int vfprintf(FILE *__stream, const char *__fmt, va_list
__ap)
vfprintf
is the central facility of the
printf
family of functions. It outputs values to
stream
under control of a format string passed in
fmt
. The actual values to print are passed as a variable argument
list ap
.
vfprintf
returns the number of characters written to
stream
, or EOF
in case of an error. Currently,
this will only happen if stream
has not been opened with write
intent.
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 properly correspond (after type promotion) with the
conversion specifier. After the %
, the following appear in
sequence:
#
The value should be converted to
an "alternate form". For c, d, i, 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 nonzero
result has the string `0x' (or `0X' for X conversions) prepended
to it.
0
(zero) Zero padding. For all
conversions, 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.
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 (d, or i).
+
A sign must always be placed
before a number produced by a signed conversion. A + overrides a
space if both are used.
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 leftadjustment 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, or the maximum number of characters to be
printed from a string for s
conversions.
An optional l
or h
length
modifier, that specifies that the argument for the d, i, o, u, x, or X
conversion is a "long int"
rather than
int
. The h
is ignored, as "short
int"
is equivalent to int
.
A character that specifies the type of conversion to be applied.
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.
p
The void *
argument is taken
as an unsigned integer, and converted similarly as a %#x
command would do.
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 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.
%
A %
is written. No argument
is converted. The complete conversion specification is "%%".
eE
The double argument is rounded and converted
in the format "[]d.ddde±dd"
where there is one digit
before the decimalpoint 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 decimalpoint character appears. An E
conversion uses the letter 'E'
(rather than
'e'
) to introduce the exponent. The exponent always
contains two digits; if the value is zero, the exponent is 00.
fF
The double argument is rounded and converted
to decimal notation in the format "[]ddd.ddd"
, where the
number of digits after the decimalpoint character is equal to the precision
specification. If the precision is missing, it is taken as 6; if the
precision is explicitly zero, no decimalpoint 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.
S
Similar to the s
format,
except the pointer is expected to point to a programmemory (ROM) string
instead of a RAM string.
In no case does a nonexistent 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.
Since the full implementation of all the mentioned features becomes fairly
large, three different flavours of vfprintf() can be selected using linker options.
The default vfprintf() implements all the mentioned
functionality except floating point conversions. A minimized version of vfprintf() is available that only implements the
very basic integer and string conversion facilities, but only the #
additional option can be specified using conversion flags (these flags are parsed
correctly from the format specification, but then simply ignored). This version can be
requested using the following compiler options:
If the full functionality including the floating point conversions is required, the following options should be used:
The specified width and precision can be at most 255.
For floatingpoint conversions, if you link
default or minimized version of vfprintf(), the symbol
? will be output and double argument will be
skiped. So you output below will not be crashed. For default
version the width field and the "pad to left" ( symbol minus
) option will work in this case.
The hh
length modifier is
ignored (char
argument is promouted to
int
). More exactly, this realization
does not check the number of h
symbols.
But the ll
length modifier will
to abort the output, as this realization does not operate
long
long
arguments.
The variable width or precision field (an
asterisk *
symbol) is not realized and will
to abort the output.