.help zsvjmp,zdojmp May84 "System Interface"
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NAME
zsvjmp, zdojmp -- non-local goto
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SYNOPSIS
.nf
include	<config.h>

zsvjmp (jumpbuf, status)		# save context for jump
zdojmp (jumpbuf, status)		# restore context and jump

int	jumpbuf[LEN_JUMPBUF]		# context saved by \fBzsvjmp\fR
int	status				# code returned by \fBzsvjmp\fR
.fi
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DESCRIPTION
These primitives are used principally to restart interpreters (e.g. the IRAF
Main and the CL) following an error abort.
When an error occurs deep in a procedure calling sequence and the interpreter
(a higher level procedure) must be restarted, the hardware stack or stacks
and registers must be restored to their earlier state.

The \fBzdojmp\fR primitive restores the context of the procedure which
originally called \fBzsvjmp\fR, causing control to return from \fBzsvjmp\fR
as if it had just been called.  The calling procedure must not itself have
returned in the interim.
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RETURN VALUE
The integer code \fIstatus\fR is zero the first time \fBzsvjmp\fR returns,
i.e., when \fBzsvjmp\fR is called by the main procedure to initialize
\fIjumpbuf\fR.  When \fBzdojmp\fR is subsequently called to "goto" the
main procedure it should be called with a nonzero \fIstatus\fR to tell
the main procedure that it has been reentered at the point immediately
following the call to \fBzsvjmp\fR.
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NOTES
Only the hardware stack and registers are restored by \fBzdojmp\fR.
Buffers which have been allocated since the first call to \fBzsvjmp\fR
will still be allocated, newly posted exception handlers will still be
posted, and so on.  It is up to the high level code to clean up following
error restart.
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EXAMPLE
Procedure A, the main (highest level) procedure, calls \fBzsvjmp\fR to
save its context for a subsequent restart, then calls procedure B.
Procedure B calls procedure C which directly or indirectly calls
\fBzdojmp\fR.  The \fIjumpbuf\fR storage area is global.

.ks
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A:	call zsvjmp (jumpbuf, status)
 99	if (status == error_code)
	    we were called from C
	call B

B:	call C

C:	call zdojmp (jumpbuf, error_code)	[e.g., goto 99]
.fi
.ke
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SEE ALSO
A discussion of the IRAF Main and error recovery.
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