# sscanf

## 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 **sscanf**().
    
           •      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).
           ┌───────────────────────────────┬───────────────┬────────────────┐
           │ **Interface** │ **Attribute** │ **Value** │
           ├───────────────────────────────┼───────────────┼────────────────┤
           │ **sscanf**(), **vsscanf**()           │ Thread safety │ MT-Safe locale │
           └───────────────────────────────┴───────────────┴────────────────┘
    


## STANDARDS



           C11, POSIX.1-2008.
    


## 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
           systems.  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.
    


* * *