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PRINTF(3)                  Linux Programmer's Manual                 PRINTF(3)



NAME
       printf(1,3,1 builtins),   fprintf,  sprintf,  snprintf,  vprintf,  vfprintf,  vsprintf,
       vsnprintf - formatted output conversion

SYNOPSIS
       #include <stdio.h>

       int printf(1,3,1 builtins)(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(1,8) the printf(1,3,1 builtins) family produce output according to a format
       as  described  below.  The functions printf(1,3,1 builtins) and vprintf write(1,2) output to
       stdout, the standard output stream; fprintf and vfprintf  write(1,2)  output
       to  the  given output stream; sprintf, snprintf, vsprintf and vsnprintf
       write(1,2) to the character string(3,n) str.

       The functions vprintf, vfprintf, vsprintf, vsnprintf are equivalent  to
       the  functions printf(1,3,1 builtins), 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(1,2) the output under the control of a format
       string(3,n) 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(1,2) 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(3,n) if(3,n) 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(8,n) is encountered, a negative  value  is
       returned.

   Format of the format string(3,n)
       The  format  string(3,n)  is a character string(3,n), beginning and ending in(1,8) its
       initial shift state, if(3,n) any.  The format string(3,n) 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(1,8) fetching zero or more subsequent arguments.  Each con-
       version(1,3,5) specification is introduced by the character %, and ends with a
       conversion  specifier.  In between there may be (in(1,8) 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(1,8)  the  order
       given,  where  each `*' and each conversion specifier asks for the next
       argument (and it is an  error(8,n)  if(3,n)  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(1,8) the argument list of the desired argument, indexed starting
       from 1. Thus,
                   printf(1,3,1 builtins)("%*d", width, num);
       and
                   printf(1,3,1 builtins)("%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(1,8)  the
       numbers  of  arguments specified using `$'; for example, if(3,n) arguments 1
       and 3 are specified, argument 2 must also be specified somewhere in(1,8) 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(3,5,7) uses `.'
       as radix character, and does not have a grouping character.  Thus,
                   printf(1,3,1 builtins)("%'.2f", 1234567.89);
       results in(1,8) `1234567.89' in(1,8) the POSIX locale(3,5,7),  in(1,8)  `1234567,89'  in(1,8)  the
       nl_NL locale(3,5,7), and in(1,8) `1.234.567,89' in(1,8) 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(3,n)  is  made
              zero (by prefixing a 0 if(3,n) it was not zero already).  For x and X
              conversions, a non-zero result has the string(3,n) `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(3,n)  no digits follow it (normally, a decimal point appears
              in(1,8) the results of those conversions only if(3,n)  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(3,n) both are given.

       ' '    (a space) A blank should be left before a  positive  number  (or
              empty string(3,n)) produced by a signed conversion.

       +      A  sign  (+ or -) always be placed before a number produced by a
              signed conversion.  By default a sign is used only for  negative
              numbers. A + overrides a space if(3,n) both are used.

       The  five  flag  characters  above  are defined in(1,8) 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(3,n) the locale(3,5,7) infor-
              mation indicates any.  Note that many  versions  of  gcc  cannot
              parse  this  option  and  will  issue a warning.  SUSv2 does not
              include %'F.

       glibc 2.2 adds one further flag character.

       I      For decimal integer conversion (i, d, u)  the  output  uses  the
              locale(3,5,7)'s  alternative output digits, if(3,n) any.  For example, since
              glibc 2.2.3 this will give Arabic-Indic digits  in(1,8)  the  Persian
              (`fa_IR') locale.

   The field width
       An  optional decimal digit string(3,n) (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(3,n) the left-adjustment flag has been given).  Instead of a deci-
       mal  digit  string(3,n) one may write(1,2) `*' or `*m$' (for some decimal integer
       m) to specify that the field width is given in(1,8) the next argument, or in(1,8)
       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(3,n) 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(1,8) the form of a period (`.')  followed by an
       optional decimal digit string.  Instead of a decimal digit  string(3,n)  one
       may write(1,2) `*' or `*m$' (for some decimal integer m) to specify that the
       precision is given in(1,8) the next  argument,  or  in(1,8)  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(3,n) for s and S conversions.

