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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 <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(): _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
              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  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 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 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 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,
              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 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.

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  GNU  C  library  supports  a nonstandard 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.

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.

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

COLOPHON
       This  page  is  part of release 3.25 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)
 

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