PRINTF(9)

💖 PRINTF(9): formatted output conversion 💖

okay so welcome to the Kernel Developer's Manual, bestie — this is the page for printf, uprintf, tprintf, and log (plus vprintf() and vlog() chilling in the synopsis). It's giving kernel-side formatted output, and honestly the vibes are immaculate. 🫶

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🌸 NAME 🌸

printf, uprintf, tprintf, log — formatted output conversion. periodt.

✨ SYNOPSIS ✨

#include <sys/types.h>
#include <sys/systm.h>

int
printf(const char *fmt, ...);

void
tprintf(struct proc *p, int pri, const char *fmt, ...);

int
uprintf(const char *fmt, ...);

int
vprintf(const char *fmt, va_list ap);

#include <sys/syslog.h>

void
log(int pri, const char *fmt, ...);

void
vlog(int pri, const char *fmt, va_list ap);

💅 DESCRIPTION 💅

The printf family of functions are similar to the printf(3) family of functions. The different functions each use a different output stream. no cap, that's the whole plot.

• The uprintf() function outputs to the current process' controlling tty.
• The printf() function writes to the console as well as to the logging facility.
• The tprintf() function outputs to the tty associated with the process p and the logging facility if pri is not -1.
• The log() function sends the message to the kernel logging facility, using the log level as indicated by pri, and to the console if no process is yet reading the log.

Each of these related functions use the fmt parameter in the same manner as printf(3). However, printf adds two other conversion specifiers and omits one. lowkey iconic behavior.

🎀 %b — bitmask decode realness 🎀

The %b identifier expects two arguments: an int and a char *. These are used as a register value and a print mask for decoding bitmasks.

The print mask is made up of two parts: the base and the arguments.

• The base value is the output base (radix) expressed as an octal value; for example, \10 gives octal and \20 gives hexadecimal.
• The arguments are made up of a sequence of bit identifiers.

Each bit identifier begins with a character specifying the number of the bit (starting from 1) this identifier describes.

• The characters from \01 to \40 can be used to specify bit numbers in the range from 1 to 32.
• The characters from \200 to \377 to specify bit numbers in the range from 1 to 128.

The rest of the identifier is a string of characters containing the name of the bit. The identifier is terminated by either the bit number at the start of the next bit identifier or by NUL for the last bit identifier.

🦋 %D — hexdump helper 🦋

The %D identifier is meant to assist in hexdumps. It requires two arguments: a u_char * pointer and a char * string.

The memory pointed to by the pointer is output in hexadecimal one byte at a time. The string is used as a delimiter between individual bytes. If present, a width directive will specify the number of bytes to display. By default, 16 bytes of data are output.

💀 %n — nope, not here 💀

The %n conversion specifier is not supported. not me looking for it, bestie, it's literally not on the menu.

🌟 log() priority tea 🌟

The log() function uses syslog(3) level values LOG_DEBUG through LOG_EMERG for its pri parameter (mistakenly called ‘priority’ here).

Alternatively, if a pri of -1 is given, the message will be appended to the last log message started by a previous call to log().

As these messages are generated by the kernel itself, the facility will always be LOG_KERN. periodt.

🦋 RETURN VALUES 🦋

The printf() and the uprintf() functions return the number of characters displayed. that's it, that's the return value. ✨

🎀 EXAMPLES 🎀

This example demonstrates the use of the %b and %D conversion specifiers. The function

void
printf_test(void)
{
        printf("reg=%b\n", 3, "\10\2BITTWO\1BITONE");
        printf("out: %4D\n", "AAZZ", ":");
}

will produce the following output:

reg=3<BITTWO,BITONE>
out: 41:41:5a:5a

The same output will be generated by the following function:

void
printf_test(void)
{
        printf("reg=%b\n", 3, "\10\201BITTWO\200BITONE");
        printf("out: %4D\n", "AAZZ", ":");
}

The call

log(LOG_DEBUG, "%s%d: been there.\n", sc->sc_name, sc->sc_unit);

will add the appropriate debug message at priority “kern.debug” to the system log.

🌈 SEE ALSO 🌈

printf(3), syslog(3)

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💖 okay that's the whole PRINTF(9) page, bestie — sourced from Debian, dated December 19, 2025. periodt. 💖