snprintf(3)



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

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

SYNOPSIS
       #include <stdio.h>

       int printf(const char *format, ...);
       int fprintf(FILE *stream, const char *format, ...);
       int sprintf(char *str, const char *format, ...);
       int snprintf(char *str, size_t size, const char *format, ...);

       #include <stdarg.h>

       int vprintf(const char *format, va_list ap);
       int vfprintf(FILE *stream, const char *format, va_list ap);
       int vsprintf(char *str, const char *format, va_list ap);
       int vsnprintf(char *str, size_t size, const char *format, va_list ap);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       snprintf(), vsnprintf():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
           _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99

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  functions  snprintf()  and  vsnprintf()  write  at most size bytes
       (including the terminating 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 (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.

   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 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 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 neg-
              ative 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.

   The field width
       An optional decimal digit string (with nonzero first digit)  specifying
       a  minimum  field  width.   If the converted value has fewer characters
       than the field width, it will be padded with spaces  on  the  left  (or
       right, if the left-adjustment flag has been given).  Instead of a 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  '.',  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.

   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.)
              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).

   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.

              (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.  The behavior is undefined if the conversion  specifica-
              tion includes any flags, a field width, or a precision.

       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 '%%'.

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.  SUSv3 and later align  their  specification  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 n;
           int size = 100;     /* Guess we need no more than 100 bytes */
           char *p, *np;
           va_list ap;

           p = malloc(size);
           if (p == 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);

               /* Check error code */

               if (n < 0) {
                   free(p);
                   return NULL;
               }

               /* If that worked, return the string */

               if (n < size)
                   return p;

               /* Else try again with more space */

               size = n + 1;       /* Precisely what is needed */

               np = realloc(p, size);
               if (np == NULL) {
                   free(p);
                   return NULL;
               } else {
                   p = np;
               }
           }
       }

       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), scanf(3), setlocale(3), wcrtomb(3),
       wprintf(3), locale(5)

COLOPHON
       This page is part of release 3.74 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
       http://www.kernel.org/doc/man-pages/.

GNU                               2014-07-08                         PRINTF(3)

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