vsnprintf - formatted output conversion

       #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; or cc -std=c99

       The functions in the printf() family produce output according to a for-
       mat 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 trailing null byte ('\0')) to 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.  Because they invoke the va_arg macro, the value of ap is  unde-
       fined 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 facilities 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 copy-
       ing 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  (not  including  the  trailing  '\0'  used  to  end  output to
       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);


           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 argu-
       ment 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
              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  con-
              versions,  the  converted  value  is  padded  on  the right with
              blanks, rather than on the left with blanks or zeros.  A - over-
              rides 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 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 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
              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-

       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

       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
              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-

       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(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  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(3) 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.

       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  con-
              version specification is '%%'.

       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 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  charac-
       ters a and A.

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

       Some programs imprudently rely on code such as the following

           sprintf(buf, "%s some further text", buf);

       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(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  secu-
       rity hole.

       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, ...)
               n = vsnprintf(p, size, fmt, 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) {
                   return NULL;
               } else {
                   p = np;

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

       This  page  is  part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

GNU                               2008-12-19                         PRINTF(3)
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