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PRINTF(3)		   Linux Programmer's Manual		     PRINTF(3)

NAME
       printf,	 fprintf,  sprintf,  snprintf,	vprintf,  vfprintf,  vsprintf,
       vsnprintf - formatted output conversion

SYNOPSIS
       #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; or cc -std=c99

DESCRIPTION
       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 trailing 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
       Upon successful return, these functions return the number of characters
       printed	(not  including	 the  trailing	'\0'  used  to	end  output to
       strings).

       The functions snprintf()	and vsnprintf()	do not write  more  than  size
       bytes  (including  the trailing '\0').  If the output was truncated due
       to this limit then the return value is the number  of  characters  (not
       including the trailing '\0') which would	have been written to the final
       string if enough	space had been available.  Thus,  a  return  value  of
       size  or	 more  means  that  the	output was truncated.  (See also below
       under NOTES.)

       If an output error is encountered, a negative value is returned.

   Format of the format	string
       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);

       and

	   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
	      conversions, a nonzero result has	the string "0x"	(or "0X" for X
	      conversions) prepended to	it.  For a, A, e, E, f,	F,  g,	and  G
	      conversions,  the	 result	 will  always contain a	decimal	point,
	      even if no digits	follow it (normally, a decimal	point  appears
	      in  the  results	of those conversions only if a digit follows).
	      For g and	G conversions, trailing	zeros are not removed from the
	      result  as  they would otherwise be.  For	other conversions, the
	      result is	undefined.

       0      The value	should be zero padded.	For d, i, o, u,	x, X, a, A, e,
	      E,  f, F,	g, and G conversions, the converted value is padded on
	      the left with zeros rather than blanks.  If the 0	 and  -	 flags
	      both  appear,  the  0  flag is ignored.  If a precision is given
	      with a numeric conversion	(d, i, o, u, x,	and X),	the 0 flag  is
	      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  '.',  or  the precision is	negative, the precision	is taken to be
       zero.  This gives the minimum number of digits to appear	for d,	i,  o,
       u, x, and X conversions,	the number of digits to	appear after the radix
       character for a,	A, e, E, f, and	F conversions, the maximum  number  of
       significant  digits  for	 g and G conversions, or the maximum number of
       characters to be	printed	from a string for s and	S conversions.

   The length modifier
       Here, "integer conversion" stands for d,	i, o, u, x, or X conversion.

       hh     A	following integer conversion corresponds to a signed  char  or
	      unsigned	char argument, or a following n	conversion corresponds
	      to a pointer to a	signed char argument.

       h      A	following integer conversion corresponds to  a	short  int  or
	      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-
	      ment.

       The  SUSv2  only	knows about 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
	      empty.

       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
	      an explicit precision 0, the output is empty.

       e, E   The double argument  is  rounded	and  converted	in  the	 style
	      [-]d.ddde+-dd  where there is one	digit before the decimal-point
	      character	and the	number of digits after it is equal to the pre-
	      cision;  if  the	precision is missing, it is taken as 6;	if the
	      precision	is zero, no decimal-point  character  appears.	 An  E
	      conversion  uses	the  letter E (rather than e) to introduce the
	      exponent.	 The exponent always contains at least two digits;  if
	      the value	is zero, the exponent is 00.

       f, F   The double argument is rounded and converted to decimal notation
	      in the style [-]ddd.ddd, where the number	of  digits  after  the
	      decimal-point character is equal to the precision	specification.
	      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+-d; 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-
	      bers.

       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
	      (each by a call to the wcrtomb(3)	function,  with	 a  conversion
	      state  starting in the initial state before the first wide char-
	      acter), up to and	including a terminating	null  wide  character.
	      The  resulting  multibyte	 characters are	written	up to (but not
	      including) the terminating null byte.  If	a precision is	speci-
	      fied,  no	 more bytes than the number specified are written, but
	      no partial multibyte characters are written.  Note that the pre-
	      cision determines	the number of bytes written, not the number of
	      wide characters or screen	positions.  The	array must  contain  a
	      terminating null wide character, unless a	precision is given and
	      it is so small that the  number  of  bytes  written  exceeds  it
	      before the end of	the array is reached.

       C      (Not in C99, but in SUSv2.)  Synonym for lc.  Don't use.

       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 '%%'.

CONFORMING TO
       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.

NOTES
       Some programs imprudently rely on code such as the following

	   sprintf(buf,	"%s some further text",	buf);

       to append text to buf.  However,	the standards explicitly note that the
       results are undefined if	source and destination	buffers	 overlap  when
       calling	sprintf(), snprintf(), vsprintf(), and vsnprintf().  Depending
       on the version of gcc(1)	used, and the compiler options employed, calls
       such as the above will not produce the expected results.

       The  glibc  implementation  of the functions snprintf() and vsnprintf()
       conforms	to the C99 standard, that  is,	behaves	 as  described	above,
       since  glibc  version 2.1.  Until glibc 2.0.6 they would	return -1 when
       the output was truncated.

BUGS
       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.

EXAMPLE
       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>

       char *
       make_message(const char *fmt, ...)
       {
	   /* Guess we need no more than 100 bytes. */
	   int n, size = 100;
	   char	*p, *np;
	   va_list ap;

	   if ((p = malloc(size)) == NULL)
	       return NULL;

	   while (1) {
	       /* Try to print in the allocated	space. */
	       va_start(ap, fmt);
	       n = vsnprintf(p,	size, fmt, ap);
	       va_end(ap);
	       /* If that worked, return the string. */
	       if (n > -1 && n < size)
		   return p;
	       /* Else try again with more space. */
	       if (n > -1)    /* glibc 2.1 */
		   size	= n+1; /* precisely what is needed */
	       else	      /* glibc 2.0 */
		   size	*= 2;  /* twice	the old	size */
	       if ((np = realloc (p, size)) == NULL) {
		   free(p);
		   return NULL;
	       } else {
		   p = np;
	       }
	   }
       }

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

COLOPHON
       This page is part of release 3.25 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at http://www.kernel.org/doc/man-pages/.

GNU				  2008-12-19			     PRINTF(3)

NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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