plan9fox/sys/man/9/error

.TH ERROR 9
.SH NAME
error, nexterror, poperror, waserror \- error handling functions
.SH SYNOPSIS
.ta \w'\fL#define 'u
.B
void	error(char*)
.PP
.B
void	nexterror(void)
.sp 0.1
.PP
.B
#define poperror() (up->nerrlab--)
.PP
.B
#define waserror() (setlabel(&up->errlab[up->nerrlab++]))
.SH DESCRIPTION
The kernel handles error conditions using non-local gotos,
similar to
.IR setjmp (2),
but using a stack of error labels to implement nested exception handling.
This simplifies many of the internal interfaces by eliminating the need
for returning and checking error codes at every level of the call stack,
at the cost of requiring kernel routines to adhere to a strict discipline.
.PP
Each process has in its defining kernel
.B Proc
structure a stack of labels,
.B NERR
(currently 64)  elements deep.
A kernel function that must perform a clean up or recovery action on an error
makes a stylised call to
.IR waserror ,
.IR nexterror
and
.IR poperror :
.IP
.EX
.DT
if(waserror()){
	/* recovery action */
	nexterror();
}
/* normal action */
poperror();
.EE
.PP
When called in the normal course of events,
.I waserror
registers an error handling block by pushing its label onto the stack,
and returns zero.
The return value of
.I waserror
should be tested as shown above.
If non-zero (true), the calling function should perform the needed
error recovery, ended by a call to
.I nexterror
to transfer control to the next location on the error stack.
Typical recovery actions include deallocating memory, unlocking resources, and
resetting state variables.
.PP
Within the recovery block,
after handling an error condition, there must normally
be a call to
.I nexterror
to transfer control to any error recovery lower down in the stack.
The main exception is in the outermost function in a process,
which must not call
.I nexterror
(there being nothing further on the stack), but calls
.I pexit
(see
.IR kproc (9))
instead,
to terminate the process.
.PP
When the need to recover a particular resource has passed,
a function that has called
.I waserror
must
remove the corresponding label from the stack by calling
.IR poperror .
This
must
be done before returning from the function; otherwise, a subsequent call to
.I error
will return to an obsolete activation record, with unpredictable but unpleasant consequences.
.PP
.I Error
copies the given error message, which is limited to
.B ERRMAX
bytes, into the
.B Proc.errstr
of the current process,
enables interrupts by calling
.I spllo
.RI ( native
only),
and finally calls
.I nexterror
to start invoking the recovery procedures currently stacked by
.IR waserror .
The file
.B /sys/src/9/port/error.h
offers a wide selection of predefined error messages, suitable for almost any occasion.
The message set by the most recent call to
.I error
can be obtained within the kernel by examining
.B up->error
and in an application, by using the
.L %r
directive of
.IR print (2).
.PP
A complex function can have nested error handlers.
A
.I waserror
block will follow the acquisition of a resource, releasing it
on error before calling
.I nexterror,
and a
.I poperror
will precede its release in the normal case.
For example:
.IP
.EX
.DT
void
outer(Thing *t)
{
    qlock(t);
    if(waserror()){      /* A */
        qunlock(t);
        nexterror();
    }
    m = mallocz(READSTR, 0);
    if(m == nil)
        error(Enomem);	/* returns to A */
    if(waserror()){     /* B */
        free(m);
        nexterror();    /* invokes A */
    }
    inner(t);
    poperror();         /* pops B */
    free(m);
    poperror();         /* pops A */
    qunlock(t);
}
.sp 1v
void
inner(Thing *t)
{
    if(t->bad)
        error(Egreg);   /* returns to B */
    t->valid++;
}
.EE
.SH SOURCE
.B /sys/src/9/port/proc.c
.SH CAVEATS
The description above has many instances of
.IR should ,
.IR will ,
.I must
and
.IR "must not" .
.SH SEE ALSO
.IR panic (9),
.IR kproc (9),
.IR splhi (9)