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



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

SYNOPSIS
       #include <&lt;stdio.h>&gt;

       int scanf(const char *format, ...);
       int fscanf(FILE *stream, const char *format, ...);
       int sscanf(const char *str, const char *format, ...);

       #include <&lt;stdarg.h>&gt;

       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(): _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE;
       or cc -std=c99

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      An  optional  'a'  character.   This is used with string conver-
              sions, and relieves the caller of the need to allocate a  corre-
              sponding  buffer to hold the input: instead, scanf() allocates a
              buffer of sufficient size, and assigns the address of this  buf-
              fer  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 subsequently
              free(3) this buffer when it is no longer required.   This  is  a
              GNU  extension;  C99  employs  the 'a' character as a conversion
              specifier (and it can also be used as such in the GNU  implemen-
              tation).

       o      An  optional  decimal  integer which specifies the maximum field
              width.  Reading of characters stops either when this maximum  is
              reached  or  when  a  non-matching character is found, whichever
              happens first.  Most conversions  discard  initial  white  space
              characters (the exceptions are noted below), and these discarded
              characters don't count towards the maximum field width.   String
              input conversions store a null terminator ('\0') to mark the end
              of the input; the maximum field width does not include this ter-
              minator.

       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 con-
              sidered 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 backwards  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 character array that is long enough
              to hold the input sequence and the  terminating  null  character
              ('\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 non-empty 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,
              although it can be suppressed with the *  assignment-suppression
              character.   The  C  standard says: "Execution of a %n directive
              does not increment the assignment count returned at the  comple-
              tion of execution" but the Corrigendum seems to contradict this.
              Probably it is wise not to make any assumptions on the effect of
              %n conversions on the return value.

RETURN VALUE
       These  functions  return the number of input items successfully matched
       and assigned, which 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  indicate  the
       error.

ERRORS
       EAGAIN  The  file  descriptor underlying stream is marked non-blocking,
       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.

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.

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.

NOTES
       The  GNU  C  library  supports a non-standard extension that causes the
       library to dynamically allocate a string of sufficient size  for  input
       strings for the %s and %a[range] conversion specifiers.  To make use of
       this feature, specify a as a length modifier (thus %as  or  %a[range]).
       The  caller must free(3) the returned string, as in the following exam-
       ple:

           char *p;
           int n;

           errno = 0;
           n = scanf("%a[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 only necessary to call free(3)  if
       the scanf() call successfully read a string.

       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  speci-
       fied),  in which case the a is interpreted as a specifier for floating-
       point numbers (see above).

       Since version 2.7, glibc also provides the m modifier for the same pur-
       pose as the a modifier.  The m modifier has the following advantages:

       * 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.)

       * It is specified in the upcoming revision of the POSIX.1 standard.

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

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-07-12                          SCANF(3)