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

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

       #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  cor‐
       responding conversion specification.

       If  the  number of conversion specifications in format exceeds the number of pointer argu‐
       ments, 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 respec‐

       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  char‐
       acter 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 suc‐
              cessful 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

       ·      An optional decimal integer which specifies the maximum field  width.   Reading  of
              characters  stops either when this maximum is reached or when a nonmatching charac‐
              ter 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  ter‐
              minating  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.

       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  intro‐
              duced 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  dis‐
              carded), and assignment does not occur.

       d      Matches an optionally signed decimal integer; the next pointer must be a pointer to

       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

       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

       u      Matches an unsigned decimal integer; the next pointer must be a pointer to unsigned

       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 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  char‐
              acters;  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 char‐
              acters 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 charac‐
              ter - is also special; when placed between two other characters, it adds all inter‐
              vening 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.

       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  success‐
       ful 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.

       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.

       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  inte‐
       ger 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  non‐
       standard 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 inter‐
       preted 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  advan‐
       tages 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 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 equiv‐
       alently 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 exam‐

           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 3.74 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/.

GNU                                         2014-01-11                                   SCANF(3)

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