Re: [PATCH] scanf.3, sscanf.3, vsscanf.3: Split the page, one for strings and one for FILEs

[Alejandro Colomar <alx.manpages@gmail.com>]

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sscanf(3)                  Library Functions Manual                  sscanf(3)

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 input according to format as de‐
        scribed  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  charac‐
        ter '%' 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’ separa‐
               tors as defined by the LC_NUMERIC category of  the  current  lo‐
               cale.   (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() allo‐
               cates 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 subse‐
               quently 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 hap‐
               pens first.  Most conversions discard initial white space  char‐
               acters  (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  conver‐
               sion to be performed.

        The conversion specifications in format are of two forms, either begin‐
        ning  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 '%' specifi‐
        cations, then these correspond in order with successive  pointer  argu‐
        ments.   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 argu‐
        ment following format.

    Conversions
        The following type modifier characters can appear in a conversion spec‐
        ification:

        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 un‐
               signed 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  ini‐
               tial  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 char‐
               acters (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  de‐
               fined 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 spe‐
               cial; when placed between two other characters, it adds all  in‐
               tervening  characters  to the set.  To include a hyphen, make it
               the last character before the  final  close  bracket.   For  in‐
               stance, [^]0-9-] means the set "everything except close bracket,
               zero  through  nine,  and hyphen".  The string ends with the ap‐
               pearance 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 * assign‐
               ment‐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 success‐
        fully 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.

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 attrib‐
        utes(7).
        ┌─────────────────────────────────────┬───────────────┬────────────────┐
        │Interface                            │ Attribute     │ Value          │
        ├─────────────────────────────────────┼───────────────┼────────────────┤
        │sscanf(), vsscanf()                  │ Thread safety │ MT‐Safe locale │
        └─────────────────────────────────────┴───────────────┴────────────────┘

STANDARDS
        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.

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 follow‐
        ing 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
        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  de‐
        fined  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  archi‐
        tectures.  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 (unreleased)        (date)                           sscanf(3)

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