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man:dprintf

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

NAME

     printf,   fprintf,   dprintf,  sprintf,  snprintf,  vprintf,  vfprintf,
     vdprintf, 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
     (including the terminating null byte ('\0')) to str.
     The   functions   vprintf(),   vfprintf(),   vdprintf(),    vsprintf(),
     vsnprintf()  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
     unchanged  to the output stream; and conversion specifications, each of
     which results in fetching zero or more subsequent arguments.  Each con-
     version specification is introduced by the character %, and ends with a
     conversion specifier.  In between there may be (in this order) zero  or
     more  flags, an optional minimum field width, an optional precision and
     an optional length modifier.
     The arguments must correspond properly (after type promotion) with  the
     conversion  specifier.  By default, the arguments are used in the order
     given, where each '*' (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
     required, by writing "%m$" instead of '%' and  "*m$"  instead  of  '*',
     where  the  decimal integer m denotes the position in the argument list
     of the desired argument, indexed starting from 1.  Thus,
         printf("%*d", width, num);
     and
         printf("%2$*1$d", width, num);
     are equivalent.  The second style allows  repeated  references  to  the
     same  argument.  The C99 standard does not include the style using '$',
     which comes from the Single UNIX Specification.  If the style using '$'
     is used, it must be used throughout for all conversions taking an argu-
     ment and all width and precision arguments, but it may  be  mixed  with
     "%%"  formats,  which do not consume an argument.  There may be no gaps
     in the numbers of arguments specified using '$'; for example, if  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
     depends 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.
  1. 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
            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.
 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,
     respectively, which must be of type int.  If the precision is given  as
     just  '.',  the precision is taken to be zero.  A negative precision is
     taken as if the precision were omitted.  This gives the minimum  number
     of digits to appear for d, i, o, u, x, and X conversions, the number of
     digits to appear after the radix character for a, A, e,  E,  f,  and  F
     conversions,  the maximum number of significant digits for g and G con-
     versions, or the maximum number of characters  to  be  printed  from  a
     string for s and S conversions.
 Length modifier
     Here, "integer conversion" stands for d, i, o, u, x, or X conversion.
     hh     A  following  integer conversion corresponds to a signed char or
            unsigned char argument, or a following n conversion  corresponds
            to a pointer to a signed char argument.
     h      A  following  integer  conversion  corresponds to a short int or
            unsigned short int argument, or a following n conversion  corre-
            sponds to a pointer to a short int argument.
     l      (ell)  A  following integer conversion corresponds to a long int
            or unsigned long int argument, or a following n conversion  cor-
            responds  to  a pointer to a long int argument, or a following c
            conversion corresponds to a wint_t argument, or  a  following  s
            conversion corresponds to a pointer to wchar_t argument.
     ll     (ell-ell).  A following integer conversion corresponds to a long
            long int or unsigned long long int argument, or  a  following  n
            conversion corresponds to a pointer to a long long int argument.
     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 before the decimal-point
            character and the number of digits after it is equal to the pre-
            cision;  if  the  precision is missing, it is taken as 6; if the
            precision is zero, no decimal-point  character  appears.   An  E
            conversion  uses  the  letter E (rather than e) to introduce the
            exponent.  The exponent always contains at least two digits;  if
            the value is zero, the exponent is 00.
     f, F   The double argument is rounded and converted to decimal notation
            in the style [-]ddd.ddd, where the number of  digits  after  the
            decimal-point character is equal to the precision specification.
            If the precision is missing, it is taken as 6; if the  precision
            is  explicitly  zero,  no decimal-point character appears.  If a
            decimal point appears, at least one digit appears before it.
            (SUSv2 does not know about F and says that character string rep-
            resentations  for infinity and NaN may be made available.  SUSv3
            adds a specification for F.  The C99 standard specifies "[-]inf"
            or  "[-]infinity" for infinity, and a string starting with "nan"
            for NaN, in the case of f conversion, and "[-]INF" or "[-]INFIN-
            ITY" or "NAN" in the case of F conversion.)
     g, G   The  double argument is converted in style f or e (or F or E for
            G conversions).  The precision specifies the number of  signifi-
            cant  digits.   If the precision is missing, 6 digits are given;
            if the precision is zero, it is treated as 1.  Style e  is  used
            if  the  exponent from its conversion is less than -4 or greater
            than or equal to the precision.  Trailing zeros are removed from
            the  fractional part of the result; a decimal point appears only
            if it is followed by at least one digit.
     a, A   (C99; not in SUSv2, but added in SUSv3) For  a  conversion,  the
            double  argument is converted to hexadecimal notation (using the
            letters abcdef) in the style [-]0xh.hhhhp+-;  for  A  conversion
            the  prefix 0X, the letters ABCDEF, and the exponent separator P
            is used.  There is one  hexadecimal  digit  before  the  decimal
            point,  and the number of digits after it is equal to the preci-
            sion.  The default precision suffices for an  exact  representa-
            tion  of  the  value if an exact representation in base 2 exists
            and otherwise is sufficiently large  to  distinguish  values  of
            type  double.  The digit before the decimal point is unspecified
            for nonnormalized numbers, and nonzero but otherwise unspecified
            for normalized numbers.
     c      If no l modifier is present, the int argument is converted to an
            unsigned char, and the resulting character is written.  If an  l
            modifier  is  present,  the  wint_t (wide character) argument is
            converted to a multibyte sequence by a call  to  the  wcrtomb(3)
            function, with a conversion state starting in the initial state,
            and the resulting multibyte string is written.
     s      If no l modifier  is  present:  the  const  char *  argument  is
            expected  to be a pointer to an array of character type (pointer
            to a string).  Characters from the array are written up to  (but
            not including) a terminating null byte ('\0'); if a precision is
            specified, no more than the number specified are written.  If  a
            precision  is given, no null byte need be present; if the preci-
            sion is not specified, or is greater than the size of the array,
            the array must contain a terminating null byte.
            If  an  l  modifier  is present: the const wchar_t * argument is
            expected to be a pointer to an array of wide  characters.   Wide
            characters  from the array are converted to multibyte characters
            (each by a call to the wcrtomb(3) function,  with  a  conversion
            state  starting in the initial state before the first wide char-
            acter), up to and including a terminating null  wide  character.
            The  resulting  multibyte  characters are written up to (but not
            including) the terminating null byte.  If a precision is  speci-
            fied,  no  more bytes than the number specified are written, but
            no partial multibyte characters are written.  Note that the pre-
            cision determines the number of bytes written, not the number of
            wide characters or screen positions.  The array must  contain  a
            terminating null wide character, unless a precision is given and
            it is so small that the  number  of  bytes  written  exceeds  it
            before the end of the array is reached.
     C      (Not  in  C99  or C11, but in SUSv2, SUSv3, and SUSv4.)  Synonym
            for lc.  Don't use.
     S      (Not in C99 or C11, but in SUSv2, SUSv3,  and  SUSv4.)   Synonym
            for ls.  Don't use.
     p      The  void * pointer argument is printed in hexadecimal (as if by
            %#x or %#lx).
     n      The number of characters written so far is stored into the inte-
            ger  pointed  to  by  the corresponding argument.  That argument
            shall be an int *, or variant whose size  matches  the  (option-
            ally)  supplied  integer  length  modifier.  No argument is con-
            verted.  (This specifier  is  not  supported  by  the  bionic  C
            library.)   The behavior is undefined if the conversion specifi-
            cation 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
     attributes(7).
     allbox; lbw23 lb lb l  l  l.   Interface Attribute Value  T{  printf(),
     fprintf(),
     sprintf(), snprintf(),
     vprintf(), vfprintf(),
     vsprintf(), vsnprintf() T}   Thread safety  MT-Safe locale

