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PANIC(9)		    Kernel Developer's Manual			PANIC(9)

NAME
     panic -- bring down system on fatal error

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

     extern char *panicstr;

     void
     panic(const char *fmt, ...);

     void
     vpanic(const char *fmt, va_list ap);

     KERNEL_PANICKED();

DESCRIPTION
     The  panic() and vpanic() functions terminate the running system.	The mes-
     sage fmt is a printf(3) style format string.  The message is printed to the
     console and panicstr is set pointing to the address of  the  message  text.
     This can be retrieved from a core dump at a later time.

     Upon entering the panic() function the panicking thread disables interrupts
     and calls critical_enter(9).  This prevents the thread from being preempted
     or interrupted while the system is still in a running state.  Next, it will
     instruct  the  other  CPUs  in  the system to stop.  This synchronizes with
     other threads to prevent concurrent panic conditions from interfering  with
     one  another.  In the unlikely event of concurrent panics, only one panick-
     ing thread will proceed.

     Control will be passed to the kernel debugger  via  kdb_enter().	This  is
     conditional   on	a   debugger   being   installed   and	enabled  by  the
     debugger_on_panic variable; see ddb(4) and gdb(4).  The debugger may initi-
     ate a system reset, or it may eventually return.

     Finally, kern_reboot(9) is called to restart the system, and a kernel  dump
     will  be  requested.   If panic() is called recursively (from the disk sync
     routines, for example), kern_reboot() will be instructed not  to  sync  the
     disks.

     The  vpanic() function implements the main body of panic().  It is suitable
     to be called by functions which perform their own variable-length	argument
     processing.  In all other cases, panic() is preferred.

     The KERNEL_PANICKED() macro is the preferred way to determine if the system
     has  panicked.   It  returns  a  boolean value.  Most often this is used to
     avoid taking an action that cannot possibly succeed in a panic context.

EXECUTION CONTEXT
     Once the panic has been initiated, code executing in  a  panic  context  is
     subject to the following restrictions:

     *	 Single-threaded  execution.   The scheduler is disabled, and other CPUs
	 are stopped/forced idle.  Functions that manipulate the scheduler state
	 must be avoided.  This includes, but is not limited to,  wakeup(9)  and
	 sleepqueue(9) functions.

     *	 Interrupts  are  disabled.   Device  I/O  (e.g. to the console) must be
	 achieved with polling.

     *	 Dynamic memory allocation cannot be relied on, and must be avoided.

     *	 Lock acquisition/release will be ignored, meaning these operations will
	 appear to succeed.

     *	 Sleeping on a resource is not strictly prohibited, but will  result  in
	 an immediate return from the sleep function.  Time-based sleeps such as
	 pause(9) may be performed as a busy-wait.

RETURN VALUES
     The panic() and vpanic() functions do not return.

SEE ALSO
     printf(3), ddb(4), gdb(4), KASSERT(9), kern_reboot(9)

FreeBSD ports 15.quarterly	 March 17, 2023 			PANIC(9)

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