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

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

       #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

       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

       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 '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.

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

       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

       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

       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.

       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

       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.

       For   an   explanation   of   the  terms  used  in  this  section,  see

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

       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.

   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.).

       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.

       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);
           } else if (errno != 0) {
           } 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.

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

       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

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