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PRINTF(3)              FreeBSD Library Functions Manual              PRINTF(3)

NAME
     printf, fprintf, sprintf, snprintf, asprintf, dprintf, vprintf, vfprintf,
     vsprintf, vsnprintf, vasprintf, vdprintf - formatted output conversion

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #define _WITH_DPRINTF

     #include <stdio.h>

     int
     printf(const char * restrict format, ...);

     int
     fprintf(FILE * restrict stream, const char * restrict format, ...);

     int
     sprintf(char * restrict str, const char * restrict format, ...);

     int
     snprintf(char * restrict str, size_t size, const char * restrict format,
         ...);

     int
     asprintf(char **ret, const char *format, ...);

     int
     dprintf(int fd, const char * restrict format, ...);

     #include <stdarg.h>

     int
     vprintf(const char * restrict format, va_list ap);

     int
     vfprintf(FILE * restrict stream, const char * restrict format,
         va_list ap);

     int
     vsprintf(char * restrict str, const char * restrict format, va_list ap);

     int
     vsnprintf(char * restrict str, size_t size, const char * restrict format,
         va_list ap);

     int
     vasprintf(char **ret, const char *format, va_list ap);

     int
     vdprintf(int fd, const char * restrict format, va_list ap);

DESCRIPTION
     The printf() family of functions produces output according to a format as
     described below.  The printf() and vprintf() functions write output to
     stdout, the standard output stream; fprintf() and vfprintf() write output
     to the given output stream; dprintf() and vdprintf() write output to the
     given file descriptor; sprintf(), snprintf(), vsprintf(), and vsnprintf()
     write to the character string str; and asprintf() and vasprintf()
     dynamically allocate a new string with malloc(3).

     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.

     The asprintf() and vasprintf() functions set *ret to be a pointer to a
     buffer sufficiently large to hold the formatted string.  This pointer
     should be passed to free(3) to release the allocated storage when it is
     no longer needed.  If sufficient space cannot be allocated, asprintf()
     and vasprintf() will return -1 and set ret to be a NULL pointer.

     The snprintf() and vsnprintf() functions will write at most size-1 of the
     characters printed into the output string (the size'th character then
     gets the terminating `\0'); if the return value is greater than or equal
     to the size argument, the string was too short and some of the printed
     characters were discarded.  The output is always null-terminated, unless
     size is 0.

     The sprintf() and vsprintf() functions effectively assume a size of
     INT_MAX + 1.

     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.  The arguments must correspond properly (after type
     promotion) with the conversion specifier.  After the %, the following
     appear in sequence:

     +o   An optional field, consisting of a decimal digit string followed by a
         $, specifying the next argument to access.  If this field is not
         provided, the argument following the last argument accessed will be
         used.  Arguments are numbered starting at 1.  If unaccessed arguments
         in the format string are interspersed with ones that are accessed the
         results will be indeterminate.

     +o   Zero or more of the following flags:

         `#'               The value should be converted to an ``alternate
                           form''.  For c, d, i, n, p, s, and u conversions,
                           this option has no effect.  For o conversions, the
                           precision of the number is increased to force the
                           first character of the output string to a zero.
                           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 conversions,
                           trailing zeros are not removed from the result as
                           they would otherwise be.

         `0' (zero)        Zero padding.  For all conversions except n, the
                           converted value is padded on the left with zeros
                           rather than blanks.  If a precision is given with a
                           numeric conversion (d, i, o, u, i, x, and X), the 0
                           flag is ignored.

         `-'               A negative field width flag; the converted value is
                           to be left adjusted on the field boundary.  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.

         ` ' (space)       A blank should be left before a positive number
                           produced by a signed conversion (a, A, d, e, E, f,
                           F, g, G, or i).

         `+'               A sign must always be placed before a number
                           produced by a signed conversion.  A + overrides a
                           space if both are used.

         `'' (apostrophe)  Decimal conversions (d, u, or i) or the integral
                           portion of a floating point conversion (f or F)
                           should be grouped and separated by thousands using
                           the non-monetary separator returned by
                           localeconv(3).

     +o   An optional decimal digit string 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) to fill out the field width.

     +o   An optional precision, in the form of a period . followed by an
         optional digit string.  If the digit string is omitted, the precision
         is taken as 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 decimal-point 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
         conversions.

     +o   An optional length modifier, that specifies the size of the argument.
         The following length modifiers are valid for the d, i, n, o, u, x, or
         X conversion:

         Modifier                 d, i               o, u, x, X                n
         hh                       signed char        unsigned char             signed char *
         h                        short              unsigned short            short *
         l (ell)                  long               unsigned long             long *
         ll (ell ell)             long long          unsigned long long        long long *
         j                        intmax_t           uintmax_t                 intmax_t *
         t                        ptrdiff_t          (see note)                ptrdiff_t *
         z                        (see note)         size_t                    (see note)
         q (deprecated)           quad_t             u_quad_t                  quad_t *

         Note: the t modifier, when applied to a o, u, x, or X conversion,
         indicates that the argument is of an unsigned type equivalent in size
         to a ptrdiff_t.  The z modifier, when applied to a d or i conversion,
         indicates that the argument is of a signed type equivalent in size to
         a size_t.  Similarly, when applied to an n conversion, it indicates
         that the argument is a pointer to a signed type equivalent in size to
         a size_t.

         The following length modifier is valid for the a, A, e, E, f, F, g,
         or G conversion:

         Modifier        a, A, e, E, f, F, g, G
         l (ell)         double (ignored, same behavior as without it)
         L               long double

         The following length modifier is valid for the c or s conversion:

         Modifier        c             s
         l (ell)         wint_t        wchar_t *

     +o   A character that specifies the type of conversion to be applied.

     A field width or precision, or both, may be indicated by an asterisk `*'
     or an asterisk followed by one or more decimal digits and a `$' instead
     of a digit string.  In this case, an int argument supplies the field
     width or precision.  A negative field width is treated as a left
     adjustment flag followed by a positive field width; a negative precision
     is treated as though it were missing.  If a single format directive mixes
     positional (nn$) and non-positional arguments, the results are undefined.

     The conversion specifiers and their meanings are:

     diouxX      The int (or appropriate variant) argument is converted to
                 signed decimal (d and i), 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.

     DOU         The long int argument is converted to signed decimal,
                 unsigned octal, or unsigned decimal, as if the format had
                 been ld, lo, or lu respectively.  These conversion characters
                 are deprecated, and will eventually disappear.

     eE          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.

                 For a, A, e, E, f, F, g, and G conversions, positive and
                 negative infinity are represented as inf and -inf
                 respectively when using the lowercase conversion character,
                 and INF and -INF respectively when using the uppercase
                 conversion character.  Similarly, NaN is represented as nan
                 when using the lowercase conversion, and NAN when using the
                 uppercase conversion.

     fF          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.

     gG          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.

     aA          The double argument is rounded and converted to hexadecimal
                 notation in the style [-]0xh.hhhp[+-]d, where the number of
                 digits after the hexadecimal-point character is equal to the
                 precision specification.  If the precision is missing, it is
                 taken as enough to represent the floating-point number
                 exactly, and no rounding occurs.  If the precision is zero,
                 no hexadecimal-point character appears.  The p is a literal
                 character `p', and the exponent consists of a positive or
                 negative sign followed by a decimal number representing an
                 exponent of 2.  The A conversion uses the prefix ``0X''
                 (rather than ``0x''), the letters ``ABCDEF'' (rather than
                 ``abcdef'') to represent the hex digits, and the letter `P'
                 (rather than `p') to separate the mantissa and exponent.

                 Note that there may be multiple valid ways to represent
                 floating-point numbers in this hexadecimal format.  For
                 example, 0x1.92p+1, 0x3.24p+0, 0x6.48p-1, and 0xc.9p-2 are
                 all equivalent.  FreeBSD 8.0 and later always prints finite
                 non-zero numbers using `1' as the digit before the
                 hexadecimal point.  Zeroes are always represented with a
                 mantissa of 0 (preceded by a `-' if appropriate) and an
                 exponent of +0.

     C           Treated as c with the l (ell) modifier.

     c           The int argument is converted to an unsigned char, and the
                 resulting character is written.

                 If the l (ell) modifier is used, the wint_t argument shall be
                 converted to a wchar_t, and the (potentially multi-byte)
                 sequence representing the single wide character is written,
                 including any shift sequences.  If a shift sequence is used,
                 the shift state is also restored to the original state after
                 the character.

     S           Treated as s with the l (ell) modifier.

     s           The 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 NUL
                 character; if a precision is specified, no more than the
                 number specified are written.  If a precision is given, no
                 null character need be present; if the precision is not
                 specified, or is greater than the size of the array, the
                 array must contain a terminating NUL character.

                 If the l (ell) modifier is used, the wchar_t * argument is
                 expected to be a pointer to an array of wide characters
                 (pointer to a wide string).  For each wide character in the
                 string, the (potentially multi-byte) sequence representing
                 the wide character is written, including any shift sequences.
                 If any shift sequence is used, the shift state is also
                 restored to the original state after the string.  Wide
                 characters from the array are written up to (but not
                 including) a terminating wide NUL character; if a precision
                 is specified, no more than the number of bytes specified are
                 written (including shift sequences).  Partial characters are
                 never written.  If a precision is given, no null character
                 need be present; if the precision is not specified, or is
                 greater than the number of bytes required to render the
                 multibyte representation of the string, the array must
                 contain a terminating wide NUL character.

     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.

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

     The decimal point character is defined in the program's locale (category
     LC_NUMERIC).

     In no case does a non-existent or small field width cause truncation of a
     numeric field; if the result of a conversion is wider than the field
     width, the field is expanded to contain the conversion result.

RETURN VALUES
     These functions return the number of characters printed (not including
     the trailing `\0' used to end output to strings), except for snprintf()
     and vsnprintf(), which return the number of characters that would have
     been printed if the size were unlimited (again, not including the final
     `\0').  These functions return a negative value if an error occurs.

EXAMPLES
     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);

     To print pi to five decimal places:

           #include <math.h>
           #include <stdio.h>
           fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

     To allocate a 128 byte string and print into it:

           #include <stdio.h>
           #include <stdlib.h>
           #include <stdarg.h>
           char *newfmt(const char *fmt, ...)
           {
                   char *p;
                   va_list ap;
                   if ((p = malloc(128)) == NULL)
                           return (NULL);
                   va_start(ap, fmt);
                   (void) vsnprintf(p, 128, fmt, ap);
                   va_end(ap);
                   return (p);
           }

COMPATIBILITY
     Many application writers used the name dprintf before the dprintf()
     function was introduced in IEEE Std 1003.1 (``POSIX.1''), so a prototype
     is not provided by default in order to avoid compatibility problems.
     Applications that wish to use the dprintf() function described herein
     should either request a strict IEEE Std 1003.1-2008 (``POSIX.1'')
     environment by defining the macro _POSIX_C_SOURCE to the value 200809 or
     greater, or by defining the macro _WITH_DPRINTF, prior to the inclusion
     of <stdio.h>.  For compatibility with GNU libc, defining either
     _BSD_SOURCE or _GNU_SOURCE prior to the inclusion of <stdio.h> will also
     make dprintf() available.

     The conversion formats %D, %O, and %U are not standard and are provided
     only for backward compatibility.  The effect of padding the %p format
     with zeros (either by the 0 flag or by specifying a precision), and the
     benign effect (i.e., none) of the # flag on %n and %p conversions, as
     well as other nonsensical combinations such as %Ld, are not standard;
     such combinations should be avoided.

ERRORS
     In addition to the errors documented for the write(2) system call, the
     printf() family of functions may fail if:

     [EILSEQ]           An invalid wide character code was encountered.

     [ENOMEM]           Insufficient storage space is available.

     [EOVERFLOW]        The size argument exceeds INT_MAX + 1, or the return
                        value would be too large to be represented by an int.

SEE ALSO
     printf(1), fmtcheck(3), scanf(3), setlocale(3), wprintf(3)

STANDARDS
     Subject to the caveats noted in the BUGS section below, the fprintf(),
     printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions
     conform to ANSI X3.159-1989 (``ANSI C89'') and ISO/IEC 9899:1999
     (``ISO C99'').  With the same reservation, the snprintf() and vsnprintf()
     functions conform to ISO/IEC 9899:1999 (``ISO C99''), while dprintf() and
     vdprintf() conform to IEEE Std 1003.1-2008 (``POSIX.1'').

HISTORY
     The functions asprintf() and vasprintf() first appeared in the GNU C
     library.  These were implemented by Peter Wemm <peter@FreeBSD.org> in
     FreeBSD 2.2, but were later replaced with a different implementation from
     OpenBSD 2.3 by Todd C. Miller <Todd.Miller@courtesan.com>.  The dprintf()
     and vdprintf() functions were added in FreeBSD 8.0.

BUGS
     The printf family of functions do not correctly handle multibyte
     characters in the format argument.

SECURITY CONSIDERATIONS
     The sprintf() and vsprintf() functions are easily misused in a manner
     which enables malicious users to arbitrarily change a running program's
     functionality through a buffer overflow attack.  Because sprintf() and
     vsprintf() assume an infinitely long string, callers must be careful not
     to overflow the actual space; this is often hard to assure.  For safety,
     programmers should use the snprintf() interface instead.  For example:

     void
     foo(const char *arbitrary_string, const char *and_another)
     {
             char onstack[8];

     #ifdef BAD
             /*
              * This first sprintf is bad behavior.  Do not use sprintf!
              */
             sprintf(onstack, "%s, %s", arbitrary_string, and_another);
     #else
             /*
              * The following two lines demonstrate better use of
              * snprintf().
              */
             snprintf(onstack, sizeof(onstack), "%s, %s", arbitrary_string,
                 and_another);
     #endif
     }

     The printf() and sprintf() family of functions are also easily misused in
     a manner allowing malicious users to arbitrarily change a running
     program's functionality by either causing the program to print
     potentially sensitive data ``left on the stack'', or causing it to
     generate a memory fault or bus error by dereferencing an invalid pointer.

     %n can be used to write arbitrary data to potentially carefully-selected
     addresses.  Programmers are therefore strongly advised to never pass
     untrusted strings as the format argument, as an attacker can put format
     specifiers in the string to mangle your stack, leading to a possible
     security hole.  This holds true even if the string was built using a
     function like snprintf(), as the resulting string may still contain user-
     supplied conversion specifiers for later interpolation by printf().

     Always use the proper secure idiom:

           snprintf(buffer, sizeof(buffer), "%s", string);

FreeBSD 11.0-PRERELEASE        December 2, 2009        FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | COMPATIBILITY | ERRORS | SEE ALSO | STANDARDS | HISTORY | BUGS | SECURITY CONSIDERATIONS

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