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RedHat 9 (Linux i386) - man page for printf (redhat section 3)

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  facili-
       ties 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 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 `$', which comes from the Single Unix Specifica-
       tion.  If the style using `$' is used, it must be used throughout for all conversions tak-
       ing  an argument and all width and precision arguments, but it may be mixed with `%%' for-
       mats 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 conver-
	      sions, 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 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 -) 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 thou-
	      sands' grouping characters if the locale information 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 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 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 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
       `.', or the precision is negative, the precision is taken to be zero.  This gives the min-
       imum 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 num-
       ber 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'. 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 argument.

       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 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+-d; 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 non-normalized 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  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 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  con-
	      verted to multibyte characters (each by a call to the wcrtomb function, with a con-
	      version 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
	      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.

       %      A  `%'  is written. No argument is converted. The complete conversion 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 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):
	      #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 conforms to the C99 stan-
       dard, i.e., behaves as described above, since glibc version 2.1. Until  glibc  2.0.6  they
       would return -1 when the output 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
       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 a new con-
       version 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 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 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)


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