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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 for‐
       mat as described below. The functions printf() and vprintf() write out‐
       put 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 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. 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 trun‐
       cated due to this limit then the return value is the number of  charac‐
       ters (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 -) 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 non-zero 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'. 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 non-normalized  num‐
	      bers, and non-zero 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()
	      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() 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 conver‐
	      sion specification is `%%'.

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;
		    }
		 }
	      }

NOTES
       The glibc implementation of the functions  snprintf()  and  vsnprintf()
       conforms	 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	 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 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 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,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 2.9.1BSD, but 2.10BSD has the
       flags #, + and <space> and no longer  mentions  D,O,U,X.	  2.11BSD  has
       vprintf(),  vfprintf(),	vsprintf(),  and  warns	 not  to  use D,O,U,X.
       4.3BSD Reno has the flag 0, the length modifiers h and L, and the  con‐
       versions	 n,  p,	 E,  G, X (with current meaning) and deprecates D,O,U.
       4.4BSD 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
       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() 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 secu‐
       rity hole.

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

Linux Manpage			  2000-10-16			     PRINTF(3)
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