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



NAME
       printf,  fprintf,  sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf - for-
       matted 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; 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
       trailing null byte ('\0')) to str.

       The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equivalent 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 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 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 to 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
       (not including the trailing '\0' used to end output to strings).

       The functions snprintf() and vsnprintf() do not write more than size bytes (includ-
       ing  the  trailing  '\0').   If the output was truncated due to this limit then the
       return value is the number of characters (not including the  trailing  '\0')  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  argu-
       ments.   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
       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 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 arguments specified using '$'; for example, if  arguments  1
       and  3  are  specified,  argument  2 must also be specified somewhere in the format
       string.

       For some numeric conversions a radix  character  ("decimal  point")  or  thousands'
       grouping  character  is  used.  The actual character used depends on the LC_NUMERIC
       part of the locale.  The POSIX locale uses '.' as radix  character,  and  does  not
       have a grouping character.  Thus,

               printf("%'.2f", 1234567.89);

       results  in  "1234567.89" in the POSIX locale, in "1234567,89" in the nl_NL locale,
       and in "1.234.567,89" in the da_DK locale.

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

       #      The value should be converted to an "alternate form".   For  o  conversions,
              the  first  character of the output string is made zero (by prefixing a 0 if
              it was not zero already).  For x and X conversions, a  non-zero  result  has
              the  string  "0x" (or "0X" for X conversions) prepended to it.  For a, A, e,
              E, f, F, g, and G 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  conver-
              sions,  trailing  zeros are not removed from the result as they would other-
              wise 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 unde-
              fined.

       -      The  converted  value  is  to  be left adjusted on the field boundary.  (The
              default is right justification.)  Except for n  conversions,  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 + over-
              rides a space if both are used.

       The  five flag characters above are defined in the C standard.  The SUSv2 specifies
       one further flag character.

       '      For decimal conversion (i, d, u, f, F, g, G) the output  is  to  be  grouped
              with thousands' grouping characters if the locale information indicates any.
              Note that many versions of gcc(1) cannot parse this option and will issue  a
              warning.  SUSv2 does not include %'F.

       glibc 2.2 adds one further flag character.

       I      For decimal integer conversion (i, d, u) the output uses the locale's alter-
              native output digits, if any.  For example, since glibc 2.2.3 this will give
              Arabic-Indic digits in the Persian ("fa_IR") locale.

   The field width
       An  optional  decimal digit string (with non-zero first digit) specifying a minimum
       field width.  If the converted value has fewer characters than the field width,  it
       will  be  padded with spaces on the left (or right, if the left-adjustment flag has
       been given).  Instead of a decimal digit string one may write  "*"  or  "*m$"  (for
       some  decimal integer m) to specify that the field width is given in the next argu-
       ment, 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 deci-
       mal  digit  string.   Instead  of a decimal digit string one may write "*" or "*m$"
       (for some decimal integer m) to specify that the precision is  given  in  the  next
       argument, or in the m-th argument, respectively, which must be of type int.  If the
       precision is given as just '.', or the precision  is  negative,  the  precision  is
       taken  to  be zero.  This gives the minimum number of digits to appear for d, i, o,
       u, x, and X conversions, the number of digits to appear after the  radix  character
       for  a, A, e, E, f, and F conversions, the maximum number of 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
              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 corresponds  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 corresponds to a pointer to a
              long int argument, or a following c conversion corresponds to a wint_t argu-
              ment, or a following s conversion corresponds to a pointer to wchar_t  argu-
              ment.

       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 dou-
              ble argument.  (C99 allows %LF, but SUSv2 does not.)

       q      ("quad". 4.4BSD and Linux libc5 only.  Don't use.)  This is  a  synonym  for
              ll.

       j      A following integer conversion corresponds to an intmax_t or uintmax_t argu-
              ment.

       z      A following integer conversion corresponds to a size_t or ssize_t  argument.
              (Linux libc5 has Z with this meaning.  Don't use it.)

       t      A following integer conversion corresponds to a ptrdiff_t argument.

       The SUSv2 only knows about the length modifiers h (in hd, hi, ho, hx, hX, hn) and l
       (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf, Lg, LG).

