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snprintf(3)

NAME

     printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf,
     vsprintf, vsnprintf, vasprintf -- 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, ...);

     #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);


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 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 out-
     put.
     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.

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

     The format string is composed of zero or more directives: ordinary char-
     acters (not %), which are copied unchanged to the output stream; and con-
     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) 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 pro-
	 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.

     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 num-
		      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 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.

	 `''	      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 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-adjust-
	 ment 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 con-
	 versions.

     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	     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 adjust-
	     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; if the precision is missing, it is taken as 6; if the
	     precision is zero, no decimal-point character appears.  An E con-
	     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 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-

     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; if a precision is specified, no more than the number speci-
	     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 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 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 indicated by the int * (or variant) pointer argument.	No
	     argument is converted.

     %	     A `%' is written.	No argument is converted.  The complete con-
	     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:

	   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 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
     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), scanf(3), setlocale(3), wprintf(3)

     The FreeBSD Security Architecture.  (See /usr/share/doc/{to be
     determined}.)


STANDARDS

     Subject to the caveats noted in the BUGS section below, the fprintf(),
     printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions con-
     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 printf family of functions do not correctly handle multibyte charac-
     ters in the format argument.

FreeBSD 5.4		       January 18, 2004 		   FreeBSD 5.4

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