.TH PRINT 2
.SH NAME
print, fprint, sprint, snprint, seprint, fmtinstall, strconv, Strconv, numbconv, fltconv, doprint \- print formatted output
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.PP
.ta \w'\fLchar* 'u
.B
int	print(char *format, ...)
.PP
.B
int	fprint(int fd, char *format, ...)
.PP
.B
int	sprint(char *s, char *format, ...)
.PP
.B
int	snprint(char *s, int len, char *format, ...)
.PP
.B
char*	seprint(char *s, char *e, char *format, ...)
.PP
.B
int	fmtinstall(int c, int (*f)(va_list*, Fconv*))
.PP
.B
void	strconv(char *s, Fconv *fp)
.PP
.B
void	Strconv(Rune *s, Fconv *fp)
.PP
.B
int	numbconv(va_list *arg, Fconv *fp)
.PP
.B
int	fltconv(va_list *arg, Fconv *fp)
.PP
.B
char*	doprint(char *s, char *es, char *format, va_list arg)
.PP
.B
extern int printcol;
.SH DESCRIPTION
.I Print
writes text to the standard output.
.I Fprint
writes to the named output
file descriptor;
a buffered form
is described in
.IR bio (2).
.I Sprint
places text
followed by the NUL character
.RB ( \e0 )
in consecutive bytes starting at
.IR s ;
it is the user's responsibility to ensure that
enough storage is available.
Each function returns the number of bytes
transmitted (not including the NUL
in the case of
.IR sprint ),
or
a negative value if an output error was encountered.
.PP
.I Snprint
is like
.IR sprint ,
but will not place more than
.I len
bytes in
.IR s .
Its result is always NUL-terminated and holds the maximal
number of complete UTF-8 characters that can fit.
.I Seprint
is like
.IR snprint ,
except that the end is indicated by a pointer
.I e
rather than a count and the return value points to the terminating NUL of the
resulting string.
.PP
Each of these functions
converts, formats, and prints its
trailing arguments
under control of a
.IR format 
string.
The
format
contains two types of objects:
plain characters, which are simply copied to the
output stream,
and conversion specifications,
each of which results in fetching of
zero or more
arguments.
The results are undefined if there are arguments of the
wrong type or too few
arguments for the format.
If the format is exhausted while
arguments remain, the excess
is ignored.
.PP
Each conversion specification has the following format:
.IP
.B "% [flags] verb
.PP
The verb is a single character and each flag is a single character or a
(decimal) numeric string.
Up to two numeric strings may be used;
the first is called
.IR f1 ,
the second
.IR f2 .
A period can be used to separate them, and if the period is
present then
.I f1
and
.I f2
are taken to be zero if missing, otherwise they are `omitted'.
Either or both of the numbers may be replaced with the character
.BR * ,
meaning that the actual number will be obtained from the argument list
as an integer.
The flags and numbers are arguments to
the
.I verb
described below.
.PP
The numeric verbs
.BR d ,
.BR o ,
.BR x ,
and
.B X
format their arguments in decimal,
octal, hexadecimal, and upper case hexadecimal.
Each interprets the flags
.BR h ,
.BR l ,
.BR u ,
.BR + ,
.BR - ,
and
.B #
to mean short, long, unsigned, always print a sign, left justified, and alternate format.
If neither
short nor long is specified,
then the argument is an
.BR int .
If unsigned is specified,
then the argument is interpreted as a
positive number and no sign is output.
If two
.B l
flags are given,
then the argument is interpreted as a
.B vlong
(a 4-byte or sometimes 8-byte integer).
If
.I f2
is not omitted, the number is padded on the left with zeros
until at least
.I f2
digits appear.
Then, if alternate format is specified,
for
.B o
conversion, the number is preceded by a
.B 0
if it doesn't already begin with one;
for
.B x
conversion, the number is preceded by
.BR 0x ;
for
.B X
conversion, the number is preceded by
.BR 0X .
Finally, if
.I f1
is not omitted, the number is padded on the left (or right, if
left justification is specified) with enough blanks to
make the field at least
.I f1
characters long.
.PP
The floating point verbs
.BR f ,
.BR e ,
.BR E ,
.BR g ,
and
.B G
take a
.B double
argument.
Each interprets the flags
.BR + ,
.BR - ,
and
.B #
to mean
always print a sign,
left justified,
and
alternate format.
.I F1
is the minimum field width and,
if the converted value takes up less than
.I f1
characters, it is padded on the left (or right, if `left justified')
with spaces.
.I F2
is the number of digits that are converted after the decimal place for
.BR e ,
.BR E ,
and
.B f
conversions,
and
.I f2
is the maximum number of significant digits for
.B g
and
.B G
conversions.
The 
.B f
verb produces output of the form
.RB [ - ] digits [ .digits\fR].
.B E
conversion appends an exponent
.BR E [ - ] digits ,
and
.B e
conversion appends an exponent
.BR e [ - ] digits .
The
.B g
verb will output the argument in either
.B e
or
.B f
with the goal of producing the smallest output.
Also, trailing zeros are omitted from the fraction part of
the output, and a trailing decimal point appears only if it is followed
by a digit.
The
.B G
verb is similar, but uses
.B E
format instead of
.BR e .
When alternate format is specified, the result will always contain a decimal point,
and for
.B g
and
.B G
conversions, trailing zeros are not removed.
.PP
The
.B s
verb copies a string
(pointer to
.BR char )
to the output.
The number of characters copied
.RI ( n )
is the minimum
of the size of the string and
.IR f2 .
These
.I n
characters are justified within a field of
.I f1
characters as described above.
The
.B S
verb is similar, but it interprets its pointer as an array
of runes (see
.IR utf (6));
the runes are converted to
.SM UTF
before output.
.PP
The
.B c
verb copies a single
.B char
(promoted to
.BR int )
justified within a field of
.I f1
characters as described above.
The
.B C
verb is similar, but works on runes.
.PP
The
.B p
verb formats a pointer value.
At the moment, it is a synonym for
.BR ux ,
but that will change once pointers and integers are different sizes.
.PP
The
.B r
verb takes no arguments; it copies the error string returned by a call to
.IR errstr (2).
.PP
.I Fmtinstall
is used to install custom verbs and flags labeled by character
.IR c ,
which must have value less than 512.
.I Fn
should be declared as
.IP
.EX
int	fn(va_list *arg, Fconv *fp)
.EE
.PP
.IB Fp ->chr
is the flag or verb character to cause
.I fn
to be called.
In
.IR fn ,
.IB fp ->f1
and
.IB fp ->f2
are the decoded flags in the conversion.
A missing
.IB fp ->f1
is denoted by the value zero.
A missing
.IB fp ->f2
is denoted by a negative number.
.IB Fp ->f3
is the bitwise OR of all the
flags seen since the most recent
.LR % .
The standard flags values are: 1
.RB ( + ),
2
.RB ( - ),
4
.RB ( # ),
8
.RB ( l ),
16
.RB ( h ),
32
.RB ( u ),
and
64
.RB ( ll ).
.I Fn
is passed a pointer
.I arg
to the argument list.
If
.IB fp ->chr
is a verb,
.I fn
should use 
.BR va_arg
to fetch its argument from the list,
then format it, and return zero.
If
.IB fp ->chr
is a flag,
.I fn
should return a negative value:
the negation of one of the above flag values, or some otherwise unused power of two.
All interpretation of
.IB fp ->f1\f1,
.IB fp ->f2\f1,
and
.IB fp-> f3
is left up to the conversion routine.
.I Fmtinstall
returns 0 if the installation succeeds, \-1 if it fails.
.PP
.IR Sprint ,
.IR snprint ,
and
.I seprint
are re-entrant; they may be called to
help prepare output in custom conversion routines.
.PP
The function
.I strconv
formats a
.SM UTF
string.
.I S
is the string,
.I fp
has the same meaning as above.
The
.I strconv
routine interprets the
.L -
flag in
.IB fp ->f3
as left-justification.
The function
.I Strconv
(with a capital S)
is like
.IR strconv ,
but its input is a rune string, which is converted to
.SM UTF
on output.
.PP
.I Printcol
indicates the position of the next output character.
Tabs, backspaces and carriage returns are interpreted appropriately.
.PP
.I Numbconv
and
.I fltconv
are used to implement the integer and floating verbs.
Their arguments are like those of the function argument to
.IR fmtinstall .
Both
.I numbconv
and
.I fltconv
use
.I strconv
to put their results into the current print buffer.
.PP
One of
.IR strconv 
or
.I Strconv
must be called to produce output; no other routine puts characters
in the output buffer.
.PP
.I Doprint
formats the argument list arg
.I arg
into the buffer starting at
.IR s ,
but it writes no characters after the address
.IR es .
It returns a pointer to the NUL
terminating the
formatted string.
.SH EXAMPLES
This function prints an error message with a variable
number of arguments and then quits.
.IP
.EX
.ta 6n +6n +6n
void fatal(char *msg, ...)
{
	char buf[1024], *out;
	va_list arg;

