scanf(3)



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

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

SYNOPSIS
       #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

DESCRIPTION
       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
       respectively.

       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.

   Conversions
       The following type modifier characters can appear in a conversion spec-
       ification:

       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
              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
              used.

       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.

RETURN VALUE
       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
       error.

ERRORS
       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.

ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).

       +---------------------+---------------+----------------+
       |Interface            | Attribute     | Value          |
       +---------------------+---------------+----------------+
       |scanf(), fscanf(),   | Thread safety | MT-Safe locale |
       |sscanf(), vscanf(),  |               |                |
       |vsscanf(), vfscanf() |               |                |
       +---------------------+---------------+----------------+

CONFORMING TO
       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.

NOTES
   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.).

BUGS
       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.

EXAMPLE
       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.

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

COLOPHON
       This page is part of release 4.13 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
       https://www.kernel.org/doc/man-pages/.

GNU                               2017-09-15                          SCANF(3)

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