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PRINTF(3)                BSD Library Functions Manual                PRINTF(3)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

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

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

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

     int
     snprintf(char *restrict s, size_t n, const char *restrict format, ...);

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

     #include <stdarg.h>
     #include <stdio.h>

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

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

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

     int
     vsnprintf(char *restrict s, size_t n, const char *restrict format,
         va_list ap);

     int
     vsprintf(char *restrict s, 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; 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 converted for out-put. output.
     put.

     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 output 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 charac-ters characters
     ters were discarded.  The output is always null-terminated.

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

     The format string is composed of zero or more directives: ordinary char-acters characters
     acters (not %), which are copied unchanged to the output stream; and con-version conversion
     version 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 promo-tion) promotion)
     tion) with the conversion specifier.  After the %, the following appear
     in sequence:

     oo   An optional field, consisting of a decimal digit string followed by a
         $, specifying the next argument to access.  If this field is not pro-vided, provided,
         vided, 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.

     oo   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 (except if a zero value is
                      printed with an explicit precision of 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 con-verted converted
                      verted 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 conversion (f or F) should be
                      grouped and separated by thousands using the non-mone-tary non-monetary
                      tary separator returned by localeconv(3).

     oo   An optional separator character (  , | ; |  : | _ ) used for separat-ing separating
         ing multiple values when printing 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.)

     oo   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-adjust-ment left-adjustment
         ment flag has been given) to fill out the field width.

     oo   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 con-versions. conversions.
         versions.

     oo   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 *

         The AltiVec Technology Programming Interface Manual also defines five
         additional length modifiers which can be used (in place of the con-ventional conventional
         ventional length modifiers) for the printing of AltiVec or SSE vec-tors: vectors:
         tors:
         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 Technology
         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.

     oo   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 `$' instead
     of a digit string.  In this case, an int argument supplies the field
     width or precision.  A negative field width is treated as a left adjust-ment adjustment
     ment flag followed by a positive field width; a negative precision is
     treated as though it were missing.  If a single format directive mixes
     positional (nn$) 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 character appears.  An E con-version conversion
             version 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 nota-tion notation
             tion 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 floatingpoint
             point numbers in this hexadecimal format.  For example,
             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 charac-ter; character;
             ter; if a precision is specified, no more than the number speci-fied specified
             fied 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 speci-fied specified
             fied 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 repre-sentation representation
             sentation 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 inte-ger integer
             ger indicated by the int * (or variant) pointer argument.  No
             argument is converted.

     %       A `%' is written.  No argument is converted.  The complete con-version conversion
             version 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() 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 pro-gram's program's
     gram'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 usersupplied
     supplied conversion specifiers for later interpolation by printf().

     Always use the proper secure idiom:

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

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), fmtcheck(3), printf_l(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 con-form conform
     form 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'').

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
     Todd C. Miller <Todd.Miller@courtesan.com> for OpenBSD 2.3.

BUGS
     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.

     The asprintf family of functions do not correctly handle multibyte char-acters characters
     acters in the format argument.

BSD                            October 16, 2004                            BSD