   The conversion specifier
       A character that specifies the type of conversion to be  applied.   The  conversion
       specifiers and their meanings are:

       d, i   The int argument is converted to signed decimal notation.  The precision, if
              any, gives the minimum number of digits that must appear; if  the  converted
              value  requires  fewer  digits,  it  is  padded on the left with zeros.  The
              default precision is 1.  When 0 is printed with an explicit precision 0, the
              output is empty.

       o, u, x, X
              The unsigned int argument is converted to unsigned octal (o), unsigned deci-
              mal (u), or unsigned hexadecimal (x and X) notation.  The letters abcdef are
              used  for x conversions; 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 pre-
              cision 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 dig-
              its  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 charac-
              ter  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.

              (The SUSv2 does not know about F and says that character string  representa-
              tions  for  infinity and NaN may be made available.  The C99 standard speci-
              fies "[-]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  conver-
              sions).   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) For a conversion, the double argument  is  converted  to
              hexadecimal notation (using the letters abcdef) in the style [-]0xh.hhhhp?d;
              for A conversion the prefix 0X, the letters ABCDEF, and the exponent separa-
              tor 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 preci-
              sion suffices for an exact representation of the value if an exact represen-
              tation in base 2 exists and otherwise is sufficiently large  to  distinguish
              values  of  type  double.  The digit before the decimal point is unspecified
              for non-normalized numbers, and non-zero but otherwise unspecified for  nor-
              malized 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 pre-
              cision 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  character), up to and including a terminating null wide charac-
              ter.  The resulting multibyte characters are written up to (but not  includ-
              ing)  the terminating null byte.  If a precision 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 writ-
              ten, 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, but in SUSv2.)  Synonym for lc.  Don't use.

       S      (Not in C99, but in SUSv2.)  Synonym for ls.  Don't use.

       p      The  void  *  pointer  argument  is  printed in hexadecimal (as if by %#x or
              %#lx).

       n      The number of characters written so far is stored into the integer indicated
              by the int * (or variant) pointer argument.  No argument is converted.

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

       %      A '%' is written.  No argument is converted.  The complete conversion speci-
              fication 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.

       Linux libc4 knows about the five C standard flags.  It knows about the length modi-
       fiers h, l, L, and the conversions c, d, e, E, f, F, g, G, i, n, o, p, s, u, x, and
       X, where F is a synonym for f.  Additionally, it accepts D, O, and  U  as  synonyms
       for  ld, lo, and lu.  (This is bad, and caused serious bugs later, when support for
       %D disappeared.)  No locale-dependent radix character, no thousands' separator,  no
       NaN or infinity, no "%m$" and "*m$".

       Linux libc5 knows about the five C standard flags and the ' flag, locale, "%m$" and
       "*m$".  It knows about the length modifiers h, l, L, Z, and q, but accepts L and  q
       both  for  long  double and for long long int (this is a bug).  It no longer recog-
       nizes F, D, O, and U, but adds the  conversion  character  m,  which  outputs  str-
       error(errno).

       glibc 2.0 adds conversion characters C and S.

       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  charac-
       ter 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 pre-
       dict.  Use snprintf() and vsnprintf() instead (or asprintf(3) and vasprintf(3)).

       Linux libc4.[45] does not have a snprintf(), but provides a libbsd that contains an
       snprintf()  equivalent  to  sprintf(), that is, one that ignores the size argument.
       Thus, the use of snprintf() with early libc4 leads to serious security problems.

       Code such as printf(foo); often indicates a bug, since foo may contain a %  charac-
       ter.   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, ...)
       {
           /* Guess we need no more than 100 bytes. */
           int n, size = 100;
           char *p, *np;
           va_list ap;

           if ((p = malloc(size)) == 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);
               /* If that worked, return the string. */
               if (n > -1 && n < size)
                   return p;
               /* Else try again with more space. */
               if (n > -1)    /* glibc 2.1 */
                   size = n+1; /* precisely what is needed */
               else           /* glibc 2.0 */
                   size *= 2;  /* twice the old size */
               if ((np = realloc (p, size)) == NULL) {
                   free(p);
                   return NULL;
               } else {
                   p = np;
               }
           }
       }

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 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



GNU                               2008-12-19                         PRINTF(3)

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