fscanf(3p) — Linux manual page

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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.1‐2017 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(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.1‐2017, Chapter 7, Locale,
       inttypes.h(0p), 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 1003.1-2017, 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 .

IEEE/The Open Group               2017                        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)