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           #include <stdio.h>
           int printf(const char *format, ...);
           int fprintf(FILE *stream, const char *format, ...);
           int sprintf(char *str, const char *format, ...);
           int snprintf(char *str, size_t size, const char *format, ...);
           #include <stdarg.h>
           int vprintf(const char *format, va_list ap);
           int vfprintf(FILE *stream, const char *format, va_list ap);
           int vsprintf(char *str, const char *format, va_list ap);
           int vsnprintf(char *str, size_t size, const char *format, va_list ap);
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
           snprintf(), vsnprintf():
               _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
               _POSIX_C_SOURCE >= 200112L;
               or cc -std=c99


           The functions in the printf() family produce output according to a for-
           mat as described below.  The functions  printf()  and  vprintf()  write
           output  to stdout, the standard output stream; fprintf() and vfprintf()
           write  output  to  the  given  output  stream;  sprintf(),  snprintf(),
           vsprintf() and vsnprintf() write to the character string str.
           The  functions  snprintf()  and  vsnprintf()  write  at most size bytes
           (including the terminating null byte ('\0')) to str.
           The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equiv-
           alent  to  the  functions  printf(),  fprintf(), sprintf(), snprintf(),
           respectively, except that they are called with a va_list instead  of  a
           variable  number  of arguments.  These functions do not call the va_end
           macro.  Because they invoke the va_arg macro, the value of ap is  unde-
           fined after the call.  See stdarg(3).
           These  eight  functions  write the output under the control of a format
           string that specifies how subsequent arguments (or  arguments  accessed
           via the variable-length argument facilities of stdarg(3)) are converted
           for output.
           C99 and POSIX.1-2001 specify that the results are undefined if  a  call
           to  sprintf(), snprintf(), vsprintf(), or vsnprintf() would cause copy-
           ing to take place between objects that overlap  (e.g.,  if  the  target
           string  array and one of the supplied input arguments refer to the same
           buffer).  See NOTES.
       Return value
           The  format  string  is a character string, beginning and ending in its
           initial shift state, if any.  The format string is composed of zero  or
           more   directives:  ordinary  characters  (not  %),  which  are  copied
           unchanged to the output stream; and conversion specifications, each  of
           which results in fetching zero or more subsequent arguments.  Each con-
           version specification is introduced by the character %, and ends with a
           conversion  specifier.  In between there may be (in this order) zero or
           more flags, an optional minimum field width, an optional precision  and
           an optional length modifier.
           The  arguments must correspond properly (after type promotion) with the
           conversion specifier.  By default, the arguments are used in the  order
           given,  where  each '*' and each conversion specifier asks for the next
           argument (and it is an  error  if  insufficiently  many  arguments  are
           given).   One  can  also specify explicitly which argument is taken, at
           each place where an argument is required, by writing "%m$"  instead  of
           '%'  and  "*m$" instead of '*', where the decimal integer m denotes the
           position in the argument list of the desired argument, indexed starting
           from 1.  Thus,
               printf("%*d", width, num);
               printf("%2$*1$d", width, num);
           are  equivalent.   The  second  style allows repeated references to the
           same argument.  The C99 standard does not include the style using  '$',
           which comes from the Single UNIX Specification.  If the style using '$'
           is used, it must be used throughout for all conversions taking an argu-
           ment  and  all  width and precision arguments, but it may be mixed with
           "%%" formats which do not consume an argument.  There may be no gaps in
           the numbers of arguments specified using '$'; for example, if arguments
           1 and 3 are specified, argument 2 must also be specified  somewhere  in
           the format string.
           For  some  numeric  conversions  a radix character ("decimal point") or
           thousands' grouping character  is  used.   The  actual  character  used
           depends  on  the  LC_NUMERIC part of the locale.  The POSIX locale uses
           '.' as radix character, and does not have a grouping character.  Thus,
                   printf("%'.2f", 1234567.89);
           results in "1234567.89" in the POSIX locale,  in  "1234567,89"  in  the
           nl_NL locale, and in "1.234.567,89" in the da_DK locale.
       The flag characters
           The character % is followed by zero or more of the following flags:
           #      The  value  should  be  converted to an "alternate form".  For o
                  conversions, the first character of the output  string  is  made
                  zero (by prefixing a 0 if it was not zero already).  For x and X
                  ignored.  For other conversions, the behavior is undefined.
           -      The converted value is to be left adjusted on the  field  bound-
                  ary.   (The  default is right justification.)  Except for n con-
                  versions, the converted  value  is  padded  on  the  right  with
                  blanks, rather than on the left with blanks or zeros.  A - over-
                  rides a 0 if both are given.
           ' '    (a space) A blank should be left before a  positive  number  (or
                  empty string) produced by a signed conversion.
           +      A sign (+ or -) should always be placed before a number produced
                  by a signed conversion.  By default a sign is used only for neg-
                  ative numbers.  A + overrides a space if both are used.
           The  five  flag  characters  above  are defined in the C standard.  The
           SUSv2 specifies one further flag character.
           '      For decimal conversion (i, d, u, f, F, g, G) the output is to be
                  grouped with thousands' grouping characters if the locale infor-
                  mation indicates any.  Note that many versions of gcc(1)  cannot
                  parse  this  option  and  will  issue a warning.  SUSv2 does not
                  include %'F.
           glibc 2.2 adds one further flag character.
           I      For decimal integer conversion (i, d, u)  the  output  uses  the
                  locale's  alternative output digits, if any.  For example, since
                  glibc 2.2.3 this will give Arabic-Indic digits  in  the  Persian
                  ("fa_IR") locale.
       The field width
           An  optional decimal digit string (with nonzero first digit) specifying
           a minimum field width.  If the converted  value  has  fewer  characters
           than  the  field  width,  it will be padded with spaces on the left (or
           right, if the left-adjustment flag has been given).  Instead of a deci-
           mal  digit  string one may write "*" or "*m$" (for some decimal integer
           m) to specify that the field width is given in the next argument, or in
           the m-th argument, respectively, which must be of type int.  A negative
           field width is taken as a '-' flag followed by a positive field  width.