   The length modifier
       Here, `integer conversion' stands for d, i, o, u, x, or X conversion.

       hh     A  following  integer conversion corresponds to a signed char or
              unsigned char argument, or a following n conversion  corresponds
              to a pointer to a signed char argument.

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

       l      (ell)  A  following integer conversion corresponds to a long int
              or unsigned long int argument, or a following n conversion  cor-
              responds  to  a pointer to a long int argument, or a following c
              conversion corresponds to a wint_t argument, or  a  following  s
              conversion corresponds to a pointer to wchar_t argument.

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

       L      A following a, A, e, E, f, F, g, or G conversion corresponds  to
              a long double argument.  (C99 allows %LF, but SUSv2 does not.)

       q      (`quad'.  BSD  4.4  and Linux libc5 only. Don't use.)  This is a
              synonym for ll.

       j      A following integer conversion corresponds  to  an  intmax_t  or
              uintmax_t argument.

       z      A  following  integer  conversion  corresponds  to  a  size_t or
              ssize_t argument. (Linux libc5 has Z with  this  meaning.  Don't
              use it.)

       t      A  following integer conversion corresponds to a ptrdiff_t argu-
              ment.

       The SUSv2 only knows about the length modifiers h (in(1,8) hd, hi,  ho,  hx,
       hX, hn) and l (in(1,8) ld(1,8), li, lo, lx, lX, ln, lc, ls) and L (in(1,8) 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(3,n) any, gives the minimum number of digits that  must
              appear;  if(3,n)  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(3,n) any,
              gives the minimum number of digits that must appear; if(3,n) 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(1,8) the style
              [-]d.dddedd where there is one digit before  the  decimal-point
              character and the number of digits after it is equal to the pre-
              cision; if(3,n) the precision is missing, it is taken as  6;  if(3,n)  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(3,n)
              the value is zero, the exponent is 00.

       f,F    The double argument is rounded and converted to decimal notation
              in(1,8)  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(3,n) 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(3,n)
              representations for infinity and NaN may be made available.  The
              C99  standard  specifies `[-]inf' or `[-]infinity' for infinity,
              and a string(3,n) starting with `nan' for NaN, in(1,8) the case of f  con-
              version(1,3,5),  and `[-]INF' or `[-]INFINITY' or `NAN*' in(1,8) the case of
              F conversion.)

       g,G    The double argument is converted in(1,8) 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(3,n)  the  precision is zero, it is treated as 1.  Style e is used
              if(3,n) the exponent from its conversion is less(1,3) 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(3,n) it is followed by at least one digit.

       a,A    (C99;  not  in(1,8)  SUSv2)  For a conversion, the double argument is
              converted to hexadecimal notation (using the letters abcdef)  in(1,8)
              the  style  [-]0xh.hhhhpd;  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(3,n) an
              exact representation in(1,8) base 2 exists and  otherwise  is  suffi-
              ciently  large  to distinguish values of type double.  The digit
              before the decimal point is unspecified for non-normalized  num-
              bers,  and nonzero but otherwise unspecified for normalized num-
              bers.

       c      If no l modifier is present, the int argument is converted to an
              unsigned  char, and the resulting character is written.  If an l
              modifier is present, the wint_t  (wide  character)  argument  is
              converted to a multibyte sequence by a call to the wcrtomb func-
              tion, with a conversion state starting in(1,8) the initial state, and
              the resulting multibyte string(3,n) 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(3,n)).  Characters from the array are written up to (but
              not including) a terminating NUL character; if(3,n)  a  precision  is
              specified,  no more than the number specified are written.  If a
              precision is given, no null character need be  present;  if(3,n)  the
              precision  is  not specified, or is greater than the size of the
              array, the array must contain a terminating NUL character.

