Provided by: manpages-dev_6.7-2_all bug

NAME

       sscanf, vsscanf - input string format conversion

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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)):

       vsscanf():
           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

DESCRIPTION

       The  sscanf()  family  of  functions  scans formatted 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 required.

       •      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.

   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 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 int).

       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      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 begin 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  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 used.

RETURN VALUE

       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.

ERRORS

       EILSEQ Input byte sequence does not form a valid character.

       EINVAL Not enough arguments; or format is NULL.

       ENOMEM Out of memory.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).
       ┌──────────────────────────────────────────────────────────────────────┬───────────────┬────────────────┐
       │ InterfaceAttributeValue          │
       ├──────────────────────────────────────────────────────────────────────┼───────────────┼────────────────┤
       │ sscanf(), vsscanf()                                                  │ Thread safety │ MT-Safe locale │
       └──────────────────────────────────────────────────────────────────────┴───────────────┴────────────────┘

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       C89, 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.

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  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.).

BUGS

   Numeric conversion specifiers
       Use of the numeric conversion  specifiers  produces  Undefined  Behavior  for  invalid  input.   See  C11
       7.21.6.2/10.   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.  Alternatively, mitigate
       it by specifying a maximum field width.

   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.

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 = sscanf(str, "%m[a-z]", &p);
           if (n == 1) {
               printf("read: %s\n", p);
               free(p);
           } else if (errno != 0) {
               perror("sscanf");
           } 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.

SEE ALSO

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

Linux man-pages 6.7                                2023-12-09                                          sscanf(3)