scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conversion
#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.
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.
- 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 (), sscanf (), vscanf (), vsscanf (), vfscanf () |
Thread safety |
MT-Safe locale |
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.
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)