	out = doprint(buf, buf+sizeof(buf), "Fatal error: ");
	va_start(arg, msg);
	out = doprint(out, buf+sizeof(buf), msg, arg);
	va_end(arg);
	write(2, buf, out-buf);
	exits("fatal error");
}
.EE
.PP
This example adds a verb to print complex numbers.
.IP
.EX
typedef
struct {
	double	r, i;
} Complex;

int
Xconv(va_list *arg, Fconv *fp)
{
	char str[50];
	Complex c;

	c = va_arg(*arg, Complex);
	sprint(str, "(%g,%g)", c.r, c.i);
	strconv(str, fp);
	return 0;
}

main(...)
{
	Complex x = (Complex){ 1.5, -2.3 };

	fmtinstall('X', Xconv);
	print("x = %X\en", x);
}
.EE
.SH SOURCE
.B /sys/src/libc/port
.SH SEE ALSO
.IR fprintf (2),
.IR utf (6),
.IR errstr (2)
.SH DIAGNOSTICS
.I Print
and
.I fprint
set
.IR errstr .
.SH BUGS
The formatting is close to that specified for ANSI
.IR fprintf (2);
the differences are:
.IP
the 
.B -
flag doesn't work
.IP
.B u
is a flag here instead of a verb
.IP
there are no
.B 0
or space flags here
.IP
there are no
.B P
or
.B n
verbs here
.PP
Also, and distinctly not a bug,
.I print
and friends generate
.SM UTF
rather than
.SM ASCII.