#include <stdio.h> int fscanf(FILE *restrict stream, const char *restrict format, ...); int scanf(const char *restrict format, ...); int sscanf(const char *restrict s, const char *restrict format, ...);
The fscanf() function shall read from the named input stream. The scanf() function shall read from the standard input stream stdin. The sscanf() function shall read from the string s. Each function reads bytes, interprets them according to a format, and stores the results in its arguments. Each expects, as arguments, a control string format described below, and a set of pointer arguments indicating where the converted input should be stored. The result is undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments shall be evaluated but otherwise ignored.
Conversions can be applied to the nth argument after the format in the argument list, rather than to the next unused argument. In this case, the conversion specifier character % (see below) is replaced by the sequence "%n$", where n is a decimal integer in the range [1,{NL_ARGMAX}]. This feature provides for the definition of format strings that select arguments in an order appropriate to specific languages. In format strings containing the "%n$" form of conversion specifications, it is unspecified whether numbered arguments in the argument list can be referenced from the format string more than once.
The format can contain either form of a conversion specification---that is, % or "%n$"---but the two forms cannot be mixed within a single format string. The only exception to this is that %% or %* can be mixed with the "%n$" form. When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N-1)th, are pointers.
The fscanf() function in all its forms shall allow detection of a language-dependent radix character in the input string. The radix character is defined in the current locale (category LC_NUMERIC). In the POSIX locale, or in a locale where the radix character is not defined, the radix character shall default to a <period> ('.').
The format is a character string, beginning and ending in its initial shift state, if any, composed of zero or more directives. Each directive is composed of one of the following: one or more white-space characters (<space>, <tab>, <newline>, <vertical-tab>, or <form-feed>); an ordinary character (neither '%' nor a white-space character); or a conversion specification. Each conversion specification is introduced by the character '%' or the character sequence "%n$", after which the following appear in sequence:
The fscanf() functions shall execute each directive of the format in turn. If a directive fails, as detailed below, the function shall return. Failures are described as input failures (due to the unavailability of input bytes) or matching failures (due to inappropriate input).
A directive composed of one or more white-space characters shall be executed by reading input until no more valid input can be read, or up to the first byte which is not a white-space character, which remains unread.
A directive that is an ordinary character shall be executed as follows: the next byte shall be read from the input and compared with the byte that comprises the directive; if the comparison shows that they are not equivalent, the directive shall fail, and the differing and subsequent bytes shall remain unread. Similarly, if end-of-file, an encoding error, or a read error prevents a character from being read, the directive shall fail.
A directive that is a conversion specification defines a set of matching input sequences, as described below for each conversion character. A conversion specification shall be executed in the following steps.
Input white-space characters (as specified by isspace()) shall be skipped, unless the conversion specification includes a [, c, C, or n conversion specifier.
An item shall be read from the input, unless the conversion specification includes an n conversion specifier. An input item shall be defined as the longest sequence of input bytes (up to any specified maximum field width, which may be measured in characters or bytes dependent on the conversion specifier) which is an initial subsequence of a matching sequence. The first byte, if any, after the input item shall remain unread. If the length of the input item is 0, the execution of the conversion specification shall fail; this condition is a matching failure, unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.
Except in the case of a % conversion specifier, the input item (or, in the case of a %n conversion specification, the count of input bytes) shall be converted to a type appropriate to the conversion character. If the input item is not a matching sequence, the execution of the conversion specification fails; this condition is a matching failure. Unless assignment suppression was indicated by a '*', the result of the conversion shall be placed in the object pointed to by the first argument following the format argument that has not already received a conversion result if the conversion specification is introduced by %, or in the nth argument if introduced by the character sequence "%n$". If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the space provided, the behavior is undefined.
The
%c,
%s,
and
%[
conversion specifiers shall accept an optional assignment-allocation
character
'm',
which shall cause a memory buffer to be allocated to hold the string
converted including a terminating null character. In such a case,
the argument corresponding to the conversion specifier should be a
reference to a pointer variable that will receive a pointer to the
allocated buffer. The system shall allocate a buffer as if
malloc()
had been called. The application shall be responsible for freeing the
memory after usage. If there is insufficient memory to allocate a buffer,
the function shall set
errno
to
[ENOMEM]
and a conversion error shall result. If the function returns EOF, any
memory successfully allocated for parameters using assignment-allocation
character
'm'
by this call shall be freed before the function returns.
The length modifiers and their meanings are:
If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.
The following conversion specifiers are valid:
If the fprintf() family of functions generates character string representations for infinity and NaN (a symbolic entity encoded in floating-point format) to support IEEE Std 754-1985, the fscanf() family of functions shall recognize them as input.