           In  no case does a nonexistent or small field width cause truncation of
           a field; if the result of a conversion is wider than the  field  width,
           the field is expanded to contain the conversion result.
       The precision
           An  optional  precision,  in the form of a period ('.')  followed by an
           optional decimal digit string.  Instead of a decimal digit  string  one
           may write "*" or "*m$" (for some decimal integer m) to specify that the
           precision is given in the next  argument,  or  in  the  m-th  argument,
           respectively,  which must be of type int.  If the precision is given as
           just '.', the precision is taken to be zero.  A negative  precision  is
           taken  as if the precision were omitted.  This gives the minimum number
                  unsigned  short int argument, or a following n conversion corre-
                  sponds to a pointer to a short int argument.
           l      (ell) A following integer conversion corresponds to a  long  int
                  or  unsigned long int argument, or a following n conversion cor-
                  responds to a pointer to a long int argument, or a  following  c
                  conversion  corresponds  to  a wint_t argument, or a following s
                  conversion corresponds to a pointer to wchar_t argument.
           ll     (ell-ell).  A following integer conversion corresponds to a long
                  long  int  or  unsigned long long int argument, or a following n
                  conversion corresponds to a pointer to a long long int argument.
           L      A  following a, A, e, E, f, F, g, or G conversion corresponds to
                  a long double argument.  (C99 allows %LF, but SUSv2 does not.)
           q      ("quad". 4.4BSD and Linux libc5 only.  Don't use.)   This  is  a
                  synonym for ll.
           j      A  following  integer  conversion  corresponds to an intmax_t or
                  uintmax_t argument.
           z      A following  integer  conversion  corresponds  to  a  size_t  or
                  ssize_t  argument.  (Linux libc5 has Z with this meaning.  Don't
                  use it.)
           t      A following integer conversion corresponds to a ptrdiff_t  argu-
           The  SUSv2  knows about only the length modifiers h (in hd, hi, ho, hx,
           hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf,
           Lg, LG).
       The conversion specifier
           A  character  that specifies the type of conversion to be applied.  The
           conversion specifiers and their meanings are:
           d, i   The int argument is converted to signed decimal  notation.   The
                  precision,  if any, gives the minimum number of digits that must
                  appear; if the converted value  requires  fewer  digits,  it  is
                  padded  on  the  left  with  zeros.  The default precision is 1.
                  When 0 is printed with an explicit precision 0,  the  output  is
           o, u, x, X
                  The  unsigned  int  argument is converted to unsigned octal (o),
                  unsigned decimal (u), or unsigned hexadecimal (x  and  X)  nota-
                  tion.   The  letters abcdef are used for x conversions; the let-
                  ters ABCDEF are used for X conversions.  The precision, if  any,
                  gives the minimum number of digits that must appear; if the con-
                  verted value requires fewer digits, it is  padded  on  the  left
                  with zeros.  The default precision is 1.  When 0 is printed with
                  If the precision is missing, it is taken as 6; if the  precision
                  is  explicitly  zero,  no decimal-point character appears.  If a
                  decimal point appears, at least one digit appears before it.
                  (The SUSv2 does not know about F and says that character  string
                  representations for infinity and NaN may be made available.  The
                  C99 standard specifies "[-]inf" or "[-]infinity"  for  infinity,
                  and  a string starting with "nan" for NaN, in the case of f con-
                  version, and "[-]INF" or "[-]INFINITY" or "NAN*" in the case  of
                  F conversion.)
           g, G   The  double argument is converted in style f or e (or F or E for
                  G conversions).  The precision specifies the number of  signifi-
                  cant  digits.   If the precision is missing, 6 digits are given;
                  if the precision is zero, it is treated as 1.  Style e  is  used
                  if  the  exponent from its conversion is less than -4 or greater
                  than or equal to the precision.  Trailing zeros are removed from
                  the  fractional part of the result; a decimal point appears only
                  if it is followed by at least one digit.
           a, A   (C99; not in SUSv2) For a conversion,  the  double  argument  is
                  converted  to hexadecimal notation (using the letters abcdef) in
                  the style [-]0xh.hhhhp?; for A conversion  the  prefix  0X,  the
                  letters  ABCDEF, and the exponent separator P is used.  There is
                  one hexadecimal digit before the decimal point, and  the  number
                  of  digits after it is equal to the precision.  The default pre-
                  cision suffices for an exact representation of the value  if  an
                  exact  representation  in  base 2 exists and otherwise is suffi-
                  ciently large to distinguish values of type double.   The  digit
                  before  the  decimal point is unspecified for nonnormalized num-
                  bers, and nonzero but otherwise unspecified for normalized  num-
           c      If no l modifier is present, the int argument is converted to an
                  unsigned char, and the resulting character is written.  If an  l
                  modifier  is  present,  the  wint_t (wide character) argument is
                  converted to a multibyte sequence by a call  to  the  wcrtomb(3)
                  function, with a conversion state starting in the initial state,
                  and the resulting multibyte string is written.
           s      If no l modifier  is  present:  The  const  char *  argument  is
                  expected  to be a pointer to an array of character type (pointer
                  to a string).  Characters from the array are written up to  (but
                  not including) a terminating null byte ('\0'); if a precision is
                  specified, no more than the number specified are written.  If  a
                  precision  is given, no null byte need be present; if the preci-
                  sion is not specified, or is greater than the size of the array,
                  the array must contain a terminating null byte.
                  If  an  l  modifier  is present: The const wchar_t * argument is
                  expected to be a pointer to an array of wide  characters.   Wide
                  characters  from the array are converted to multibyte characters
           S      (Not in C99, but in SUSv2.)  Synonym for ls.  Don't use.
           p      The  void * pointer argument is printed in hexadecimal (as if by
                  %#x or %#lx).
           n      The number of characters written so far is stored into the inte-
                  ger  indicated  by  the int * (or variant) pointer argument.  No
                  argument is converted.
           m      (Glibc extension.)  Print output of strerror(errno).   No  argu-
                  ment is required.
           %      A  '%' is written.  No argument is converted.  The complete con-
                  version specification is '%%'.


