PRINTF(9)              FreeBSD Kernel Developer's Manual             PRINTF(9)

NAME

     printf, uprintf, tprintf, log - formatted output conversion

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.
     The uprintf() function outputs to the current process' controlling tty,
     while printf() 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.

     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 and 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.

     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.

     The %n conversion specifier is not supported.

     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.

RETURN VALUES

     The printf() and the uprintf() functions return the number of characters
     displayed.

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)

FreeBSD 15.1-STABLE-HBSD       December 19, 2025                     PRINTF(9)