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SNPRINTB(3) NetBSD Library Functions Manual SNPRINTB(3)
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snprintb, snprintb_m -- bitmask output conversion
System Utilities Library (libutil, -lutil)
snprintb(char *buf, size_t buflen, const char *fmt, uint64_t val);
snprintb_m(char *buf, size_t buflen, const char *fmt, uint64_t val,
The snprintb() function formats a bitmask into a mnemonic form suitable
This conversion is useful for decoding bit fields in device registers.
It formats the integer val into the buffer buf, of size 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 buf is too small to hold the formatted output, snprintb() will
fill as much as it can, and return the number of bytes that it would have
written if the buffer were long enough excluding the terminating NUL.
The decoding directive string fmt describes how the bitfield is to be
interpreted and displayed. It follows two possible syntaxes, referred to
as ``old'' and ``new''. The main advantage of the ``new'' formatting is
that it is capable of handling multi-bit fields.
The first character of fmt may be \177, indicating that the remainder of
the format string follows the ``new'' syntax. The second character (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 (in C escape-character format) are \10
(octal), \12 (decimal), and \20 (hexadecimal).
The remaining characters in fmt are interpreted as a list of bit-posi-
tion-description pairs. From here the syntaxes diverge.
The ``old'' format syntax is series of bit-position-description 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 (octal 40, hexadecimal
20, the ASCII space character) describes the most significant bit.
The remaining characters in a bit-position-description pair are the char-
acters to print should the bit being described be set. Description
strings are delimited by the next bit position value character encoun-
tered (distinguishable by its value being <= 32), or the end of the
decoding directive string itself.
For the ``new'' format syntax, a bit-position-description 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 ``old'' syntax
where it is one.
b\B Describes a bit position. The bit-position B indicates the corre-
sponding bit, as in the ``old'' format.
f\B\L Describes a multi-bit field beginning at bit-position B and having
a bit-length of L. The remaining characters are printed as a
description of the field followed by `=' 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 fmt.
F\B\L Describes a multi-bit field like `f', but just extracts the value
for use with the `=' and `:' formatting directives described
=\V The field previously extracted by the last `f' or `F' operator is
compared to the byte `V' (for values 0 through 255). If they are
equal, `=' followed by the string following `V' is printed. This
and the `:' operator may be repeated to annotate multiple possible
:\V Operates like the `=' operator, but omits the leading `='.
*FMT This provides a ``default'' case that prints FMT using printf(3)
when other `:' or `=' have not matched. FMT may contain a
uintmax_t format specification that prints the value that did not
match, since the field can be more than 32 bits wide.
Finally, each field is delimited by a NUL (`\0') character. By conven-
tion, the format string has an additional NUL character at the end, fol-
lowing that delimiting the last bit-position-description pair.
The snprintb_m() function accepts an additional max argument. If this
argument is zero, the snprintb_m() function returns exactly the same
results in the buf as the snprintb() function. If the 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 max
characters, the snprintb_m() function returns multiple formatted strings
in the output buffer buf. Each string is NUL-terminated, and the last
string is followed by an additional NUL character (or, if you prefer, a
The snprintb() and 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
snprintb_m(), the NUL characters terminating each individual string are
included in the total number of bytes.
Two examples of the old formatting style:
snprintb(buf, buflen, "\10\2BITTWO\1BITONE", 3)
"\x0bLORES\x0a" "FPA\x09" "DIAG\x07" "CACHE"
An example of the new formatting style:
A more complex example from <sys/mman.h> that uses the both bit position
`b' formatting as well as the `F' multi-field formatting with a default
#define MAP_FMT "\177\020\
snprintb(buf, buflen, MAP_FMT, 0x0d001234)
snprintb(buf, buflen, MAP_FMT, 0x2e000000)
An example using snprintb_m:
snprintb() will fail if:
[EINVAL] The leading character does not describe a supported
format, or snprintf() failed.
The snprintb() function was originally implemented as a non-standard %b
format string for the kernel printf() function in NetBSD 1.5 and earlier
releases. It was called bitmask_snprintf() in NetBSD 5.0 and earlier
The ``new'' format was the invention of Chris Torek.
NetBSD 9.99 December 6, 2019 NetBSD 9.99