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       sscanf, vsscanf - input string format conversion


       Standard C library (libc, -lc)


       #include <stdio.h>

       int sscanf(const char *restrict str,
                  const char *restrict format, ...);

       #include <stdarg.h>

       int vsscanf(const char *restrict str,
                  const char *restrict format, va_list ap);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L


       The sscanf() 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.

       sscanf() These functions read their input from the string pointed to by str.

       The vsscanf() function is analogous to vsprintf(3).

       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 sscanf() 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: sscanf() 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().

       •      For decimal conversions, an optional quote character (').  This specifies that  the
              input  number  may  include  thousands'  separators  as  defined  by the LC_NUMERIC
              category of the current locale.   (See  setlocale(3).)   The  quote  character  may
              precede or follow the '*' assignment-suppression character.

       •      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,  sscanf()  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
              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  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.

       •      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 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 or unsigned short (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 or unsigned long (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).  If used  with  %c  or  %s,  the  corresponding  parameter  is
              considered as a pointer to a wide character or wide-character string respectively.

       ll     (ell-ell)  Indicates  that  the conversion will be one of b, d, i, o, u, x, X, or n
              and the next pointer is a pointer to a long long or unsigned long long (rather than

       L      Indicates  that  the conversion will be either e, f, or g and the next pointer is a
              pointer to long double or (as a GNU extension) 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  initial  white  space  characters  are
              discarded), and assignment does not occur.

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

       i      Deprecated.  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      Deprecated.   Matches an unsigned octal integer; the next pointer must be a pointer
              to unsigned int.

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

       x      Deprecated.   Matches  an  unsigned  hexadecimal integer (that may optionally begin
              with a prefix of 0x or 0X, which is discarded); the next pointer must be a  pointer
              to unsigned int.

       X      Deprecated.  Equivalent to x.

       f      Deprecated.   Matches  an optionally signed floating-point number; the next pointer
              must be a pointer to float.

       e      Deprecated.  Equivalent to f.

       g      Deprecated.  Equivalent to f.

       E      Deprecated.  Equivalent to f.

       a      Deprecated.  (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 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
              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,  or
              variant whose size matches the (optionally) supplied integer length modifier.  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


       On  success,  these  functions  return  the number of input items successfully 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.


       EILSEQ Input byte sequence does not form a valid character.

       EINVAL Not enough arguments; or format is NULL.

       ENOMEM Out of memory.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue          │
       │sscanf(), vsscanf()                                     │ Thread safety │ MT-Safe locale │


       These functions conform to C99 and POSIX.1-2001.

       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 sscanf() allocate a buffer for
       a string, with a pointer to that buffer being returned in *buf:

           char *buf;
           sscanf(str, "%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 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
       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.).


   Numeric conversion specifiers
       Use of the numeric conversion specifiers produces Undefined Behavior  for  invalid  input.
       See  C11 ⟨⟩.  This is a
       bug in the ISO C standard, and not an  inherent  design  issue  with  the  API.   However,
       current  implementations are not safe from that bug, so it is not recommended to use them.
       Instead, programs should use functions such as strtol(3) to  parse  numeric  input.   This
       manual  page  deprecates  use of the numeric conversion specifiers until they are fixed by
       ISO C.

   Nonstandard modifiers
       These functions are fully C99 conformant, but provide the additional modifiers q and a  as
       well as an additional behavior of the L and ll modifiers.  The latter may be considered to
       be a bug, as it changes the behavior of modifiers defined in C99.

       Some combinations of the type modifiers and conversion specifiers defined by  C99  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 C99 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

           char *p;
           int n;

           errno = 0;
           n = sscanf(str, "%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 sscanf() call
       successfully read a string.


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