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PRINTF(3)                BSD 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
     #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 s;
     and asprintf() and vasprintf() dynamically allocate a new string with malloc(3).

     Extended locale versions of these functions are documented in printf_l(3).  See xlocale(3) for more
     information.

     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 con-verted converted
     verted for output.

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

     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 n-1 of the characters printed into the out-put output
     put string (the n'th character then gets the terminating `\0'); if the return value is greater than or
     equal to the n argument, the string was too short and some of the printed characters were discarded.
     The output is always null-terminated.

     The sprintf() and vsprintf() functions effectively assume an infinite n.

     For those routines that write to a user-provided character string, that string and the format strings
     should not overlap, as the behavior is undefined.

     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:

     •   An optional field, consisting of a decimal digit string followed by a $, specifying the next argu-ment argument
         ment 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.

     •   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 num-ber number
                      ber 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.

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

     •   An optional separator character (  , | ; |  : | _ ) used for separating multiple values when print-ing printing
         ing an AltiVec or SSE vector, or other multi-value unit.

         NOTE: This is an extension to the printf() specification.  Behaviour of these values for printf()
         is only defined for operating systems conforming to the AltiVec Technology Programming Interface
         Manual.  (At time of writing this includes only Mac OS X 10.2 and later.)

     •   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.

     •   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 decimalpoint
         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.

     •   An optional length modifier, that specifies the size of the argument.  The following length modi-fiers modifiers
         fiers 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 con-version, conversion,
         version, indicates that the argument is of a signed type equivalent in size to a size_t.  Simi-larly, Similarly,
         larly, 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 *

         The AltiVec Technology Programming Interface Manual also defines five additional length modifiers
         which can be used (in place of the conventional length modifiers) for the printing of AltiVec or
         SSE vectors:
         v           Treat the argument as a vector value, unit length will be determined by the conversion
                     specifier (default = 16 8-bit units for all integer conversions, 4 32-bit units for
                     floating point conversions).
         vh, hv      Treat the argument as a vector of 8 16-bit units.
         vl, lv      Treat the argument as a vector of 4 32-bit units.

         NOTE: The vector length specifiers are extensions to the printf() specification.  Behaviour of
         these values for printf() is only defined for operating systems conforming to the AltiVec Technol-ogy Technology
         ogy Programming Interface Manual.  (At time of writing this includes only Mac OS X 10.2 and later.)

         As a further extension, for SSE2 64-bit units:
         vll, llv    Treat the argument as a vector of 2 64-bit units.

     •   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 pre-cision; precision;
             cision; if the precision is missing, it is taken as 6; if the precision is zero, no decimal-point decimalpoint
             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 decimalpoint
             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 conver-sion conversion
             sion 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 hexa-decimal-point hexadecimal-point
             decimal-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 hexadec-imal hexadecimal
             imal format.  For example, 0x1.92p+1, 0x3.24p+0, 0x6.48p-1, and 0xc.9p-2 are all equivalent.
             The format chosen depends on the internal representation of the number, but the implementation
             guarantees that the length of the mantissa will be minimized.  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 origi-nal original
             nal state after the string.  Wide characters from the array are written up to (but not includ-ing) including)
             ing) 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.

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);
           }

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() inter-face interface
     face 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);

COMPATIBILITY
     The conversion formats %D, %O, and %U are not standard and are provided only for backward compatibil-ity. compatibility.
     ity.  The effect of padding the %p format with zeros (either by the 0 flag or by specifying a preci-sion), precision),
     sion), and the benign effect (i.e., none) of the # flag on %n and %p conversions, as well as other non-sensical nonsensical
     sensical 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.

SEE ALSO
     printf(1), printf_l(3), fmtcheck(3), scanf(3), setlocale(3), stdarg(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 implementa-tion implementation
     tion from Todd C. Miller <Todd.Miller@courtesan.com> for OpenBSD 2.3.  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.

BSD                            December 2, 2009                            BSD

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