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NAME

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

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 || _POSIX_C_SOURCE >= 200112L;
           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:

       ·      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.

       ·      An  ordinary  character  (i.e., one other than white space or '%').  This character
              must exactly match the next character of input.

       ·      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  character  '%'  or  the
       character sequence "%n$" (see below for the distinction) followed by:

       ·      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().

       ·      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() 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.

       ·      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 happens  first.   Most  conversions  discard  initial
              white  space  characters  (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.

       ·      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.

       ·      A conversion specifier that specifies the type of input conversion to be performed.

       The conversion specifications in format are of two forms, either  beginning  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
       '%'  specifications, then these correspond in order with successive pointer arguments.  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 argument following format.

   Conversions
       The following type modifier characters can appear in a conversion specification:

       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  initial  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 characters (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  characters  if  the  first  character  after the open bracket is a
              circumflex (^).  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 successfully 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).

       ┌─────────────────────┬───────────────┬────────────────┐
       │InterfaceAttributeValue          │
       ├─────────────────────┼───────────────┼────────────────┤
       │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 following 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 architectures.  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 4.04 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 http://www.kernel.org/doc/man-pages/.