If an l (ell) qualifier is present, the input is a sequence of characters that begins in the initial shift state. Each character shall be converted to a wide character as if by a call to the mbrtowc() function, with the conversion state described by an mbstate_t object initialized to zero before the first character is converted. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to an array of wchar_t large enough to accept the sequence and the terminating null wide character, which shall be added automatically. Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a wchar_t.
If an
l
(ell) qualifier is present, the input is a sequence of characters that
begins in the initial shift state. Each character in the sequence shall
be converted to a wide character as if by a call to the
mbrtowc()
function, with the conversion state described by an
mbstate_t
object initialized to zero before the first character is converted.
If the
'm'
assignment-allocation character is not specified, the application shall
ensure that the corresponding argument is a pointer to an array of
wchar_t
large enough to accept the sequence and the terminating null wide
character, which shall be added automatically.
Otherwise, the application shall ensure that the corresponding
argument is a pointer to a pointer to a
wchar_t.
The conversion specification includes all subsequent bytes in the format string up to and including the matching <right-square-bracket> (']'). The bytes between the square brackets (the scanlist) comprise the scanset, unless the byte after the <left-square-bracket> is a <circumflex> ('ha'), in which case the scanset contains all bytes that do not appear in the scanlist between the <circumflex> and the <right-square-bracket>. If the conversion specification begins with "[]" or "[ha]", the <right-square-bracket> is included in the scanlist and the next <right-square-bracket> is the matching <right-square-bracket> that ends the conversion specification; otherwise, the first <right-square-bracket> is the one that ends the conversion specification. If a '-' is in the scanlist and is not the first character, nor the second where the first character is a 'ha', nor the last character, the behavior is implementation-defined.
If an l (ell) qualifier is present, the input shall be a sequence of characters that begins in the initial shift state. Each character in the sequence is converted to a wide character as if by a call to the mbrtowc() function, with the conversion state described by an mbstate_t object initialized to zero before the first character is converted. No null wide character is added. If the 'm' assignment-allocation character is not specified, the application shall ensure that the corresponding argument is a pointer to an array of wchar_t large enough to accept the resulting sequence of wide characters. Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer to a wchar_t.
If a conversion specification is invalid, the behavior is undefined.
The conversion specifiers A, E, F, G, and X are also valid and shall be equivalent to a, e, f, g, and x, respectively.
If end-of-file is encountered during input, conversion shall be terminated. If end-of-file occurs before any bytes matching the current conversion specification (except for %n) have been read (other than leading white-space characters, where permitted), execution of the current conversion specification shall terminate with an input failure. Otherwise, unless execution of the current conversion specification is terminated with a matching failure, execution of the following conversion specification (if any) shall be terminated with an input failure.
Reaching the end of the string in sscanf() shall be equivalent to encountering end-of-file for fscanf().
If conversion terminates on a conflicting input, the offending input is left unread in the input. Any trailing white space (including <newline> characters) shall be left unread unless matched by a conversion specification. The success of literal matches and suppressed assignments is only directly determinable via the %n conversion specification.
The fscanf() and scanf() functions may mark the last data access timestamp of the file associated with stream for update. The last data access timestamp shall be marked for update by the first successful execution of fgetc(), fgets(), fread(), getc(), getchar(), getdelim(), getline(), gets(), fscanf(), or scanf() using stream that returns data not supplied by a prior call to ungetc().
In addition, the fscanf() function shall fail if:
In addition, the fscanf() function may fail if:
The following sections are informative.
int i, n; float x; char name[50];
n = scanf("%d%f%s", &i, &x, name);
with the input line:
25 54.32E-1 Hamster
assigns to n the value 3, to i the value 25, to x the value 5.432, and name contains the string "Hamster".
The call:
int i; float x; char name[50];
(void) scanf("%2d%f%*d %[0123456789]", &i, &x, name);
with input:
56789 0123 56a72
assigns 56 to i, 789.0 to x, skips 0123, and places the string "56\0" in name. The next call to getchar() shall return the character 'a'.
The following call uses fscanf() to read three floating-point numbers from standard input into the input array.
float input[3]; fscanf (stdin, "%f %f %f", input, input+1, input+2);
For functions that allocate memory as if by malloc(), the application should release such memory when it is no longer required by a call to free(). For fscanf(), this is memory allocated via use of the 'm' assignment-allocation character.
The Base Definitions volume of POSIX.1-2017, Chapter 7, Locale, <inttypes.h>, <langinfo.h>, <stdio.h>, <wchar.h>
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .