man_3_printf

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 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 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 vari‐
       able-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 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.

   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 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 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  preci‐
       sion  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.  (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  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 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) 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 information indicates any.
              (See setlocale(3).)  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.

   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.

   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 conversions, 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 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 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 argu‐
              ment.

       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 argument, 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 compliant 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  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.

       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  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 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  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.

              (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 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 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  includ‐
              ing)  the  terminating null byte.  If a precision is specified, no more bytes than the number specified are written, but no partial multibyte charac‐
              ters 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.  (This specifier is not supported by the
              bionic C library.)  The behavior is undefined if the conversion specification 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 conversion 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).

       ┌────────────────────────┬───────────────┬────────────────┐
       │Interface               │ Attribute     │ Value          │
       ├────────────────────────┼───────────────┼────────────────┤
       │printf(), fprintf(),    │ Thread safety │ MT-Safe locale │
       │sprintf(), snprintf(),  │               │                │
       │vprintf(), vfprintf(),  │               │                │
       │vsprintf(), vsnprintf() │               │                │
       └────────────────────────┴───────────────┴────────────────┘

CONFORMING TO
       fprintf(), printf(), sprintf(), vprintf(), vfprintf(), vsprintf(): POSIX.1-2001, POSIX.1-2008, C89, C99.

       snprintf(), vsnprintf(): POSIX.1-2001, POSIX.1-2008, C99.

       The dprintf() and vdprintf() functions were originally GNU extensions that were later standardized in POSIX.1-2008.

       Concerning the return value of snprintf(), SUSv2 and C99 contradict each other: when snprintf() is called with size=0 then SUSv2 stipulates  an  unspecified
       return  value  less than 1, while C99 allows str to be NULL in this case, and gives the return value (as always) as the number of characters that would have
       been written in case the output string has been large enough.  POSIX.1-2001 and later align their 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 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 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 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)