From: Paul Brannan <pbrannan@...>
Date: 2001-10-17T23:26:58+09:00
Subject: [ruby-talk:22673] Re: Exception handling in c

On Wed, 17 Oct 2001 vrabel@burncd.sk wrote:

> Hi,
>
> is there another way, how to handle exception in c, apart of the rb_rescue?
>
> Thanx
>
>   Peter Vrabel

Generally speaking, you don't need to actually catch the exception; in
fact, rb_ensure() will do just fine.  It's not 100% convenient, because
you still have to break your function into multiple little functions.

Very often, I find that it is possible to lay out my code in such a way
that I don't ever need to explicitly handle Ruby exceptions.  For example:

  VALUE foo(VALUE self, VALUE param1, VALUE param2) {
    Foo *foo;
    int x;
    char *y;
    Bar *b1, *b2;
    FILE *fp;

    /* I think this can throw, but it won't hurt anything */
    Data_Get_Struct(self, Foo, foo);

    /* This can throw too, but it also won't hurt anything */
    x = NUM2INT(param1);

    /* If I were to use malloc() here, then I would need to use rb_ensure
     * to make sure that the memory gets freed.
     */
    b1 = ALLOC(Bar);

    /* Here's another option, but is faster since the memory is allocated
     * on the stack.
     */
    b2 = ALLOCA_N(Bar, 1);

    /* Now I'm opening a file, but I don't want to use the Ruby file I/O.
     * I must be sure to close the file when I am done.
     */
    fp = fopen("foo", "w+"); /* Assume this is successful */
    Data_Wrap_Struct(rb_cObject, NULL, fclose, fp);

    /* Do some processing here ... */

    /* This can throw, but it's OK */
    y = STR2CSTR(param2);
    return INT2NUM(atoi(y));
}

In C++, exception safety is much more complex, especially if you want to
be able to declare automatic objects on the stack.  The trickiest problem
in C, though, is not leaking memory/resources, but leaving all your
objects in a consistent state.  In this example, if I were to raise an
exception in the middle of my processing, then even though the file would
get closed, I would probably be leaving the file half-written.  In that
case, I cannot be certain that the data in the file is correct.

Paul