SNPRINTB(3) Library Functions Manual SNPRINTB(3)

snprintb, snprintb_m — bitmask output conversion

System Utilities Library (libutil, -lutil)

#include <util.h>

int
snprintb(char *buf, size_t bufsize, const char *fmt, uint64_t val);

int
snprintb_m(char *buf, size_t bufsize, const char *fmt, uint64_t val, size_t max);

The () function formats a bitmask into a mnemonic form suitable for printing.

It formats the integer val into the buffer buf, of size bufsize, interpreting the bits within that integer as flags or groups of bits. The buffer is always NUL-terminated. If the buffer buf is too small to hold the formatted output, () 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. If bufsize is zero, nothing is written and buf may be a null pointer.

The () function accepts an additional max argument. If this argument is zero, the snprintb_m() function behaves exactly like the snprintb() function. If the max argument 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 zero-length string).

The decoding directive in fmt describes how the bitfield is to be interpreted and displayed. It follows two possible formats, referred to as “old” and “new”. The “old” format is limited to describing single bits in a 32-bit value, the bit positions are 1-based. The “new” format supports multi-bit fields and 64-bit values, the bit positions are 0-based.

If the first character of fmt is (in C escape-character format) ‘\177’ or ‘\x7f’, the remainder of the fmt argument follows the “new” format.

The next character (the first for the “old” format) specifies the numeral base in which to print the numbers in the output. The possible values are ‘\010’ or ‘\x08’ for octal, ‘\012’ or ‘\x0a’ for decimal, and ‘\020’ or ‘\x10’ for hexadecimal.

The remaining characters in the fmt argument represent the formatting conversions, according to the “old” or “new” format.

In the “old” format, each conversion specifies a bit position and a description that is printed if the corresponding bit is set.

The bit position is a 1-based single-byte binary value, ranging from ‘\001’ or ‘\x01’ (1) for the least significant bit up to ‘\040’ or ‘\x20’ (32) for the most significant bit.

The description is delimited by the next character whose value is 32 or less (see ascii(7)), or by the end of the format string itself.

In the “new” format, each conversion begins with a conversion type, followed by type-specific parameters, each encoded as a single byte, followed by a NUL-terminated description. The bit positions are 0-based, ranging from ‘\000’ or ‘\x00’ (0) for the least significant bit to ‘\077’ or ‘\x3f’ (63) for the most significant bit.

bit descr
Prints the description from descr if the bit at the position bit is set.
lsb width descr
Prints the description from descr, a delimiting ‘=’ and the numerical value of the multi-bit field whose least significant bit is at lsb and that spans width bits. To print individual values of the field, see the ‘=’ and ‘*’ conversions below.
cmp descr
Compares the field value from the previous ‘f’ conversion to the single-byte value cmp, ranging from ‘\000’ or ‘\x00’ (0) to ‘\377’ or ‘\xff’ (255). If they are equal, prints ‘=’ followed by the description from descr. This conversion may be repeated.
lsb width [descr]
Describes a multi-bit field like ‘f’, but just extracts the value for use with the ‘:’ and ‘*’ conversions below. The description from descr is ignored, it is only present for uniformity with the other conversions.
cmp descr
Compares the field value from the previous ‘F’ conversion to the single-byte value cmp, ranging from ‘\000’ or ‘\x00’ (0) to ‘\377’ or ‘\xff’ (255). If they are equal, prints the description from descr. This conversion may be repeated.
fmt
If none of the previous ‘=’ or ‘:’ conversions matched, prints the format string fmt via snprintf(3). The format string fmt may contain a single uintmax_t conversion specification to print the field value that did not match.

The new format is terminated by an additional NUL character at the end, following that delimiting the last conversion. This NUL is supplied by the compiler to terminate the string literal and doesn't need to be written explicitly.

The snprintb() and snprintb_m() functions return the number of bytes that they 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, bufsize, "\010\002BITTWO\001BITONE", 3)
⇒ "03<BITTWO,BITONE>"

snprintb(buf, bufsize,
    "\x10"
    "\x10" "NOTBOOT"
    "\x0f" "FPP"
    "\x0e" "SDVMA"
    "\x0c" "VIDEO"
    "\x0b" "LORES"
    "\x0a" "FPA"
    "\x09" "DIAG"
    "\x07" "CACHE"
    "\x06" "IOCACHE"
    "\x05" "LOOPBACK"
    "\x04" "DBGCACHE",
    0xe860)
⇒ "0xe860<NOTBOOT,FPP,SDVMA,VIDEO,CACHE,IOCACHE>"

