Provided by: manpages-dev_6.8-2_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 ⟨https://port70.net/%7Ensz/c/c11/n1570.html#7.21.6.2p10⟩.  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)