4 ms·

The section on callbacks should probably mention that it's best to allow some sort of context cookie to be passed along to the callback. This allows callbacks t

The section on callbacks should probably mention that it's best to allow some sort of context cookie to be passed along to the callback. This allows callbacks to be re-entrant and to target any side effects to some particular context. Compare, e.g. qsort() to qsort_r().

BulgarianIdiot 5y ago
Functions are re-entrant unless they refer to and modify state outside themselves. If you mean recursive calls, that happens rarely in frameworks which deal with a very 'flat' processing pipeline.
- smcameron 5y ago
  No, I don't mean recursive. I mean the callback might need to read (or write) some state that's different than the state used by other, concurrent instances of the callback, or other arbitrary threads, so the framework should provide a means for it to get at that state that doesn't rely on say, global variables, so that multiple instances of the callback may be running concurrently with different state. Typically this is done with a cookie (in C, a "void *" parameter is generally used for this.) Basically, if you write a framework that has callbacks, and you don't make allowance for passing such a context cookie, you're imposing a constraint on your users that you might not mean to. Maybe "re-entrant" wasn't quite the right word, but in my defense, the qsort_r() man page contains this: "In this way, the comparison function does not need to use global variables to pass through arbitrary arguments, and is therefore reentrant and safe to use in threads." In addition to allowing one to write re-entrant callbacks, it also allows passing in arbitrary data of whatever kind, not just whatever parameters the framework author happened to think of.
  - ptx 5y ago
    This isn't a problem in any of the languages the article mentions (Python, Ruby, JavaScript, Java) so that might be why the author didn't bring it up. Callbacks in these languages are objects that can carry along their own data.
  - garethrowlands 5y ago
    This is because in C, a function pointer is just a bare pointer to code, and cannot imply extra data such as your cookie. Usually a 'function' can do just that, either by capturing data (a 'closure') or by partial application.

BulgarianIdiot 5y ago

Functions are re-entrant unless they refer to and modify state outside themselves. If you mean recursive calls, that happens rarely in frameworks which deal with a very 'flat' processing pipeline.

smcameron 5y ago
No, I don't mean recursive. I mean the callback might need to read (or write) some state that's different than the state used by other, concurrent instances of the callback, or other arbitrary threads, so the framework should provide a means for it to get at that state that doesn't rely on say, global variables, so that multiple instances of the callback may be running concurrently with different state. Typically this is done with a cookie (in C, a "void *" parameter is generally used for this.) Basically, if you write a framework that has callbacks, and you don't make allowance for passing such a context cookie, you're imposing a constraint on your users that you might not mean to. Maybe "re-entrant" wasn't quite the right word, but in my defense, the qsort_r() man page contains this: "In this way, the comparison function does not need to use global variables to pass through arbitrary arguments, and is therefore reentrant and safe to use in threads." In addition to allowing one to write re-entrant callbacks, it also allows passing in arbitrary data of whatever kind, not just whatever parameters the framework author happened to think of.
- ptx 5y ago
  This isn't a problem in any of the languages the article mentions (Python, Ruby, JavaScript, Java) so that might be why the author didn't bring it up. Callbacks in these languages are objects that can carry along their own data.
- garethrowlands 5y ago
  This is because in C, a function pointer is just a bare pointer to code, and cannot imply extra data such as your cookie. Usually a 'function' can do just that, either by capturing data (a 'closure') or by partial application.