Provided by: manpages-dev_3.01-1_all bug

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;
       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  'a'  character.   This   is   used   with   string
              conversions,  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.
              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
              implementation).

       ·      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 whitespace
              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
              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 diouxX 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 diouxX 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 efg
              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 efg and the next
              pointer is a pointer to long double or the  conversion  will  be
              dioux 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,  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
              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 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
              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,
              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
              completion 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.

       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 POSX.1-2001.  These standard 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
       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 diouxX 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
       example:

           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
       specified),  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
       purpose   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 3.01 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/.