.\"     $NetBSD: kprintf.9,v 1.8 2000/02/07 12:37:02 abs 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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.Dd September 1, 1998
.Dt KPRINTF 9
.Os
.Sh NAME
.Nm printf, sprintf, vprintf, uprintf, ttyprintf
.Nd kernel formatted output conversion
.Sh SYNOPSIS
.Fd #include <sys/systm.h>
.Ft void
.Fo "printf"
.Fa "const char *format"
.Fa "..."
.Fc
.Ft int
.Fo "sprintf"
.Fa "char *buf"
.Fa "const char *format"
.Fa "..."
.Fc
.Ft void
.Fo "vprintf"
.Fa "const char *format"
.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
.Sh DESCRIPTION
The
.Fn printf ,
.Fn sprintf ,
.Fn vprintf ,
.Fn uprintf ,
and
.Fn ttyprintf
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.
The functions
.Fn uprintf
and
.Fn ttyprintf
send formatted strings to the current process's controlling tty and a specific
tty,
respectively.
.Pp
Since each of these kernel functions is a scaled down version of its user space
counterpart
.Po
.Xr printf 3 ,
.Xr sprintf 3 ,
and
.Xr vprintf 3
.Pc ,
this page describes only the differences between the user space and kernel
versions, rather than describing all of their functional details.
.Ss FORMAT OPTIONS
In addition to the formatting specifiers that the user space functions
accept in the format string
.Fa format ,
the kernel functions accept the following format specifiers.
.Pp
.Bl -tag -width "\e177"
.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
.Pq "as an integer"
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
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
.Pq "distinguishable by its value being \(<= 32" ,
or the end of the decoding directive string itself.
.It Li %:
Inline format continuation.
This format specifier allows for recursive formatted output.
Its argument is the new formatting string to obey and the argument which
follows it is a
.Va va_list
argument vector from which to obtain the data to be formatted.
The
.Li %:
specifier is non portable, and its use is strongly discouraged, even in
NetBSD specific code (it is not supported on all ports).
.It Li %r
Integer value using current
.Tn DDB
radix.
.Bf -emphasis
This format specifier is only availble in kernels enabled with the
.Tn DDB
debugger.
.Po
See
.Xr ddb 4
.Pc .
.Ef
Displays an integer using the current
.Tn DDB
radix.
.It Li %z
Signed hexadecimal with
.Ql 0x
prefix.
.Bf -emphasis
This format specifier is only available in kernels enabled with the
.Tn DDB
debugger.
.Po
See
.Xr ddb 4
.Pc .
.Ef
Displays a signed integer using the C-style hexadecimal constant format.
.El
.Sh RETURN VALUES
The
.Fn sprintf
function returns the number of characters it placed in the buffer
.Fa buf .
.Sh EXAMPLES
An example 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
.Pp
An example use of the
.Li %:
format specifier for recursive formatting.
.Bd -literal -offset indent
void
bail(char *fmt, ...)
{
        va_list ap;
        va_start (ap, fmt);
        printf("bailing out: %:\en", fmt, ap);
        va_end(ap);
}

bail("descriptor %d is invalid.", 5)
\(rA "bailing out: descriptor 5 is invalid"
.Ed
.Sh SEE ALSO
.\" The following page has not been written, but I am including a cross-
.\" reference to it in the hopes that it will inspire someone to write it.
.Xr printf 3 ,
.Xr printf 1 ,
.Xr ddb 4 ,
.Xr tprintf 9
.Sh CODE REFERENCES
.Pa sys/kern/subr_prf.c
.Sh BUGS
The
.Li %b
format specifier cannot be used to decode integers greater than 32 bits in
size.
The
.Fn uprintf
and
.Fn ttyprintf
functions should be used sparingly, if at all.