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FSCANF(3P)                POSIX Programmer's Manual               FSCANF(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       fscanf, scanf, sscanf — convert formatted input

SYNOPSIS         top

       #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         top

       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.1‐2008 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(3p)) 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         top

       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         top

       For the conditions under which the fscanf() functions fail and may
       fail, refer to fgetc(3p) or fgetwc(3p).

       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         top

       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         top

       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         top

       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         top

       None.

SEE ALSO         top

       Section 2.5, Standard I/O Streams, fprintf(3p), getc(3p),
       setlocale(3p), strtod(3p), strtol(3p), strtoul(3p), wcrtomb(3p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 7, Locale,
       langinfo.h(0p), stdio.h(0p), wchar.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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 .

IEEE/The Open Group                 2013                          FSCANF(3P)

Pages that refer to this page: stdio.h(0p)fgetc(3p)fgets(3p)fprintf(3p)fread(3p)localeconv(3p)scanf(3p)setlocale(3p)sscanf(3p)stdin(3p)strptime(3p)strtod(3p)strtol(3p)strtoul(3p)vfscanf(3p)wcstod(3p)wcstol(3p)wcstoul(3p)