              If an l modifier is present: The const  wchar_t  *  argument  is
              expected  to  be a pointer to an array of wide characters.  Wide
              characters from the array are converted to multibyte  characters
              (each by a call to the wcrtomb function, with a conversion state
              starting in(1,8) the initial state before the first wide  character),
              up  to  and  including  a  terminating  null wide character. The
              resulting multibyte  characters  are  written  up  to  (but  not
              including)  the  terminating null byte. If a precision is speci-
              fied, no more bytes than the number specified are  written,  but
              no  partial multibyte characters are written. Note that the pre-
              cision determines the number of bytes written, not the number of
              wide  characters  or screen positions.  The array must contain a
              terminating null wide character, unless a precision is given and
              it  is  so  small  that  the  number of bytes written exceeds it
              before the end of the array is reached.

       C      (Not in(1,8) C99, but in(1,8) SUSv2.)  Synonym for lc.  Don't use.

       S      (Not in(1,8) C99, but in(1,8) SUSv2.)  Synonym for ls.  Don't use.

       p      The void * pointer argument is printed in(1,8) hexadecimal (as if(3,n)  by
              %#x or %#lx).

       n      The number of characters written so far is stored into the inte-
              ger indicated by the int * (or variant)  pointer  argument.   No
              argument is converted.

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


EXAMPLES
       To print pi to five decimal places:
              #include <math.h>
              #include <stdio.h>
              fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

       To print a date and time(1,2,n) in(1,8) 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(1,3,5) must be able to print the arguments in(1,8) an order  specified
       by the format:
              #include <stdio.h>
              fprintf(stdout, format,
                   weekday, month, day, hour, min);
       where format depends on locale(3,5,7), 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(3,n) 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(3,n) ((p = malloc (size)) == NULL)
                    return NULL;
                 while (1) {
                    /* Try to print in(1,8) the allocated space. */
                    va_start(ap, fmt);
                    n = vsnprintf (p, size, fmt, ap);
                    va_end(ap);
                    /* If that worked, return the string. */
                    if(3,n) (n > -1 && n < size)
                       return p;
                    /* Else try again with more space. */
                    if(3,n) (n > -1)    /* glibc 2.1 */
                       size = n+1; /* precisely what is needed */
                    else           /* glibc 2.0 */
                       size *= 2;  /* twice the old size */
                    if(3,n) ((p = realloc (p, size)) == NULL)
                       return NULL;
                 }
              }


NOTES
       The  glibc  implementation of the functions snprintf and vsnprintf con-
       forms to the C99 standard, i.e.,  behaves  as  described  above,  since
       glibc version(1,3,5) 2.1. Until glibc 2.0.6 they would return -1 when the out-
       put was truncated.

CONFORMING TO
       The fprintf, printf(1,3,1 builtins), 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(1,3) than 1, while C99 allows
       str to be NULL in(1,8) this case, and gives the return value (as always)  as
       the  number of characters that would have been written in(1,8) case the out-
       put string(3,n) 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(1,8),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(3,5,7), %m$ and *m$.  It knows about the  length  modifiers  h,l,L,Z,q,
       but  accepts  L  and q both for long doubles and for long long integers
       (this is a bug).  It no longer recognizes FDOU, but adds a new  conver-
       sion character m, which outputs strerror(errno).

       glibc 2.0 adds conversion characters C and S.

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

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

HISTORY
       Unix  V7  defines  the three routines printf(1,3,1 builtins), 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(1,8),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(1,n) descriptor instead of a stream.

BUGS
       Because sprintf and vsprintf assume an arbitrarily long string(3,n), callers
       must  be careful not to overflow the actual space; this is often impos-
       sible to assure. Note that  the  length  of  the  strings  produced  is
       locale-dependent  and difficult to predict.  Use snprintf and vsnprintf
       instead (or asprintf and vasprintf).

       Linux libc4.[45] does not have a snprintf, but provides a  libbsd  that
       contains  an snprintf equivalent to sprintf, i.e., one that ignores the
       size argument.  Thus, the use of snprintf with  early  libc4  leads  to
       serious security problems.

       Code  such as printf(1,3,1 builtins)(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(1,3,1 builtins) call to write(1,2) to memory and creating a security
       hole.


SEE ALSO
       printf(1,3,1 builtins)(1), asprintf(3), dprintf(3), scanf(3),  wcrtomb(3),  wprintf(3),
       locale(3,5,7)(5)



Linux Manpage                     2000-10-16                         PRINTF(3)

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