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)