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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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vscanf(), vsscanf(), vfscanf():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
DESCRIPTION
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 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 or unsigned short (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 or unsigned long (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.
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 (that may optionally begin with a prefix of 0x or 0X,
which is discarded); 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.
RETURN VALUE
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.
ERRORS
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.
ATTRIBUTES
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() │ │ │
└──────────────────────┴───────────────┴────────────────┘
CONFORMING TO
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.
NOTES
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.).
BUGS
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.
EXAMPLES
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.
SEE ALSO
getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
COLOPHON
This page is part of release 5.10 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 2020-08-13 SCANF(3)