Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
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 ||
	   _POSIX_C_SOURCE >= 200112L;
	   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 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
       Upon successful return, these functions return the number of characters
       printed (excluding the null byte	used to	end output to strings).

       The  functions  snprintf()  and vsnprintf() do not write	more than size
       bytes (including	the terminating	null byte ('\0')).  If the output  was
       truncated  due  to  this	 limit	then the return	value is the number of
       characters (excluding the terminating null byte)	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 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
	      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+-;  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):

       If truncation occurs in glibc versions prior to 2.0.6, this is  treated
       as an error instead of being handled gracefully.

       #include	<stdio.h>
       #include	<stdlib.h>
       #include	<stdarg.h>

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

	       /* 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 */

	       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 3.53 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				  2013-03-05			     PRINTF(3)

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

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=sprintf&sektion=3&manpath=CentOS+7.1>

home | help