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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; 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  out-
       put 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  vari-
       able 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 argu-
       ments  (or  arguments accessed via the variable-length argument facilities of stdarg(3)) are converted for out-
       put.

       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 (including 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 preci-
       sion 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 argu-
       ment 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 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 charac-
       ter, 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 (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 con-
              verted 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.)
              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 conver-
              sion.

       +      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 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  char-
              acters  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 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 non-zero first digit) specifying a minimum field width.  If the con-
       verted 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 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  conver-
       sions, 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 fol-
              lowing n conversion corresponds to a pointer to a long int argument, or a following c conversion  corre-
              sponds 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.)

       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 argument.

       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 num-
              ber  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 hexadec-
              imal (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 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 precision; if  the  preci-
              sion  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 num-
              ber of digits after the decimal-point character is equal to the precision specification.  If the  preci-
              sion  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 representations for infinity and NaN may
              be  made  available.   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) 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 separator P is used.  There is one hexadecimal digit before the decimal point, and the num-
              ber of digits after it is equal to the precision.  The default precision suffices for an exact represen-
              tation  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 non-normalized
              numbers, and non-zero 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 char-
              acter 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  char-
              acter  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  writ-
              ten.   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 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 character.  The resulting multibyte characters
              are written up to (but not including) 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 written, not the number of wide characters or screen  posi-
              tions.   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 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.

       Linux  libc4  knows about the five C standard flags.  It knows about the length modifiers h, l, L, and the con-
       versions 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  recognizes  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 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)).

       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  %  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, ...)
       {
           /* 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 informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-12-19                         PRINTF(3)