fscanf - get input from a file
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conversion
#include <stdio.h>
int fscanf(FILE *stream, const char *format, ...);
Note that ...
represents zero or more additional arguments.
#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)):
This function “scans” a file for values of specified types. It expects as input the pointer to a FILE
that was returned by fopen, a “format string” that specifies what to expect, and zero or more subsequent arguments, each of which should be a location in memory. The format string should typically contain “conversion specifications,” placeholders that begin with %
that specify what types of values to expect. Subsequent arguments will be assigned those values. For instance, if n
is a int
, this function can get an int
from a user using %i
:
scanf("%i", &n);
Among this function’s supported conversion specifications are:
Conversion Specification | Type |
---|---|
%c |
char |
%f |
double |
%f |
float |
%i |
int |
%li |
long |
To get a single word (i.e., a sequence of non-whitespace characters), use %s
. But it is only safe to use this function to get a word from a file using %s
if that word is of some maximal length. For instance, if file
is a pointer to a FILE
that was returned by fopen and buffer
is an array of 3 bytes, this function could be user to get "hi"
, including its '\0'
, but not "hi!"
, as follows:
fscanf(file, "%s", buffer);
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
.
The following type modifier characters
can appear in a
conversion specification:
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
).
As for h, but the next pointer is a pointer to a
signed char
or unsigned char
.
As for h, but the next pointer is a pointer to an
intmax_t
or a uintmax_t
. This modifier was introduced
in C99.
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.
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
.
equivalent to L. This specifier does not exist in ANSI C.
As for h, but the next pointer is a pointer to a
ptrdiff_t
. This modifier was introduced in C99.
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.
Matches an optionally signed decimal integer; the next pointer must
be a pointer to int
.
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.
Matches an unsigned octal integer; the next pointer must be a pointer
to unsigned int
.
Matches an unsigned decimal integer; the next pointer must be a
pointer to unsigned int
.
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
.
Equivalent to x.
Matches an optionally signed floating-point number; the next pointer
must be a pointer to float
.
Equivalent to f.
Equivalent to f.
Equivalent to f.
(C99) Equivalent to f.
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.
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.
Matches a pointer value (as printed by %p in
printf(3)); the next pointer must be a pointer to a
pointer to void
.
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.
This function returns the number of arguments that were assigned values or EOF
, a constant defined in stdio.h
, if the end of the file has been reached.
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.
#include <stdio.h>
int main(void)
{
FILE *file = fopen("hi.txt", "r");
if (file != NULL)
{
char buffer[3];
fscanf(file, "%s", buffer);
fclose(file);
printf("%s\n", buffer);
}
}
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.
The file descriptor underlying stream
is marked nonblocking,
and the read operation would block.
The file descriptor underlying stream
is invalid, or not
open for reading.
Input byte sequence does not form a valid character.
The read operation was interrupted by a signal; see signal(7).
Not enough arguments; or format
is NULL.
Out of memory.
The result of an integer conversion would exceed the size that can be stored in the corresponding integer type.
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.
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