lib/libutil/snprintb.3

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.Dd May 7, 2009
.Dt SNPRINTB 3
.Os
.Sh NAME
.Nm snprintb
.Nd bitmask output conversion
.Sh LIBRARY
.Lb libutil
.Sh SYNOPSIS
.In util.h
.Ft int
.Fn "snprintb" "char *buf" "size_t buflen" "const char *fmt" "uint64_t val"
.Ft int
.Fn "snprintb_m" "char *buf" "size_t buflen" "const char *fmt" "uint64_t val" \
"size_t max"
.Sh DESCRIPTION
The
.Fn snprintb
function formats a bitmask into a mnemonic form suitable for printing.
.Pp
This conversion is useful for decoding bit fields in device registers.
It formats the integer
.Fa val
into the buffer
.Fa buf ,
of size
.Fa buflen ,
using a specified radix and an interpretation of
the bits within that integer as though they were flags.
The buffer is always NUL-terminated.
If the buffer
.Fa buf
is too small to hold the formatted output,
.Fn snprintb
will fill as much as it can, and return the number of bytes
that would have written if the buffer was long enough excluding the
terminating NUL.
.Pp
The decoding directive string
.Fa fmt
describes how the bitfield is to be interpreted and displayed.
It follows two possible syntaxes, referred to as
.Dq old
and
.Dq new .
The main advantage of the
.Dq new
formatting is that it is capable of handling multi-bit fields.
.Pp
The first character of
.Fa fmt
may be
.Li \e177 ,
indicating that the remainder of the format string follows the
.Dq new
syntax.
The second character
.Pq the first for the old format
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
.Fa fmt
are interpreted as a list of bit-position\(endescription pairs.
From here the syntaxes diverge.
.Pp
The
.Dq old
format syntax is series of bit-position\(endescription pairs.
Each 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 \*[Le] 32 ,
or the end of the decoding directive string itself.
.Pp
For the
.Dq new
format syntax, a bit-position\(endescription begins with a field type
followed by a binary bit-position and possibly a field length.
The least significant bit is bit-position zero, unlike the
.Dq old
syntax where it is one.
.Bl -tag -width "xxxxx"
.It Cm b\eB
Describes a bit position.
The bit-position
.Fa B
indicates the corresponding bit, as in the
.Dq old
format.
.It Cm f\eB\eL
Describes a multi-bit field beginning at bit-position
.Fa B
and having a bit-length of
.Fa L .
The remaining characters are printed as a description of the field
followed by
.Sq \&=
and the value of the field.
The value of the field is printed in the base specified as the second
character of the decoding directive string
.Ar fmt .
.It Cm F\eB\eL
Describes a multi-bit field like
.Sq f ,
but just extracts the value for use with the
.Sq \&=
and
.Sq \&:
formatting directives described below.
.It Cm \&=\eV
The field previously extracted by the last
.Sq f
or
.Sq F
operator is compared to the byte
.Sq Cm V
.Pq for values 0 through 255 .
If they are equal,
.Sq \&=
followed by the string following
.Sq Cm V
is printed.
This and the
.Sq \&:
operator may be repeated to annotate multiple possible values.
.It Cm :\eV
Operates like the
.Sq \&=
operator, but omits the leading
.Sq \&= .
.El
.Pp
Finally, each field is delimited by a NUL
.Pq Sq \e0
character.
By convention, the format string has an additional NUL character at
the end, following that delimiting the last bit-position\(endescription
pair.
.Pp
The
.Fn snprintb_m
function accepts an additional
.Fa max
argument.
If this argument is zero, the
.Fn snprintb_m
function returns exactly the same results in the
.Fa buf
as the
.Fn snprintb
function.
If the
.Fa max
argument is present and has a non-zero value, it represents the maximum
length of a formatted string.
If the formatted string would require more than
.Fa max
characters, the
.Fn snprintb_m
function returns multiple formatted strings in the output buffer
.Fa buf .
Each string is NUL-terminated, and the last string is followed by an
additional NUL character (or, if you prefer, a zero-length string).
.Sh RETURN VALUES
The
.Fn snprintb
and
.Fn snprintb_m
functions return the number of bytes that would have written to the buffer
if there was adequate space, excluding the final terminating NUL, or \-1 in
case an error occurred.
For
.Fn snprintb_m ,
the NUL characters terminating each individual string are included in the
total number of bytes.
.Sh EXAMPLES
Two examples of the old formatting style:
.Bd -literal -offset indent
snprintb(buf, buflen, "\e10\e2BITTWO\e1BITONE", 3)
\(rA "03<BITTWO,BITONE>"

snprintb(buf, buflen,
       "\e20\ex10NOTBOOT\ex0f" "FPP\ex0eSDVMA\ex0cVIDEO"
       "\ex0bLORES\ex0a" "FPA\ex09" "DIAG\ex07" "CACHE"
       "\ex06IOCACHE\ex05LOOPBACK\ex04" "DBGCACHE",
       0xe860)
\(rA "0xe860<NOTBOOT,FPP,SDVMA,VIDEO,CACHE,IOCACHE>"
.Ed
.Pp
An example of the new formatting style:
.Bd -literal -offset indent
snprintb(buf, buflen,
       "\e177\e020b\e0LSB\e0b\e1_BITONE\e0f\e4\e4NIBBLE2\e0"
       "f\ex10\e4BURST\e0=\e4FOUR\e0=\exfSIXTEEN\e0"
       "b\ex1fMSB\e0\e0",
       0x800f0701)
\(rA "0x800f0701<LSB,NIBBLE2=0x0,BURST=0xf=SIXTEEN,MSB>"
.Ed
.Pp
An example using snprintb_m:
.Bd -literal -offset indent
snprintb_m(buf, buflen,
       "\e177\e020b\e0LSB\e0b\e1_BITONE\e0f\e4\e4NIBBLE2\e0"
       "f\ex10\e4BURST\e0=\e4FOUR\e0=\exfSIXTEEN\e0"
       "b\ex1fMSB\e0\e0",
       0x800f0701, 34)
\(rA "0x800f0701<LSB,NIBBLE2=0x0>\e00x800f0701<BURST=0xf=SIXTEEN,MSB>\e0"
.Ed
.Sh ERRORS
.Fn snprintb
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The leading character does not describe a supported format,
or
.Fn snprintf
failed.
.El
.Sh SEE ALSO
.Xr printf 3 ,
.Xr snprintf 3
.Sh HISTORY
The
.Fn snprintb
function was originally implemented as a non-standard
.Li %b
format string for the kernel
.Fn printf
function in
.Nx 1.5
and earlier releases.
It was called
.Fn bitmask_snprintf
in
.Nx 5.0
and earlier releases.
.Sh AUTHORS
The
.Dq new
format was the invention of
.An Chris Torek .