FAIL(9) 						   BSD Kernel Developer's Manual						   FAIL(9)

NAME

     KFAIL_POINT_CODE, KFAIL_POINT_RETURN, KFAIL_POINT_RETURN_VOID, KFAIL_POINT_ERROR, KFAIL_POINT_GOTO, fail_point, DEBUG_FP -- fail points

SYNOPSIS

     #include <sys/fail.h>

     KFAIL_POINT_CODE(parent, name, code);

     KFAIL_POINT_RETURN(parent, name);

     KFAIL_POINT_RETURN_VOID(parent, name);

     KFAIL_POINT_ERROR(parent, name, error_var);

     KFAIL_POINT_GOTO(parent, name, error_var, label);

DESCRIPTION

     Fail points are used to add code points where errors may be injected in a user controlled fashion.  Fail points provide a convenient wrapper
     around user-provided error injection code, providing a sysctl(9) MIB, and a parser for that MIB that describes how the error injection code
     should fire.

     The base fail point macro is KFAIL_POINT_CODE() where parent is a sysctl tree (frequently DEBUG_FP for kernel fail points, but various sub-
     systems may wish to provide their own fail point trees), and name is the name of the MIB in that tree, and code is the error injection code.
     The code argument does not require braces, but it is considered good style to use braces for any multi-line code arguments.  Inside the code
     argument, the evaluation of RETURN_VALUE is derived from the return() value set in the sysctl MIB.  See SYSCTL VARIABLES below.

     The remaining KFAIL_POINT_*() macros are wrappers around common error injection paths:

     KFAIL_POINT_RETURN(parent, name)
       is the equivalent of KFAIL_POINT_CODE(..., return RETURN_VALUE)

     KFAIL_POINT_RETURN_VOID(parent, name)
       is the equivalent of KFAIL_POINT_CODE(..., return)

     KFAIL_POINT_ERROR(parent, name, error_var)
       is the equivalent of KFAIL_POINT_CODE(..., error_var = RETURN_VALUE)

     KFAIL_POINT_GOTO(parent, name, error_var, label)
       is the equivalent of KFAIL_POINT_CODE(...,
	 { error_var = RETURN_VALUE; goto label;})

SYSCTL VARIABLES

     The KFAIL_POINT_*() macros add sysctl MIBs where specified.  Many base kernel MIBs can be found in the debug.fail_point tree (referenced in
     code by DEBUG_FP).

     The sysctl variable may be set using the following grammar:

       <fail_point> ::
	   <term> ( "->" <term> )*

       <term> ::
	   ( (<float> "%") | (<integer> "*" ) )*
	   <type>
	   [ "(" <integer> ")" ]

       <float> ::
	   <integer> [ "." <integer> ] |
	   "." <integer>

       <type> ::
	   "off" | "return" | "sleep" | "panic" | "break" | "print"

     The <type> argument specifies which action to take:

     off     Take no action (does not trigger fail point code)

     return  Trigger fail point code with specified argument

     sleep   Sleep the specified number of milliseconds

     panic   Panic

     break   Break into the debugger, or trap if there is no debugger support

     print   Print that the fail point executed

     The <float>% and <integer>* modifiers prior to <type> control when <type> is executed.  The <float>% form (e.g. "1.2%") can be used to spec-
     ify a probability that <type> will execute.  The <integer>* form (e.g. "5*") can be used to specify the number of times <type> should be exe-
     cuted before this <term> is disabled.  Only the last probability and the last count are used if multiple are specified, i.e. "1.2%2%" is the
     same as "2%".  When both a probability and a count are specified, the probability is evaluated before the count, i.e. "2%5*" means "2% of the
     time, but only 5 times total".

     The operator -> can be used to express cascading terms.  If you specify <term1>-><term2>, it means that if <term1> does not 'execute',
     <term2> is evaluated.  For the purpose of this operator, the return() and print() operators are the only types that cascade.  A return() term
     only cascades if the code executes, and a print() term only cascades when passed a non-zero argument.

EXAMPLES

     sysctl debug.fail_point.foobar="2.1%return(5)"
	     21/1000ths of the time, execute code with RETURN_VALUE set to 5.

     sysctl debug.fail_point.foobar="2%return(5)->5%return(22)"
	     2/100ths of the time, execute code with RETURN_VALUE set to 5.  If that does not happen, 5% of the time execute code with
	     RETURN_VALUE set to 22.

     sysctl debug.fail_point.foobar="5*return(5)->0.1%return(22)"
	     For 5 times, return 5.  After that, 1/1000th of the time, return 22.

     sysctl debug.fail_point.foobar="0.1%5*return(5)"
	     Return 5 for 1 in 1000 executions, but only 5 times total.

     sysctl debug.fail_point.foobar="1%*sleep(50)"
	     1/100th of the time, sleep 50ms.

CAVEATS

     It is easy to shoot yourself in the foot by setting fail points too aggressively or setting too many in combination.  For example, forcing
     malloc() to fail consistently is potentially harmful to uptime.

     The sleep() sysctl setting may not be appropriate in all situations.  Currently, fail_point_eval() does not verify whether the context is
     appropriate for calling msleep().

AUTHORS

     This manual page was written by Zach Loafman <zml@FreeBSD.org>.

BSD
								   May 10, 2009 							       BSD