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
     vsnprintf() instead (or asprintf(3) and vasprintf(3)).
     Code such as printf(foo); often indicates a bug, since foo may  contain
     a  % character.  If foo comes from untrusted user input, it may contain
     %n, causing the printf() call to write to memory and creating  a  secu-
     rity hole.

EXAMPLE

     To print Pi to five decimal places:
         #include <math.h> #include <stdio.h> fprintf(stdout, "pi = %.5f\n",
         4 * atan(1.0));
     To print a date and time in the form "Sunday,  July  3,  10:02",  where
     weekday and month are pointers to strings:
         #include <stdio.h> fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
                 weekday, month, day, hour, min);
     Many  countries use the day-month-year order.  Hence, an international-
     ized version must be able to print the arguments in an order  specified
     by the format:
         #include <stdio.h> fprintf(stdout, format,
                 weekday, month, day, hour, min);
     where  format  depends  on locale, and may permute the arguments.  With
     the value:
         "%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
     one might obtain "Sonntag, 3. Juli, 10:02".
     To allocate a sufficiently large string and print into it (code correct
     for both glibc 2.0 and glibc 2.1):
     #include <stdio.h> #include <stdlib.h> #include <stdarg.h>
     char * make_message(const char *fmt, ...)  {
         int size = 0;
         char *p = NULL;
         va_list ap;
         /* Determine required size */
         va_start(ap, fmt);
         size = vsnprintf(p, size, fmt, ap);
         va_end(ap);
         if (size < 0)
             return NULL;
         size++;             /* For '\0' */
         p = malloc(size);
         if (p == NULL)
             return NULL;
         va_start(ap, fmt);
         size = vsnprintf(p, size, fmt, ap);
         va_end(ap);
         if (size < 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 4.16 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 2017-09-15 PRINTF(3)

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