SCANF(3)



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():
           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

DESCRIPTION
       The scanf() family of functions scans input according to format as  de-
       scribed  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      For decimal conversions, an optional quote character (').   This
              specifies  that  the input number may include thousands' separa-
              tors as defined by the LC_NUMERIC category of  the  current  lo-
              cale.   (See  setlocale(3).)  The quote character may precede or
              follow the '*' assignment-suppression character.

       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.

   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 un-
              signed 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 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.

       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 (that may optionally be-
              gin  with  a  prefix  of 0x or 0X, which is discarded); 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  de-
              fined 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  spe-
              cial;  when placed between two other characters, it adds all in-
              tervening characters to the set.  To include a hyphen,  make  it
              the  last  character  before  the  final close bracket.  For in-
              stance, [^]0-9-] means the set "everything except close bracket,
              zero  through  nine,  and hyphen".  The string ends with the ap-
              pearance 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 as-
              signment 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  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
       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 at-
       tributes(7).

       +---------------------+---------------+----------------+
       |Interface            | Attribute     | Value          |
       +---------------------+---------------+----------------+
       |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 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.).

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  de-
       fined  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.

EXAMPLES
       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 5.07 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
       https://www.kernel.org/doc/man-pages/.

GNU                               2020-04-11                          SCANF(3)

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