           The  fprintf(),  printf(),  sprintf(),   vprintf(),   vfprintf(),   and
           vsprintf()  functions  conform  to  C89  and  C99.   The snprintf() and
           vsnprintf() functions conform to C99.
           Concerning the return value of snprintf(),  SUSv2  and  C99  contradict
           each other: when snprintf() is called with size=0 then SUSv2 stipulates
           an unspecified return value less than 1, while C99  allows  str  to  be
           NULL in this case, and gives the return value (as always) as the number
           of characters that would have been written in case  the  output  string
           has been large enough.
           Linux  libc4 knows about the five C standard flags.  It knows about the
           length modifiers h, l, L, and the conversions c, d, e, E, f, F,  g,  G,
           i,  n, o, p, s, u, x, and X, where F is a synonym for f.  Additionally,
           it accepts D, O, and U as synonyms for ld, lo, and lu.  (This  is  bad,
           and  caused  serious  bugs later, when support for %D disappeared.)  No
           locale-dependent radix character, no thousands' separator,  no  NaN  or
           infinity, no "%m$" and "*m$".
           Linux  libc5  knows  about  the  five  C standard flags and the ' flag,
           locale, "%m$" and "*m$".  It knows about the length modifiers h, l,  L,
           Z,  and  q,  but accepts L and q both for long double and for long long
           int (this is a bug).  It no longer recognizes F, D, O, and U, but  adds
           the conversion character m, which outputs strerror(errno).
           glibc 2.0 adds conversion characters C and S.
           glibc  2.1 adds length modifiers hh, j, t, and z and conversion charac-
           ters a and A.
           glibc 2.2 adds the conversion character F with C99 semantics,  and  the
           flag character I.


