Handling Errors in a Shore Application
Version 0.1

The Shore Project Group
Computer Sciences Department
Madison, WI

1 Introduction

This document describes error handling facilities for Shore applications. Shore application writers are encouraged to read Building Shore Applications and look at the example applications before reading this document. The SDL manual and the example programs show the standard ways of handling errors in Shore. This document is intended for application writers interested in more details.

This document has three sections. The first section describes the shrc (Shore return code) type, which is used by most Shore methods. The second section describes transaction macros, and the third section describes how to install an error handler function.

2 The shrc Class

Most methods in Shore return an error indicator of type shrc. The shrc type contains not only an integer error code, but also a stack trace showing where the error occurred and the series of function calls that triggered the error. (In the current implementation, the stack trace contains only the bottom three levels of the stack.) The stack trace may be of limited value to application programmers, but it can be very useful to Shore implementors, if included in a Shore bug report.

Shrc is a C++ class, and it contains various member functions that application programmers may find useful. The following is a portion of the class definition of shrc:

    class shrc

        bool is_error() const;
        int  err_num() const;
             operator const void*() const;

        friend ostream &operator<<(ostream& o, const shrc &rc);

If a Shore method completes successfully, then it returns the constant RCOK. If a Shore method returns something other than RCOK, then an application can use then shrc::err_num method to get the integer error code corresponding to the error. The possible error code values can be found in the following header files: OC_error.h, e_error.h, fc_error.h, opt_error_def.h, svas_error_def.h, and unix_error.h. These header files are in the include directory of the Shore release.

Shrc includes operator const void*() to allow applications to say things like shrc rc = ...; if(rc).... Shrc also includes an overloading of the C++ << operator.

3 Macros for Transaction Management

The most common way to handle errors in database systems is to abort the transaction that caused the error. Application programs can abort their transactions by calling Shore::abort_transaction. While this method causes any changes to persistent data to be be undone, it does not affect the state of the application program, itself. Shore provides macros to begin, commit, and abort transactions that can help to address part of this problem. The macros employ ANSI C setjmp and longjmp facilities.

The four macros are:

    SH_BEGIN_TRANSACTION(rc)        // rc is an ``out'' parameter
    SH_ABORT_TRANSACTION(rc)        // rc is an ``in''  parameter
    SH_DO(rc)                       // rc is an ``in''  parameter

The argument to SH_BEGIN_TRANSACTION is of type shrc. This argument must be an lvalue, as the macro will assign to it. SH_BEGIN_TRANSACTION calls setjmp, and can therefore return from either of two different contexts: a direct call to SH_BEGIN_TRANSACTION, or a call to SH_ABORT_TRANSACTION (described below). A direct call to SH_BEGIN_TRANSACTION calls Shore::begin_transaction. Upon return, rc will be set to the return value of Shore::begin_transaction.. When SH_BEGIN_TRANSACTION returns because of a call to SH_ABORT_TRANSACTION, then the value of rc is whatever was passed to SH_ABORT_TRANSACTION.

SH_COMMIT_TRANSACTION is equivalent to Shore::commit_transaction.

SH_ABORT_TRANSACTION takes as parameter an expression of type shrc. (Unlike SH_BEGIN_TRANSACTION, this argument is an rvalue, not an lvalue). It calls Shore::abort_transaction and then performs a longjmp, which returns control back to the line where SH_BEGIN_TRANSACTION was called. The rc passed to SH_ABORT_TRANSACTION will be the return value of SH_BEGIN_TRANSACTION.

SH_DO takes an expression of type shrc. If this expression evaluates to RCOK then the macro returns. Otherwise, SH_ABORT_TRANSACTION is called with the value of the given expression as its argument.

The following code fragment the use of the transaction macros.

    shrc rc;

    // Begin a transaction.  A subsequent call to SH_ABORT_TRANSACTION
    // will return us here.


        // Some error occurred.  The rc indicates why new transaction could
        // not be started or why the transaction was aborted.
        cerr << rc << endl;

    else {

        // We successfully started a transaction.  The main body of the
        // transaction goes here. 

        SH_DO(operation 1);
        SH_DO(operation 2);

        // If we completed the main body of the transaction without
        // errors, then we try to commit the transaction.  Note that
        // if the commit fails, then SH_DO will call SH_ABORT_TRANSACTION
        // for us.  The shrc returned by SH_COMMIT_TRANSACTION will become
        // the the return value of SH_BEGIN_TRANSACTION (above).


Note that because these macros make use of setjmp and longjmp, the function containing the call to SH_BEGIN_TRANSACTION must not have terminated before SH_DO or SH_ABORT_TRANSACTION is called. However, a call to SH_COMMIT_TRANSACTION, SH_DO, or SH_ABORT_TRANSACTION does not have to be in the same function as the call to SH_BEGIN_TRANSACTION. In particular, it may be useful to call SH_ABORT_TRANSACTION from within the Shore error handler function, which is described in the next section.

4 The Shore Error Handler Function

In certain cases, it is impossible or impractical to return a shrc. For example, REF(T)::operator-> does not have an opportunity to return an error code, because of the way C++ defines operator->. Ideally, errors from overloaded operators would raise an exception that could be caught and handled by the application. Unfortunately, C++ exceptions are not stable enough to be used. Therefore, in situations where it is impractical to return a shrc, Shore calls a handler function, passing a shrc as its argument. The default handler function simply prints the shrc and exits by calling _exit (this form of exit does not call global destructors).

Applications can install their own error handler by calling Shore::set_error_handler. This method takes a pointer to a handler function as its argument. It installs the given function as the new handler and returns the old handler function. Passing a zero argument to Shore::set_error_handler reinstalls the default handler.

By installing a handler function, an application gives itself an opportunity to clean up its transient state before aborting a transaction. Once the transient state has been cleaned up, the handler function can call SH_ABORT_TRANSACTION to abort the transaction and longjmp back to the call to SH_BEGIN_TRANSACTION. Alternately, the handler function can exit the process with _exit. A handler function should not return, however. If it does, then Shore will terminate the process by calling _exit.

Thu Nov 3 14:19:42 CST 1994