(), │ │ │
│vsscanf(), vfscanf() │ │ │
└─────────────────────┴───────────────┴────────────────┘
The functions fscanf(), scanf(), and sscanf() conform to C89 and C99
and POSIX.1-2001. These standards do not specify the ERANGE error.
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.
The 'a' assignment-allocation modifier
Originally, the GNU C library supported dynamic allocation for string
inputs (as a nonstandard extension) via the a character. (This
feature is present at least as far back as glibc 2.0.) Thus, one
could write the following to have scanf() allocate a buffer for an
input string, with a pointer to that buffer being returned in *buf:
char *buf;
scanf("%as", &buf);
The use of the letter a for this purpose was problematic, since a is
also specified by the ISO C standard as a synonym for f (floating-
point input). POSIX.1-2008 instead specifies the m modifier for
assignment allocation (as documented in DESCRIPTION, above).
Note that the a modifier is not available if the program is compiled
with gcc -std=c99 or gcc -D_ISOC99_SOURCE (unless _GNU_SOURCE is also
specified), in which case the a is interpreted as a specifier for
floating-point numbers (see above).
Support for the m modifier was added to glibc starting with version
2.7, and new programs should use that modifier instead of a.
As well as being standardized by POSIX, the m modifier has the
following further advantages over the use of a:
* It may also be applied to %c conversion specifiers (e.g., %3mc).
* It avoids ambiguity with respect to the %a floating-point
conversion specifier (and is unaffected by gcc -std=c99 etc.).
All functions are fully C89 conformant, but provide the additional
specifiers q and a as well as an additional behavior of the L and l
specifiers. The latter may be considered to be a bug, as it changes
the behavior 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 behavior 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, that is, use q instead of L in
combination with d, i, o, u, x, and X 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.
To use the dynamic allocation conversion specifier, specify m as a
length modifier (thus %ms or %m[range]). The caller must free(3) the
returned string, as in the following example:
char *p;
int n;
errno = 0;
n = scanf("%m[a-z]", &p);
if (n == 1) {
printf("read: %s\n", p);
free(p);
} else if (errno != 0) {
perror("scanf");
} else {
fprintf(stderr, "No matching characters\n");
}
As shown in the above example, it is necessary to call free(3) only
if the scanf() call successfully read a string.
getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-03-13 SCANF(3)
Pages that refer to this page: curs_scanw(3x), fgetc(3), getline(3), gets(3), lber-decode(3), pmextractvalue(3), printf(3), puts(3), strptime(3), proc(5), locale(7)