           Some programs imprudently rely on code such as the following


           Because sprintf() and vsprintf() assume  an  arbitrarily  long  string,
           callers must be careful not to overflow the actual space; this is often
           impossible to assure.  Note that the length of the strings produced  is
           locale-dependent   and   difficult  to  predict.   Use  snprintf()  and
           vsnprintf() instead (or asprintf(3) and vasprintf(3)).
           Linux libc4.[45] does not have a snprintf(), but provides a libbsd that
           contains  an  snprintf()  equivalent  to  sprintf(),  that is, one that
           ignores the size argument.  Thus, the  use  of  snprintf()  with  early
           libc4 leads to serious security problems.
           Code  such as printf(foo); often indicates a bug, since foo may contain
           a % character.  If foo comes from untrusted user input, it may  contain
           %n,  causing  the printf() call to write to memory and creating a secu-
           rity hole.


           To print Pi to five decimal places:
               #include <math.h>
               #include <stdio.h>
               fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));
           To print a date and time in the form "Sunday,  July  3,  10:02",  where
           weekday and month are pointers to strings:
               #include <stdio.h>
               fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
                       weekday, month, day, hour, min);
           Many  countries use the day-month-year order.  Hence, an international-
           ized version must be able to print the arguments in an order  specified
           by the format:
               #include <stdio.h>
               fprintf(stdout, format,
                       weekday, month, day, hour, min);
           where  format  depends  on locale, and may permute the arguments.  With
           the value:
               "%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
           one might obtain "Sonntag, 3. Juli, 10:02".
           To allocate a sufficiently large string and print into it (code correct
           for both glibc 2.0 and glibc 2.1):
           #include <stdio.h>
           #include <stdlib.h>
           #include <stdarg.h>
                   /* Try to print in the allocated space */
                   va_start(ap, fmt);
                   n = vsnprintf(p, size, fmt, ap);
                   /* Check error code */
                   if (n < 0) {
                       return NULL;
                   /* If that worked, return the string */
                   if (n < size)
                       return p;
                   /* Else try again with more space */
                   size = n + 1;       /* Precisely what is needed */
                   np = realloc(p, size);
                   if (np == NULL) {
                       return NULL;
                   } else {
                       p = np;
           If  truncation occurs in glibc versions prior to 2.0.6, this is treated
           as an error instead of being handled gracefully.


           printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcrtomb(3),
           wprintf(3), locale(5)

    GNU 2013-12-30 PRINTF(3)


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