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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 || _POSIX_C_SOURCE >= 200112L;
	   or cc -std=c99

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 snprintf() and vsnprintf() write at most size bytes (including the terminat-
       ing null byte ('\0')) to str.

       The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equivalent to  the  func-
       tions  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 undefined
       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 facili-
       ties 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 copying 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 conversion
       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  speci-
       fier.  By default, the arguments are used in the order given, where each '*' and each con-
       version 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 '$', which comes from the Single UNIX Speci-
       fication.  If the style using '$' 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  argu-
       ments  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  char-
       acter.  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  (nor-
	      mally,  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 boundary.  (The default is
	      right justification.)  Except for n conversions, 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)  pro-
	      duced by a signed conversion.

       +      A  sign  (+ or -) should always be placed before a number produced by a signed con-
	      version.	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 thou-
	      sands' grouping characters if the locale information 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 decimal 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 dec-
       imal  integer m) to specify that the precision is given in the next argument, or in the m-
       th argument, respectively, which must be of type int.  If the precision is given  as  just
       '.', the precision is taken to be zero.	A negative precision is taken as if the precision
       were omitted.  This gives the minimum number 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 argu-
	      ment, or a following n conversion corresponds to a pointer to a signed  char  argu-
	      ment.

       h      A  following  integer  conversion  corresponds to a short int or unsigned short int
	      argument, or a following n conversion corresponds to a pointer to a short int argu-
	      ment.

       l      (ell) A following integer conversion corresponds to a long int or unsigned long int
	      argument, or a following n conversion corresponds to a pointer to a long int  argu-
	      ment,  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 argu-
	      ment.  (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 argument.

       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 speci-
       fiers 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) notation.  The letters abcdef are used for x con-
	      versions;  the  letters  ABCDEF are used for X conversions.  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.

       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 precision; 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 conversion, and "[-]INF" or "[-]INFINITY" or "NAN*" in the case of  F  conver-
	      sion.)

       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 significant  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 hexadeci-
	      mal  notation (using the letters abcdef) in the style [-]0xh.hhhhp+-; for A conver-
	      sion 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 precision suffices  for	an  exact
	      representation  of the value if an exact representation in base 2 exists and other-
	      wise is sufficiently large to distinguish values of type double.	The digit  before
	      the  decimal point is unspecified for nonnormalized numbers, and nonzero but other-
	      wise unspecified for normalized numbers.

       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  wcr-
	        tomb(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 precision 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  con-
	      verted  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 character), up
	      to  and including a terminating null wide character.  The resulting multibyte char-
	      acters are written up to (but not including) the terminating null byte.  If a  pre-
	      cision  is  specified,  no more bytes than the number specified are written, but no
	      partial multibyte characters are written.  Note that the precision  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 integer indicated by the
	      int * (or variant) pointer argument.  No argument is converted.

       m      (Glibc extension.)  Print output of strerror(errno).  No argument is required.

       %      A '%' is written.  No argument is converted.  The complete conversion specification
	      is '%%'.

CONFORMING TO
       The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf()  functions  con-
       form 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 characters 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 unde-
       fined 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  care-
       ful  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 security 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 internationalized 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 3.55 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					    2013-09-04					  PRINTF(3)
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