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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);

DESCRIPTION
       The functions in	the printf family produce output according to a	format
       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 vprintf, vfprintf,	vsprintf, vsnprintf are	equivalent  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. Consequently,
       the value of ap is undefined after the  call.  The  application	should
       call va_end(ap) itself afterwards.

       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.

   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 argument
       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 non-zero 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 conver-
	      sions, the converted value is padded on the right	 with  blanks,
	      rather than on the left with blanks or zeros.  A - overrides 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 -)	always be placed before	a number produced by a
	      signed conversion.  By default a sign is used only for  negative
	      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	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,	Arabic
	      digits).	However, it does not include  any  locale  definitions
	      with such	outdigits defined.

   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 non-existent 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'.	BSD  4.4  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 non-normalized  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 func-
	      tion, 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 NUL character; if  a	 precision  is
	      specified,  no more than the number specified are	written.  If a
	      precision	is given, no null character need be  present;  if  the
	      precision	 is  not specified, or is greater than the size	of the
	      array, the array must contain a terminating NUL character.

	      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 function, with a conversion state
	      starting in the initial state before the first wide  character),
	      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.

       %      A	`%' is written.	No argument is converted. The complete conver-
	      sion specification is `%%'.

EXAMPLES
       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;
		 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 ((p = realloc (p, size))	== NULL)
		       return NULL;
		 }
	      }

NOTES
       The  glibc  implementation of the functions snprintf and	vsnprintf con-
       forms to	the C99	standard, i.e.,	 behaves  as  described	 above,	 since
       glibc version 2.1. Until	glibc 2.0.6 they would return -1 when the out-
       put was truncated.

CONFORMING TO
       The fprintf, printf, sprintf, vprintf, vfprintf,	and vsprintf functions
       conform	to  ANSI X3.159-1989 (``ANSI C'') and ISO/IEC 9899:1999	(``ISO
       C99'').	The  snprintf  and  vsnprintf  functions  conform  to  ISO/IEC
       9899:1999.

       Concerning the return value of snprintf,	the SUSv2 and the C99 standard
       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 out-
       put 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 cdeEfFgGinopsuxX, where F
       is a synonym for	f.  Additionally, it accepts  D,O,U  as	 synonyms  for
       ld,lo,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,q,
       but  accepts  L	and q both for long doubles and	for long long integers
       (this is	a bug).	 It no longer recognizes FDOU, but adds	a new  conver-
       sion character m, which outputs strerror(errno).

       glibc 2.0 adds conversion characters C and S.

       glibc 2.1 adds length modifiers hh,j,t,z	and conversion characters a,A.

       glibc 2.2 adds the conversion character F with C99 semantics,  and  the
       flag character I.

HISTORY
       Unix  V7	 defines  the three routines printf, fprintf, sprintf, and has
       the flag	-, the width or	precision *, the length	modifier  l,  and  the
       conversions  doxfegcsu,	and  also D,O,U,X as synonyms for ld,lo,lu,lx.
       This is still true for BSD 2.9.1, but BSD 2.10 has the flags #,	+  and
       <space>	and  no	 longer	 mentions  D,O,U,X.   BSD  2.11	 has  vprintf,
       vfprintf, vsprintf, and warns not to use	D,O,U,X.  BSD 4.3 Reno has the
       flag 0, the length modifiers h and L, and the conversions n, p, E, G, X
       (with current meaning) and deprecates D,O,U.  BSD  4.4  introduces  the
       functions  snprintf  and	vsnprintf, and the length modifier q.  FreeBSD
       also has	functions asprintf and vasprintf, that allocate	a buffer large
       enough  for sprintf.  In	glibc there are	functions dprintf and vdprintf
       that print to a file descriptor instead of a stream.

BUGS
       Because sprintf and vsprintf assume an arbitrarily long string, callers
       must  be	careful	not to overflow	the actual space; this is often	impos-
       sible to	assure.	Note that  the	length	of  the	 strings  produced  is
       locale-dependent	 and difficult to predict.  Use	snprintf and vsnprintf
       instead (or asprintf and	vasprintf).

       Linux libc4.[45]	does not have a	snprintf, but provides a  libbsd  that
       contains	 an snprintf equivalent	to sprintf, i.e., 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 security
       hole.

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

Linux Manpage			  2000-10-16			     PRINTF(3)

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | NOTES | CONFORMING TO | HISTORY | BUGS | SEE ALSO

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