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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  format  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 terminat‐
       ing null byte ('\0')) to str.

       The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equivalent to  the  func‐
       tions  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 undefined
       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 facili‐
       ties 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 copying 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 conversion
       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  speci‐
       fier.  By default, the arguments are used in the order given, where each '*' and each con‐
       version 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 '$', which comes from the Single UNIX Speci‐
       fication.  If the style using '$' is used, it must be used throughout for all  conversions
       taking  an  argument  and all width and precision arguments, but it may be mixed with "%%"
       formats which do not consume an argument.  There may be no gaps in the  numbers  of  argu‐
       ments  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  char‐
       acter.  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  (nor‐
              mally,  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 boundary.  (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) pro‐
              duced by a signed conversion.

       +      A sign (+ or -) should always be placed before a number produced by a  signed  con‐
              version.   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  Specifi‐
       cation specifies one further flag character.

       '      For decimal conversion (i, d, u, f, F, g, G) the output is to be grouped with thou‐
              sands' grouping characters if the locale information 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 decimal 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 dec‐
       imal 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 conversions, or
       the maximum number of characters to be printed from a string for s and S conversions.

   The length modifier
       Here, "integer conversion" stands for d, i, o, u, x, or X conversion.

       hh     A following integer conversion corresponds to a signed char or unsigned char  argu‐
              ment,  or  a following n conversion corresponds to a pointer to a signed char argu‐
              ment.

       h      A following integer conversion corresponds to a short int  or  unsigned  short  int
              argument, or a following n conversion corresponds to a pointer to a short int argu‐
              ment.

       l      (ell) A following integer conversion corresponds to a long int or unsigned long int
              argument,  or a following n conversion corresponds to a pointer to a long int argu‐
              ment, 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 argu‐
              ment.  (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 argument, 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 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  speci‐
       fiers 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) notation.  The letters abcdef are used for x con‐
              versions; the letters ABCDEF are used for X conversions.  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.

       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 precision; 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  representations  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 "[-]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 significant 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 precision.  The default precision
              suffices for an exact representation 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  num‐
              bers, 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 wcr‐
              tomb(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 precision 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 con‐
              verted 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 character), up
              to and including a terminating null wide character.  The resulting multibyte  char‐
              acters  are written up to (but not including) the terminating null byte.  If a pre‐
              cision is specified, no more bytes than the number specified are  written,  but  no
              partial  multibyte  characters are written.  Note that the precision 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 integer pointed to by
              the corresponding argument.  That argument shall be an int *  ,  or  variant  whose
              size  matches  the  (optionally)  supplied integer length modifier.  No argument is
              converted.  The behavior is undefined if the conversion specification includes  any
              flags, a field width, or a precision.

       m      (Glibc extension.)  Print output of strerror(errno).  No argument is required.

       %      A '%' is written.  No argument is converted.  The complete conversion specification
              is '%%'.

CONFORMING TO
       The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf()  functions  con‐
       form 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 characters 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 unde‐
       fined 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  care‐
       ful  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 security 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 internationalized 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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