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



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 vari-
       able 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 correspond-
              ing  pointer  argument.  If the next item of input does not match the conversion specification, the con-
              version 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 specifi-
              cation 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 white space characters (the exceptions are noted below), and these discarded
              characters don't count towards the maximum field width.  String input conversions store a null  termina-
              tor ('\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$".
       The two forms should not be mixed in the same format string, except that a string containing  "%n$"  specifica-
       tions can include %% and %*.  If format contains '%' specifications then these correspond in order with succes-
       sive 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 con-
              version 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 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 termi-
              nating 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  interven-
              ing  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 direc-
              tive does not increment the assignment count returned at the completion of execution" but the  Corrigen-
              dum  seems to contradict this.  Probably it is wise not to make any assumptions on the effect of %n con-
              versions 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; 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 inte-
              ger type.

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 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.

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.

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-07-12                          SCANF(3)