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.Li -> .Vt where appropriate;
from josiah frentsos, tweaked by schwarze

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.\"	$OpenBSD: printf.9,v 1.26 2022/09/11 06:38:11 jmc Exp $
.\"     $NetBSD: kprintf.9,v 1.6 1999/03/16 00:40:47 garbled Exp $
.\"
.\" Copyright (c) 1998 The NetBSD Foundation, Inc.
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.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Jeremy Cooper.
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.Dd $Mdocdate: September 11 2022 $
.Dt PRINTF 9
.Os
.Sh NAME
.Nm printf ,
.Nm snprintf ,
.Nm vprintf ,
.Nm vsnprintf ,
.Nm uprintf ,
.Nm ttyprintf ,
.Nm db_printf ,
.Nm db_vprintf
.Nd kernel formatted output conversion
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Ft int
.Fo printf
.Fa "const char *format"
.Fa "..."
.Fc
.Ft int
.Fo snprintf
.Fa "char *buf"
.Fa "size_t size"
.Fa "const char *format"
.Fa "..."
.Fc
.Ft int
.Fo vprintf
.Fa "const char *format"
.Fa "va_list ap"
.Fc
.Ft int
.Fo vsnprintf
.Fa "char *buf"
.Fa "size_t size"
.Fa "const char *fmt"
.Fa "va_list ap"
.Fc
.Ft void
.Fo uprintf
.Fa "const char *format"
.Fa "..."
.Fc
.Ft void
.Fo ttyprintf
.Fa "struct tty *tty"
.Fa "const char *format"
.Fa "..."
.Fc
.In ddb/db_output.h
.Ft void
.Fn db_printf "const char *format" ...
.Ft void
.Fn db_vprintf "const char *format" "va_list ap"
.Sh DESCRIPTION
The
.Fn printf ,
.Fn snprintf ,
.Fn vprintf ,
.Fn vsnprintf ,
.Fn uprintf ,
.Fn ttyprintf ,
.Fn db_printf ,
and
.Fn db_vprintf
functions allow the kernel to send formatted messages to various output
devices.
The functions
.Fn printf
and
.Fn vprintf
send formatted strings to the system console and to the system log.
The functions
.Fn uprintf
and
.Fn ttyprintf
send formatted strings to the current process's controlling tty and a specific
tty,
respectively.
The functions
.Fn db_printf
and
.Fn db_vprintf
send formatted strings to the ddb console, and are only used to implement
.Xr ddb 4 .
.Pp
Since each of these kernel functions is a variant of its user space
counterpart, this page describes only the differences between the user
space and kernel versions.
.Pp
Only a subset of the user space conversion specification is available to the
kernel version:
.Bd -filled -offset indent
.Sm off
.Cm %
.Op Ar width
.Op Ar size
.Ar conversion
.Sm on
.Ed
.Pp
Refer to
.Xr printf 3
for functional details.
.Ss FORMAT OPTIONS
The kernel functions don't support as many formatting specifiers as their
user space counterparts.
In addition to the floating point formatting specifiers,
the following integer type specifiers are not supported:
.Bl -tag -width 5n
.It Li %hh
Argument of
.Vt char
type.
This format specifier is accepted by the kernel but will be handled as
.Li %h .
.It Li %j
Argument of
.Vt intmax_t
or
.Vt uintmax_t
type.
.It Li %t
Argument of
.Vt ptrdiff_t
type.
.El
.Pp
The kernel functions support some additional formatting specifiers:
.Bl -tag -width 5n
.It Li %b
Bit field expansion.
This format specifier is useful for decoding bit fields in device registers.
It displays an integer using a specified radix
.Pq base
and an interpretation of
the bits within that integer as though they were flags.
It requires two arguments from the argument vector, the first argument being
the bit field to be decoded
(of type
.Vt int ,
unless a width modifier has been specified)
and the second being a decoding directive string.
.Pp
The decoding directive string describes how the bitfield is to be interpreted
and displayed.
The first character of the string is a binary character representation of the
output numeral base in which the bitfield will be printed before it is decoded.
Recognized radix values
.Pq "in C escape-character format"
are
.Li \e10
.Pq octal ,
.Li \e12
.Pq decimal ,
and
.Li \e20
.Pq hexadecimal .
.Pp
The remaining characters in the decoding directive string are interpreted as a
list of bit-position\(endescription pairs.
A bit-position\(endescription pair begins with a binary character value
that represents the position of the bit being described.
A bit position value of one describes the least significant bit.
Whereas a position value of 32
.Pq "octal 40, hexadecimal 20, the ASCII space character"
describes the most significant bit.
.Pp
To deal with more than 32 bits, the characters 128
.Pq "octal 200, hexadecimal 80"
through 255
.Pq "octal 377, hexadecimal FF"
are used.
The value 127 is subtracted from the character to determine the
bit position (1 is least significant, and 128 is most significant).
.Pp
The remaining characters in a bit-position\(endescription pair are the
characters to print should the bit being described be set.
Description strings are delimited by the next bit position value character
encountered
.Po
distinguishable by its value being \*(Le 32 or \*(Ge 128
.Pc ,
or the end of the decoding directive string itself.
.El
.Sh RETURN VALUES
The
.Fn printf
and
.Fn vprintf
functions return the number of characters printed.
.Pp
The
.Fn snprintf
and
.Fn vsnprintf
functions return the number of characters that would have been put into
the buffer
.Fa buf
if the
.Fa size
were unlimited.
.Sh EXAMPLES
Use of the
.Li %b
format specifier for decoding device registers.
.Bd -literal -offset indent
printf("reg=%b\en", 3, "\e10\e2BITTWO\e1BITONE")
\(rA "reg=3<BITTWO,BITONE>"

printf("enablereg=%b\en", 0xe860,
       "\e20\ex10NOTBOOT\ex0fFPP\ex0eSDVMA\ex0cVIDEO"
       "\ex0bLORES\ex0aFPA\ex09DIAG\ex07CACHE"
       "\ex06IOCACHE\ex05LOOPBACK\ex04DBGCACHE")
\(rA "enablereg=e860<NOTBOOT,FPP,SDVMA,VIDEO,CACHE,IOCACHE>"
.Ed
.Sh CODE REFERENCES
.Pa sys/kern/subr_prf.c
.Sh SEE ALSO
.Xr revoke 2 ,
.Xr printf 3 ,
.Xr ddb 4 ,
.Xr log 9