When To Use Goto When Programming in C

Although the use of goto is almost always bad programming practice (surely you can find a better way of doing XYZ), there are times when it really isn't a bad choice. Some might even argue that, when it is useful, it's the best choice.



Most of what I have to say about goto really only applies to C. If you're using C++, there's no sound reason to use goto in place of exceptions. In C, however, you don't have the power of an exception handling mechanism, so if you want to separate out error handling from the rest of your program logic, and you want to avoid rewriting clean up code multiple times throughout your code, then goto can be a good choice.

What do I mean? You might have some code that looks like this:

int big_function()
{
    /* do some work */
    if([error])
    {
        /* clean up*/
        return [error];
    }
    /* do some more work */
    if([error])
    {
        /* clean up*/
        return [error];
    }
    /* do some more work */
    if([error])
    {
        /* clean up*/
        return [error];
    }
    /* do some more work */
    if([error])
    {
        /* clean up*/
        return [error];
    }
    /* clean up*/
    return [success];
}
This is fine until you realize that you need to change your cleanup code. Then you have to go through and make 4 changes. Now, you might decide that you can just encapsulate all of the cleanup into a single function; that's not a bad idea. But it does mean that you'll need to be careful with pointers -- if you plan to free a pointer in your cleanup function, there's no way to set it to then point to NULL unless you pass in a pointer to a pointer. In a lot of cases, you won't be using that pointer again anyway, so that may not be a major concern. On the other hand, if you add in a new pointer, file handle, or other thing that needs cleanup, then you'll need to change your cleanup function again; and then you'll need to change the arguments to that function.

In some cases, this might be acceptable -- but if you're making a lot of changes to your code, adding in new variables, etc. -- it may not be worth the time or the extra lines of code to make that function call.

Instead, since you know that you're going to be executing only one piece of code and then returning from the function, you might as well use a goto to jump to the very end of the function, where you have your cleanup code.

Goto is a pretty simple keyword: you just need to include a "label" placed above the target location (followed by a colon), and then direct the program to go to the label. Note that this only works within the same function; you can't just enter one function from another.
goto label;
/* Code
   ...
*/
label:
Now, using this, if we want to handle all our errors in one place, we'll need to add in a variable to track the return value of our function so we can return it:
int big_function()
{
    int ret_val = [success];
    /* do some work */
    if([error])
    {
        ret_val = [error];
        goto end;
    }
    /* do some more work */
    if([error])
    {
        ret_val = [error];
        goto end;
    }
    /* do some more work */
    if([error])
    {
        ret_val = [error];
        goto end;
    }
    /* do some more work */
    if([error])
    {
        ret_val = [error];
        goto end;
    }
end:
    /* clean up*/
    return ret_val;
}
The benefit here is that your code following end has access to everything it will need to perform cleanup, and you've managed to reduce the number of change points considerably. Another benefit is that you've gone from having multiple exit points for your function to just one; there's no chance you'll accidentally return from the function without cleaning up.

Moreover, since goto is only being used to jump to a single point, it's not as though you're creating a mass of spaghetti code jumping back and forth in an attempt to simulate function calls. Rather, goto actually helps write more structured code.

There is one thing to be aware of: while your cleanup code should be able to free all of the memory you use, there may be times when you actually want to free that memory yourself and possibly reallocate it later. In these cases, if you do call free on a ptr and then have an if([error]) between that call to free and the subsequent call to malloc, you should definitely set the pointer to point to NULL! This will prevent your jumping to the cleanup code and then calling free on that pointer a second time, which can result in a security hole (the "double free" problem).

Goto should always be used sparingly, and as a last resort -- but there is a time and a place for it. The question should be not "do you have to use it" but "is it the best choice" to use it.
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Exception Handling in C++