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NAME

       fscanf, scanf, sscanf — convert formatted input

SYNOPSIS

       #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, ...);

DESCRIPTION

The functionality described on this reference page is aligned with the ISO C standard. Any conflict
between the requirements described here and the ISO C standard is unintentional. This volume of
POSIX.1‐2017 defers to the ISO C standard.

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:

* An optional assignment-suppressing character '*'.

* An optional non-zero decimal integer that specifies the maximum field width.

* An optional assignment-allocation character 'm'.

* An option length modifier that specifies the size of the receiving object.

* A conversion specifier character that specifies the type of conversion to be applied. The valid
conversion specifiers are described below.

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:

hh Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to signed char or unsigned char.

h Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to short or unsigned short.

l (ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to long or unsigned long; that a following a, A, e, E, f, F, g, or G conversion
specifier applies to an argument with type pointer to double; or that a following c, s, or [
conversion specifier applies to an argument with type pointer to wchar_t. If the 'm' assignment-
allocation character is specified, the conversion applies to an argument with the type pointer to
a pointer to wchar_t.

ll (ell-ell)
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to long long or unsigned long long.

j Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to intmax_t or uintmax_t.

z Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to size_t or the corresponding signed integer type.

t Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument
with type pointer to ptrdiff_t or the corresponding unsigned type.

L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument
with type pointer to long double.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is
undefined.

The following conversion specifiers are valid:

d Matches an optionally signed decimal integer, whose format is the same as expected for the
subject sequence of strtol() with the value 10 for the base argument. In the absence of a size
modifier, the application shall ensure that the corresponding argument is a pointer to int.

i Matches an optionally signed integer, whose format is the same as expected for the subject
sequence of strtol() with 0 for the base argument. In the absence of a size modifier, the
application shall ensure that the corresponding argument is a pointer to int.

o Matches an optionally signed octal integer, whose format is the same as expected for the subject
sequence of strtoul() with the value 8 for the base argument. In the absence of a size modifier,
the application shall ensure that the corresponding argument is a pointer to unsigned.

u Matches an optionally signed decimal integer, whose format is the same as expected for the
subject sequence of strtoul() with the value 10 for the base argument. In the absence of a size
modifier, the application shall ensure that the corresponding argument is a pointer to unsigned.

x Matches an optionally signed hexadecimal integer, whose format is the same as expected for the
subject sequence of strtoul() with the value 16 for the base argument. In the absence of a size
modifier, the application shall ensure that the corresponding argument is a pointer to unsigned.

a, e, f, g
Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as
expected for the subject sequence of strtod(). In the absence of a size modifier, the
application shall ensure that the corresponding argument is a pointer to float.

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.

s Matches a sequence of bytes that are not white-space characters. If the 'm' assignment-allocation
character is not specified, the application shall ensure that the corresponding argument is a
pointer to the initial byte of an array of char, signed char, or unsigned char large enough to
accept the sequence and a terminating null character code, which shall be added automatically.
Otherwise, the application shall ensure that the corresponding argument is a pointer to a pointer
to a char.

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.

[ Matches a non-empty sequence of bytes from a set of expected bytes (the scanset). The normal
skip over white-space characters shall be suppressed in this case. If the 'm' assignment-
allocation character is not specified, the application shall ensure that the corresponding
argument is a pointer to the initial byte of an array of char, signed char, or unsigned char
large enough to accept the sequence and a terminating null byte, which shall be added
automatically. Otherwise, the application shall ensure that the corresponding argument is a
pointer to a pointer to a char.

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>
('^'), 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 "[^]", 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 '^', nor the last character, the behavior is implementation-defined.

c Matches a sequence of bytes of the number specified by the field width (1 if no field width is
present in the conversion specification). No null byte is added. The normal skip over white-space
characters shall be suppressed in this case. If the 'm' assignment-allocation character is not
specified, the application shall ensure that the corresponding argument is a pointer to the
initial byte of an array of char, signed char, or unsigned char large enough to accept the
sequence. Otherwise, the application shall ensure that the corresponding argument is a pointer
to a pointer to a char.

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.

p Matches an implementation-defined set of sequences, which shall be the same as the set of
sequences that is produced by the %p conversion specification of the corresponding fprintf()
functions. The application shall ensure that the corresponding argument is a pointer to a pointer
to void. The interpretation of the input item is implementation-defined. If the input item is a
value converted earlier during the same program execution, the pointer that results shall compare
equal to that value; otherwise, the behavior of the %p conversion specification is undefined.

n No input is consumed. The application shall ensure that the corresponding argument is a pointer
to the integer into which shall be written the number of bytes read from the input so far by this
call to the fscanf() functions. Execution of a %n conversion specification shall not increment
the assignment count returned at the completion of execution of the function. No argument shall
be converted, but one shall be consumed. If the conversion specification includes an assignment-
suppressing character or a field width, the behavior is undefined.

C Equivalent to lc.

S Equivalent to ls.

% Matches a single '%' character; no conversion or assignment occurs. The complete conversion
specification shall be %%.

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().

RETURN VALUE

       Upon  successful completion, these functions shall return the number of successfully matched and assigned
       input items; this number can be zero in the event of an early matching failure. If the input ends  before
       the first conversion (if any) has completed, and without a matching failure having occurred, EOF shall be
       returned. If an error occurs before the first conversion (if any) has completed, and without  a  matching
       failure  having  occurred, EOF shall be returned and errno shall be set to indicate the error.  If a read
       error occurs, the error indicator for the stream shall be set.

ERRORS

       For the conditions under which the fscanf() functions fail and may fail, refer to fgetc() or fgetwc().

       In addition, the fscanf() function shall fail if:

       EILSEQ Input byte sequence does not form a valid character.

       ENOMEM Insufficient storage space is available.

       In addition, the fscanf() function may fail if:

       EINVAL There are insufficient arguments.

       The following sections are informative.

EXAMPLES

       The call:

           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'.

   Reading Data into an Array
       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);

APPLICATION USAGE

       If  the  application calling fscanf() has any objects of type wint_t or wchar_t, it must also include the
       <wchar.h> header to have these objects defined.

       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.

RATIONALE

       This function is aligned with the ISO/IEC 9899:1999 standard, and in doing so a  few  ``obvious''  things
       were  not  included.  Specifically,  the set of characters allowed in a scanset is limited to single-byte
       characters.  In other similar places, multi-byte characters have been permitted, but for  alignment  with
       the  ISO/IEC 9899:1999  standard,  it  has  not  been  done here. Applications needing this could use the
       corresponding wide-character functions to achieve the desired results.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical  and  Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       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 .