An example of the new formatting style:

snprintb(buf, bufsize,
    "\177\020"
    "b\000" "LSB\0"
    "b\001" "BITONE\0"
    "f\004\004" "NIBBLE2\0"
    "f\020\004" "BURST\0"
        "=\x04" "FOUR\0"
        "=\x0f" "FIFTEEN\0"
    "b\037" "MSB\0",
    0x800f0701)
⇒ "0x800f0701<LSB,NIBBLE2=0,BURST=0xf=FIFTEEN,MSB>"

The same example using snprintb_m:

snprintb_m(buf, bufsize,
    "\177\020"
    "b\000" "LSB\0"
    "b\001" "BITONE\0"
    "f\004\004" "NIBBLE2\0"
    "f\020\004" "BURST\0"
        "=\x04" "FOUR\0"
        "=\x0f" "FIFTEEN\0"
    "b\037" "MSB\0",
    0x800f0701, 34)
⇒ "0x800f0701<LSB,NIBBLE2=0>\0"
   "0x800f0701<BURST=0xf=FIFTEEN,MSB>\0"
   ""

A more complex example from <sys/mman.h> that uses both the single-bit ‘b’ formatting as well as the multi-bit field ‘F’ formatting with a default ‘*’:

#define MAP_FMT "\177\020"                      \
        "b\0"  "SHARED\0"                       \
        "b\1"  "PRIVATE\0"                      \
        "b\2"  "COPY\0"                         \
        "b\4"  "FIXED\0"                        \
        "b\5"  "RENAME\0"                       \
        "b\6"  "NORESERVE\0"                    \
        "b\7"  "INHERIT\0"                      \
        "b\11" "HASSEMAPHORE\0"                 \
        "b\12" "TRYFIXED\0"                     \
        "b\13" "WIRED\0"                        \
        "F\14\1\0"                              \
                ":\0" "FILE\0"                  \
                ":\1" "ANONYMOUS\0"             \
        "b\15" "STACK\0"                        \
        "F\30\010\0"                            \
                ":\000" "ALIGN=NONE\0"          \
                ":\012" "ALIGN=1KB\0"           \
                ":\013" "ALIGN=2KB\0"           \
                ":\014" "ALIGN=4KB\0"           \
                ":\015" "ALIGN=8KB\0"           \
                ":\016" "ALIGN=16KB\0"          \
                ":\017" "ALIGN=32KB\0"          \
                ":\020" "ALIGN=64KB\0"          \
                ":\021" "ALIGN=128KB\0"         \
                ":\022" "ALIGN=256KB\0"         \
                ":\023" "ALIGN=512KB\0"         \
                ":\024" "ALIGN=1MB\0"           \
                ":\025" "ALIGN=2MB\0"           \
                ":\026" "ALIGN=4MB\0"           \
                ":\027" "ALIGN=8MB\0"           \
                ":\030" "ALIGN=16MB\0"          \
                ":\034" "ALIGN=256MB\0"         \
                ":\040" "ALIGN=4GB\0"           \
                ":\044" "ALIGN=64GB\0"          \
                ":\050" "ALIGN=1TB\0"           \
                ":\054" "ALIGN=16TB\0"          \
                ":\060" "ALIGN=256TB\0"         \
                ":\064" "ALIGN=4PB\0"           \
                ":\070" "ALIGN=64PB\0"          \
                ":\074" "ALIGN=1EB\0"           \
                "*"     "ALIGN=2^%ju\0"

snprintb(buf, bufsize, MAP_FMT, 0x0d001234)
⇒ "0xd001234<COPY,FIXED,RENAME,HASSEMAPHORE,ANONYMOUS,ALIGN=8KB>"

snprintb(buf, bufsize, MAP_FMT, 0x2e000000)
⇒ "0x2e000000<FILE,ALIGN=2^46>"

snprintb() will fail if:

[]
The leading character (for the “old” format) or the second character (for the “new” format) does not describe a supported numeral base, or a bit number in the fmt argument is out of bounds, or the sequence of conversions in the fmt argument is invalid, or snprintf() failed.

snprintf(3)

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 releases.

The “new” format was the invention of Chris Torek.

When using hexadecimal character escapes for bit positions or field widths, if a following description starts with one of the letters A to F, that letter is considered part of the character escape. In such a situation, the character escape and the description must be put into separate string literals, as in "\x0f" "FIFTEEN".

April 8, 2024 NetBSD 11.0