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SCANF(3) Linux Programmer's Manual SCANF(3)


     scanf,  fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conver-


     #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)):
     vscanf(), vsscanf(), vfscanf():
         _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L


     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
     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:
     o      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.
     o      An ordinary character (i.e., one other than white space or '%').
            This character must exactly match the next character of input.
     o      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 charac-
     ter '%' or the character sequence "%n$" (see below for the distinction)
     followed by:
     o      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().
     o      For  decimal conversions, an optional quote character (').  This
            specifies that the input number may include  thousands'  separa-
            tors  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.
     o      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() allo-
            cates 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 subse-
            quently free(3) this buffer when it is no longer required.
     o      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 hap-
            pens  first.  Most conversions discard initial white space char-
            acters (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.
     o      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  int
            rather than a pointer to an int.
     o      A  conversion specifier that specifies the type of input conver-
            sion to be performed.
     The conversion specifications in format are of two forms, either begin-
     ning  with  '%'  or  beginning with "%n$".  The two forms should not be
     mixed in the same format string, except that a string containing  "%n$"
     specifications  can include %% and %*.  If format contains '%' specifi-
     cations, then these correspond in order with successive  pointer  argu-
     ments.   In the "%n$" 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 argu-
     ment following format.
     The following type modifier characters can appear in a conversion spec-
     h      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  int  or
            unsigned short int (rather than int).
     hh     As  for h, but the next pointer is a pointer to a signed char or
            unsigned char.
     j      As for h, but the next pointer is a pointer to an intmax_t or  a
            uintmax_t.  This modifier was introduced in C99.
     l      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  int  or
            unsigned long int (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.
     L      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
     q      equivalent to L.  This specifier does not exist in ANSI C.
     t      As  for  h,  but  the  next pointer is a pointer to a ptrdiff_t.
            This modifier was introduced in C99.
     z      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  ini-
            tial  white space characters are discarded), and assignment does
            not occur.
     d      Matches an optionally signed decimal integer; the  next  pointer
            must be a pointer to int.
     D      Equivalent  to  ld; this exists only for backward compatibility.
            (Note: thus only in  libc4.   In  libc5  and  glibc  the  %D  is
            silently ignored, causing old programs to fail mysteriously.)
     i      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
     o      Matches  an  unsigned  octal integer; the next pointer must be a
            pointer to unsigned int.
     u      Matches an unsigned decimal integer; the next pointer must be  a
            pointer to unsigned int.
     x      Matches  an  unsigned hexadecimal integer; the next pointer must
            be a pointer to unsigned int.
     X      Equivalent to x.
     f      Matches an optionally signed  floating-point  number;  the  next
            pointer must be a pointer to float.
     e      Equivalent to f.
     g      Equivalent to f.
     E      Equivalent to f.
     a      (C99) Equivalent to f.
     s      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.
     c      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 char-
            acters  (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 charac-
            ters  if the first character after the open bracket is a circum-
            flex (^).  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.
     p      Matches a pointer value (as printed by %p in printf(3); the next
            pointer must be a pointer to a pointer to void.
     n      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.


     On success, these functions return the number of input  items  success-
     fully  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


     EAGAIN The file descriptor underlying stream is marked nonblocking, and
            the read operation would block.
     EBADF  The  file  descriptor  underlying stream is invalid, or not open
            for reading.
     EILSEQ Input byte sequence does not form a valid character.
     EINTR  The read operation was interrupted by a signal; see signal(7).
     EINVAL Not enough arguments; or format is NULL.
     ENOMEM Out of memory.
     ERANGE The result of an integer conversion would exceed the  size  that
            can be stored in the corresponding integer type.


     For   an   explanation   of   the  terms  used  in  this  section,  see
     allbox; lbw20 lb lb  l  l  l.   Interface Attribute Value  T{  scanf(),
     sscanf(), vscanf(),
     vsscanf(), vfscanf() T}   Thread safety  MT-Safe locale


     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.


 The 'a' assignment-allocation modifier
     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  follow-
     ing 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 archi-
     tectures.  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.


     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.


     getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)


     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at

GNU 2017-09-15 SCANF(3)

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