code: 9ferno

ref: b39057dffedac61fd6c028908a9aaadf4426b446
dir: /man/2/sys-print/

.TH SYS-PRINT 2
.SH NAME
print, aprint, fprint, sprint \- print formatted output
.SH SYNOPSIS
.EX
include "sys.m";
sys := load Sys Sys->PATH;

aprint: fn(format: string, *): array of byte;
fprint: fn(fd: ref FD, format: string, *): int;
print:  fn(format: string, *): int;
sprint: fn(format: string, *): string;
.EE
.SH DESCRIPTION
These functions format and print their arguments as
.SM UTF
text.
.B Print
writes text to the standard output.
.B Fprint
writes to the named output
file descriptor.
.B Sprint
places text
in a string, which it returns.
.B Aprint
is similar but returns the text in
.IR utf (6)
representation as an array of bytes.
.B Print
and
.B fprint
return the number of bytes transmitted
or
a negative value if an  error was encountered when writing the output.
.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 Limbo compiler recognizes calls to these functions
and checks that the arguments match the format specifications in number and type.
.PP
Each conversion specification has the following format:
.IP
.BI "%" " \fR[\fPflags\fR]\fP 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 .
They can be separated by
.RB ` . ',
and if one is present, then
.I f1
and
.I f2
are taken to be zero if missing, otherwise they are considered `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
.TP
.BR d ", " o ", " x ", " X
The numeric verbs
.BR d ,
.BR o ,
and
.B x
format their
.B int
arguments in decimal, octal, and hexadecimal (with hex digits in lower-case).
The flag
.B b
is required when the corresponding value is a Limbo
.BR  big ,
not an
.BR int .
Arguments are taken to be signed, unless the
.B u
flag is given, to force them to be treated as unsigned.
Each interprets the flags
.B \- ,
.B , ,
and
.B #
to mean left justified, commas every three digits, and alternative format.
If
.I f2
is not omitted, the number is padded on the left with zeros
until at least
.I f2
digits appear.
Then, if alternative format is specified
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.
The verb
.B X
is similar to
.BR x ,
except that the hexadecimal digits are displayed in upper-case,
and in alternative format, the number is preceded by
.BR 0X .
.PP
.TP
.BR e ", " f ", " g
The floating point verbs
.BR e ,
.BR f ,
and
.BR g
take a
.B real
argument.
Each interprets the flags
.BR + ,
.BR \- ,
and
.B #
to mean
always print a sign,
left justified,
and
alternative 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
and
.B f
conversions,
and
.I f2
is the maximum number of significant digits for
.B g
conversions.
The 
.B f
verb produces output of the form
.RB [ \- ]\c
.IR digits [\c
.BI \&. digits\fR].
The
.B e
conversion appends an exponent
.BR e [ \- ]\c
.IR 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.
When alternative format is specified, the result will always contain a decimal point,
and for
.B g
conversions, trailing zeros are not removed.
.TP
.BR E ", " G
These are the same as
.B e
and
.B g
respectively, but use
.B E
not
.B e
to specify an exponent when one appears.
.TP
.B c
The
.B c
verb converts a single Unicode character
from an
.B int
argument to a UTF encoding,
justified within a field of
.I f1
characters as described above.
.TP
.B r
The
.B r
verb takes no arguments; it prints the error string
associated with the most recent system error.
.TP
.B s
The
.B s
verb copies a
.B string
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.
.TP
.B q
The
.B q
verb copies a
.B string
to the output as for
.BR s ,
but quotes the string in the style of
.IR sh (1)
only if necessary to avoid ambiguity (for instance
if the string contains quotes or spaces).
If the format string, however, includes the specifier
.B #
(for example
.BR %#q ),
the printed string will always be quoted.
.SH SOURCE
.B /libinterp/runt.c:/^xprint
.br
.B /os/port/print.c
.br
.B /lib9/print.c
.SH SEE ALSO
.IR sys-intro (2),
.IR sys-open (2)
.SH BUGS
The
.B x
verb does not apply the
.B 0x
prefix when
.I f2
is present.
The prefix should probably be
.B 16r
anyway.