snprintf(3)



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

NAME
       printf,  fprintf,  dprintf,  sprintf,  snprintf, vprintf, vfprintf, vd-
       printf, 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() 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 (in-
       cluding the terminating null byte ('\0')) to str.

       The  functions  vprintf(),  vfprintf(),  vdprintf(),  vsprintf(),   vs-
       nprintf()   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  un-
       changed  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 re-
       quired, 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 de-
       pends on the LC_NUMERIC part of the locale.  (See  setlocale(3).)   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.  (See setlocale(3).)  Note that many ver-
              sions of gcc(1) cannot parse this option and will issue a  warn-
              ing.  (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 lo-
              cale'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, re-
       spectively, 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 un-
              signed 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.

       q      A synonym for ll.  This is a nonstandard extension, derived from
              BSD; avoid its use in new code.

       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.)

       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.

       Z      A  nonstandard  synonym for z that predates the appearance of z.
              Do not use in new code.

       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, except for those modifiers explicitly
       noted as being nonstandard extensions.  SUSv2 specified only 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).

       As  a nonstandard extension, the GNU implementations treats ll and L as
       synonyms, so that one can, for example, write llg (as a synonym for the
       standards-compliant  Lg) and Ld (as a synonym for the standards compli-
       ant lld).  Such usage is nonportable.

   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 (which is nonzero if  the
              argument  is nonzero) before the decimal-point character and the
              number of digits after it is equal to the precision; if the pre-
              cision  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 expo-
              nent 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  ex-
              pected 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 ex-
              pected 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  be-
              fore 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  li-
              brary.)   The behavior is undefined if the conversion specifica-
              tion 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 at-
       tributes(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  vs-
       nprintf() 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.

EXAMPLES
       To print Pi to five decimal places:

           #include <math.h>
           #include <stdio.h>
           fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

       To print a date and time 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 = 0;
           size_t size = 0;
           char *p = NULL;
           va_list ap;

           /* Determine required size */

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

           if (n < 0)
               return NULL;

           /* One extra byte for '\0' */

           size = (size_t) n + 1;
           p = malloc(size);
           if (p == NULL)
               return NULL;

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

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

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

COLOPHON
       This  page  is  part of release 5.07 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                               2020-06-09                         PRINTF(3)

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