vsnprintf(3)



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

NAME
       printf,   fprintf,   dprintf,  sprintf,  snprintf,  vprintf,  vfprintf,
       vdprintf, vsprintf, vsnprintf - formatted output conversion

SYNOPSIS
       #include <stdio.h>

       int printf(const char *format, ...);
       int fprintf(FILE *stream, const char *format, ...);
       int dprintf(int fd, 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 vdprintf(int fd, 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():
           _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE

       dprintf(), vdprintf():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _GNU_SOURCE

DESCRIPTION
       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 function dprintf() is the same as fprintf(3) except that it outputs
       to a file descriptor, fd, instead of to a stdio stream.

       The  functions  snprintf()  and  vsnprintf()  write  at most size bytes
       (including the terminating null byte ('\0')) to str.

       The   functions   vprintf(),   vfprintf(),   vdprintf(),    vsprintf(),
       vsnprintf()  are  equivalent  to  the  functions  printf(),  fprintf(),
       dprintf(), sprintf(), snprintf(), respectively, except  that  they  are
       called with a va_list instead of a variable number of arguments.  These
       functions do not call the va_end macro.  Because they invoke the va_arg
       macro, the value of ap is undefined after the call.  See stdarg(3).

       All  of  these 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.

   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 '*' (see Field width and Precision  below)  and  each
       conversion  specifier asks for the next argument (and it is an error if
       insufficiently many arguments are given).  One can also specify explic-
       itly  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 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  argu-
       ments  1  and  3 are specified, argument 2 must also be specified some-
       where 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.

   Flag characters
       The character % is followed by zero or more of the following flags:

       #      The value should be converted to an  "alternate  form".   For  o
              conversions,  the  first  character of the output string is made
              zero (by prefixing a 0 if it was not zero already).  For x and X
              conversions, a nonzero result has the string "0x" (or "0X" for X
              conversions) prepended to it.  For a, A, e, E, f, F,  g,  and  G
              conversions,  the  result  will  always contain a decimal point,
              even if no digits follow it (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.)  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
              negative numbers.  A + overrides a space if both are used.

       The  five  flag  characters above are defined in the C99 standard.  The
       Single UNIX Specification 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 did not
              include %'F, but SUSv3 added it.)

       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.

   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 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.

   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 '.', the precision is taken to be zero.  A negative  precision  is
       taken  as if the precision were omitted.  This gives the minimum number
       of digits to appear for d, i, o, u, x, and X conversions, the number of
       digits  to  appear  after  the radix character for a, A, e, E, f, and F
       conversions, the maximum number of significant digits for g and G  con-
       versions,  or  the  maximum  number  of characters to be printed from a
       string for s and S conversions.

   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.)
              This is a synonym for ll.

       j      A following integer conversion corresponds  to  an  intmax_t  or
              uintmax_t argument, or a following n conversion corresponds to a
              pointer to an intmax_t argument.

       z      A following  integer  conversion  corresponds  to  a  size_t  or
              ssize_t  argument,  or a following n conversion corresponds to a
              pointer to a size_t argument.

       t      A following integer conversion corresponds to a ptrdiff_t  argu-
              ment,  or a following n conversion corresponds to a pointer to a
              ptrdiff_t argument.

       SUSv3 specifies all of the above.  SUSv2 specified only the length mod-
       ifiers  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).

   Conversion specifiers
       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.

              (SUSv2 does not know about F and says that character string rep-
              resentations for infinity and NaN may be made available.   SUSv3
              adds a specification for F.  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 "[-]INFIN-
              ITY" 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, but added in SUSv3) For a conversion, the
              double argument is converted to hexadecimal notation (using  the
              letters  abcdef)  in  the style [-]0xh.hhhhp+-; 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  preci-
              sion.   The  default precision suffices for an exact representa-
              tion of the value if an exact representation in  base  2  exists
              and  otherwise  is  sufficiently  large to distinguish values of
              type double.  The digit before the decimal point is  unspecified
              for nonnormalized numbers, and nonzero but otherwise 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(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.
              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 or C11, but in SUSv2, SUSv3,  and  SUSv4.)   Synonym
              for lc.  Don't use.

