vfprintf man page on OpenBSD

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PRINTF(3)		  OpenBSD Programmer's Manual		     PRINTF(3)

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

SYNOPSIS
     #include <stdio.h>

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

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

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

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

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

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

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

DESCRIPTION
     The printf() family of functions produce output according to the given
     format as described below.	 This format may contain ``conversion
     specifiers''; the results of such conversions, if any, depend on the
     arguments following the format string.

     The printf() and vprintf() functions write output to the standard output
     stream, stdout; fprintf() and vfprintf() write output to the supplied
     stream pointer stream; sprintf(), snprintf(), vsprintf(), and vsnprintf()
     write to the character string str; asprintf() and vasprintf() write to a
     dynamically allocated string that is stored in ret.

     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.

     snprintf() and vsnprintf() 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.	 If size is zero, str may be a null pointer
     and no characters will be written; the number of bytes that would have
     been written excluding the terminating `\0' byte, or -1 on error, will be
     returned.

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

     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.

     o	 Zero or more of the following flags:

	 -   A hash `#' character specifying that 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 (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.

	 -   A zero `0' character specifying 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, x, and X), the `0' flag is
	     ignored.

	 -   A negative field width flag `-' indicates 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.

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

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

     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 an 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    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.

	     If the argument is infinity, it will be converted to [-]inf (e)
	     or [-]INF (E), respectively.  If the argument is not-a-number
	     (NaN), it will be converted to [-]nan (e) or [-]NAN (E),
	     respectively.

     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.

	     If the argument is infinity, it will be converted to [-]inf (f)
	     or [-]INF (F), respectively.  If the argument is not-a-number
	     (NaN), it will be converted to [-]nan (f) or [-]NAN (F),
	     respectively.

     gG	     The double argument is converted in style f or e (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.

	     If the argument is infinity, it will be converted to [-]inf (g)
	     or [-]INF (G), respectively.  If the argument is not-a-number
	     (NaN), it will be converted to [-]nan (g) or [-]NAN (G),
	     respectively.

     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,
	     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.

	     If the argument is infinity, it will be converted to [-]inf (a)
	     or [-]INF (A), respectively.  If the argument is not-a-number
	     (NaN), it will be converted to [-]nan (a) or [-]NAN (A),
	     respectively.

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

     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 NUL 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.

     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 `%%'.

     In no case does a non-existent 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.

RETURN VALUES
     For all these functions if an output or encoding error occurs, a value of
     -1 is returned.

     The printf(), fprintf(), sprintf(), vprintf(), vfprintf(), vsprintf(),
     asprintf(), and vasprintf() functions return the number of characters
     printed (not including the trailing `\0' used to end output to strings).

     The snprintf() and vsnprintf() functions return the number of characters
     that would have been output if the size were unlimited (again, not
     including the final `\0'.).

     The asprintf() and vasprintf() functions return the number of characters
     that were output to the newly allocated string (excluding the final
     `\0').  A pointer to the newly allocated string is returned in ret; it
     should be passed to free(3) to release the allocated storage when it is
     no longer needed.	If sufficient space cannot be allocated, these
     functions will return -1.	The value of ret in this situation is
     implementation-dependent (on OpenBSD, ret will be set to the null
     pointer, but this behavior should not be relied upon).

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 <stdarg.h>
	   #include <stdio.h>
	   #include <stdlib.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);
	   }

SEE ALSO
     printf(1), scanf(3)

STANDARDS
     The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf()
     functions conform to ANSI X3.159-1989 (``ANSI C'').

HISTORY
     The functions snprintf() and vsnprintf() first appeared in 4.4BSD.

     The functions asprintf() and vasprintf() first appeared in the GNU C
     library.  This implementation first appeared in OpenBSD 2.3.

CAVEATS
     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.

     Because sprintf() and vsprintf() assume an infinitely long string,
     callers must be careful not to overflow the actual space; this is often
     impossible to assure.  For safety, programmers should use the snprintf()
     and asprintf() family of interfaces instead.  Unfortunately, the
     snprintf() interface is not available on older systems and the asprintf()
     interface is not portable.

     It is important never to pass a string with user-supplied data as a
     format without using `%s'.	 An attacker can put format specifiers in the
     string to mangle the stack, leading to a possible security hole.  This
     holds true even if the string has been built ``by hand'' using a function
     like snprintf(), as the resulting string may still contain user-supplied
     conversion specifiers for later interpolation by printf().

     Be sure to use the proper secure idiom:

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

     There is no way for printf() to know the size of each argument passed.
     If positional arguments are used, care must be taken to ensure that all
     parameters, up to the last positionally specified parameter, are used in
     the format string.	 This allows for the format string to be parsed for
     this information.	Failure to do this will mean the code is non-portable
     and liable to fail.

OpenBSD 4.9			March 26, 2010			   OpenBSD 4.9
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