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@(#)ch10.n 6.3 (Berkeley) 04/17/91
." $Header: /na/franz/doc/RCS/ch10.n,v 1.1 83/01/31 07:08:20 jkf Exp $ .Lc Exception Handling 10 .sh 2 Errset and Error Handler Functions \n(ch 1 .pp .Fr allows the user to handle in a number of ways the errors which arise during computation. One way is through the use of the .i errset function. If an error occurs during the evaluation of the .i errset 's first argument, then the locus of control will return to the errset which will return nil (except in special cases, such as .i err ). The other method of error handling is through an error handler function. When an error occurs, the error handler is called and is given as an argument a description of the error which just occurred. The error handler may take one of the following actions: .nr $p 0 .np it could take some drastic action like a .i reset or a .i throw . .np it could, assuming that the error is continuable, return to the function which noticed the error. The error handler indicates that it wants to return a value from the error by returning a list whose .i car is the value it wants to return. .np it could decide not to handle the error and return a non-list to indicate this fact. .sh 2 "The Anatomy of an error" .pp Each error is described by a list of these items: .nr $p 0 .np error type - This is a symbol which indicates the general classification of the error. This classification may determine which function handles this error. .np unique id - This is a fixnum unique to this error. .np continuable - If this is non-nil then this error is continuable. There are some who feel that every error should be continuable and the reason that some (in fact most) errors in .Fr are not continuable is due to the laziness of the programmers. .np message string - This is a symbol whose print name is a message describing the error. .np data - There may be from zero to three lisp values which help describe this particular error. For example, the unbound variable error contains one datum value, the symbol whose value is unbound. The list describing that error might look like:
(ER%misc 0 t |Unbound Variable:| foobar) .sh 2 "Error handling algorithm" .pp This is the sequence of operations which is done when an error occurs: .nr $p 0 .np If the symbol .b ER%all has a non nil value then this value is the name of an error handler function. That function is called with a description of the error. If that function returns (and of course it may choose not to) and the value is a list and this error is continuable, then we return the .i car of the list to the function which called the error. Presumably the function will use this value to retry the operation. On the other hand, if the error handler returns a non list, then it has chosen not to handle this error, so we go on to step (2). Something special happens before we call the .b ER%all error handler which does not happen in any of the other cases we will describe below. To help insure that we don't get infinitely recursive errors if .b ER%all is set to a bad value, the value of .b ER%all is set to nil before the handler is called. Thus it is the responsibility of the .b ER%all handler to `reenable' itself by storing its name in .b ER%all. .np Next the specific error handler for the type of error which just occurred is called (if one exists) to see if it wants to handle the error. The names of the handlers for the specific types of errors are stored as the values of the symbols whose names are the types. For example the handler for miscellaneous errors is stored as the value of .b ER%misc. Of course, if .b ER%misc has a value of nil, then there is no error handler for this type of error. Appendix B contains list of all error types. The process of classifying the errors is not complete and thus most errors are lumped into the ER%misc category. Just as in step (1), the error handler function may choose not to handle the error by returning a non-list, and then we go to step (3). .np Next a check is made to see if there is an .i errset surrounding this error. If so the second argument to the .i errset call is examined. If the second argument was not given or is non nil then the error message associated with this error is printed. Finally the stack is popped to the context of the .i errset and then the .i errset returns nil. If there was no .i errset we go to step (4). .np If the symbol .b ER%tpl has a value then it is the name of an error handler which is called in a manner similar to that discussed above. If it chooses not to handle the error, we go to step (5). .np At this point it has been determined that the user doesn't want to handle this error. Thus the error message is printed out and a .i reset is done to send the flow of control to the top-level. .pp To summarize the error handling system: When an error occurs, you have two chances to handle it before the search for an .i errset is done. Then, if there is no .i errset , you have one more chance to handle the error before control jumps to the top level. Every error handler works in the same way: It is given a description of the error (as described in the previous section). It may or may not return. If it returns, then it returns either a list or a non-list. If it returns a list and the error is continuable, then the .i car of the list is returned to the function which noticed the error. Otherwise the error handler has decided not to handle the error and we go on to something else. .sh 2 "Default aids" .pp There are two standard error handlers which will probably handle the needs of most users. One of these is the lisp coded function .i break-err-handler which is the default value of .b ER%tpl. Thus when all other handlers have ignored an error, .i break-err-handler will take over. It will print out the error message and go into a read-eval-print loop. The other standard error handler is .i debug-err-handler . This handler is designed to be connected to .b ER%all and is useful if your program uses .i errset and you want to look at the error before it is thrown up to the .i errset . .sh +0 Autoloading .pp When .i eval , .i apply or .i funcall are told to call an undefined function, an ER%undef error is signaled. The default handler for this error is .i undef-func-handler . This function checks the property list of the undefined function for the indicator autoload. If present, the value of that indicator should be the name of the file which contains the definition of the undefined function. .i Undef-func-handler will load the file and check if it has defined the function which caused the error. If it has, the error handler will return and the computation will continue as if the error did not occur. This provides a way for the user to tell the lisp system about the location of commonly used functions. The trace package sets up an autoload property to point to /usr/lib/lisp/trace. .sh +0 Interrupt processing .pp The UNIX operating system provides one user interrupt character which defaults to ^C.\*[\(dg\*] .(f \*[\(dg\*]Actually there are two but the lisp system does not allow you to catch the QUIT interrupt. .)f The user may select a lisp function to run when an interrupt occurs. Since this interrupt could occur at any time, and in particular could occur at a time when the internal stack pointers were in an inconsistent state, the processing of the interrupt may be delayed until a safe time. When the first ^C is typed, the lisp system sets a flag that an interrupt has been requested. This flag is checked at safe places within the interpreter and in the .i qlinker function. If the lisp system doesn't respond to the first ^C, another ^C should be typed. This will cause all of the transfer tables to be cleared forcing all calls from compiled code to go through the .i qlinker function where the interrupt flag will be checked. If the lisp system still doesn't respond, a third ^C will cause an immediate interrupt. This interrupt will not necessarily be in a safe place so the user should .i reset the lisp system as soon as possible.