       S      (Not  in  C99  or C11, but in SUSv2, SUSv3, and SUSv4.)  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 pointed to by the  corresponding  argument.   That  argument
              shall  be  an  int *, or variant whose size matches the (option-
              ally) supplied integer length modifier.   No  argument  is  con-
              verted.   (This  specifier  is  not  supported  by  the bionic C
              library.)  The behavior is undefined if the conversion  specifi-
              cation includes any flags, a field width, or a precision.

       m      (Glibc  extension;  supported by uClibc and musl.)  Print output
              of strerror(errno).  No argument is required.

       %      A '%' is written.  No argument is converted.  The complete  con-
              version specification is '%%'.

RETURN VALUE
       Upon successful return, these functions return the number of characters
       printed (excluding the null byte used to end output to strings).

       The functions snprintf() and vsnprintf() do not write  more  than  size
       bytes  (including the terminating null byte ('\0')).  If the output was
       truncated due to this limit, then the return value  is  the  number  of
       characters  (excluding the terminating null byte) which would have been
       written to the final string if enough space had been available.   Thus,
       a  return  value  of  size or more means that the output was truncated.
       (See also below under NOTES.)

       If an output error is encountered, a negative value is returned.

ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).

       +------------------------+---------------+----------------+
       |Interface               | Attribute     | Value          |
       +------------------------+---------------+----------------+
       |printf(), fprintf(),    | Thread safety | MT-Safe locale |
       |sprintf(), snprintf(),  |               |                |
       |vprintf(), vfprintf(),  |               |                |
       |vsprintf(), vsnprintf() |               |                |
       +------------------------+---------------+----------------+

CONFORMING TO
       fprintf(),  printf(),  sprintf(),  vprintf(),  vfprintf(),  vsprintf():
       POSIX.1-2001, POSIX.1-2008, C89, C99.

       snprintf(), vsnprintf(): POSIX.1-2001, POSIX.1-2008, C99.

       The dprintf() and vdprintf() functions were originally  GNU  extensions
       that were later standardized in POSIX.1-2008.

       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.  POSIX.1-2001 and later align  their  specifica-
       tion of snprintf() with C99.

       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.

NOTES
       Some programs imprudently rely on code such as the following

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

       to append text to buf.  However, the standards explicitly note that the
       results are undefined if source and destination  buffers  overlap  when
       calling  sprintf(), snprintf(), vsprintf(), and vsnprintf().  Depending
       on the version of gcc(1) used, and the compiler options employed, calls
       such as the above will not produce the expected results.

       The  glibc  implementation  of the functions snprintf() and vsnprintf()
       conforms to the C99 standard, that  is,  behaves  as  described  above,
       since  glibc version 2.1.  Until glibc 2.0.6, they would return -1 when
       the output was truncated.

BUGS
       Because sprintf() and vsprintf() assume  an  arbitrarily  long  string,
       callers must be 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)).

       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.

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, ...)
       {
           int size = 0;
           char *p = NULL;
           va_list ap;

           /* Determine required size */

           va_start(ap, fmt);
           size = vsnprintf(p, size, fmt, ap);
           va_end(ap);

           if (size < 0)
               return NULL;

           size++;             /* For '\0' */
           p = malloc(size);
           if (p == NULL)
               return NULL;

           va_start(ap, fmt);
           size = vsnprintf(p, size, fmt, ap);
           if (size < 0) {
               free(p);
               return NULL;
           }
           va_end(ap);

           return p;
       }

       If truncation occurs in glibc versions prior to 2.0.6, this is  treated
       as an error instead of being handled gracefully.

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

COLOPHON
       This page is part of release 4.08 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

GNU                               2016-07-17                         PRINTF(3)

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