4 ms·

I'm kinda surprised that the assignment-during-test thing doesn't generate a warning in the case of a switch statement. It does in other cases (if and while do

by rosser 12y ago

I'm kinda surprised that the assignment-during-test thing doesn't generate a warning in the case of a switch statement. It does in other cases (if and while do under gcc and clang, at least).

cperciva 12y ago
If it creates warnings, you can silence them by adding extra parentheses, e.g., if ((buf = malloc(buflen)) == NULL) goto outofmemory;
- rosser 12y ago
  I was originally going to comment saying exactly that — under gcc and clang, at least, just the extra parens will disable the warning; no comparison necessary — but thought to test that the warning is generated in case of a switch statement (and is then disabled by the extra set of parens), which led to realizing the behavior is different. Just surprised by the inconsistency.
- loup-vaillant 12y ago
  Careful with that example. The parentheses here don't just remove the warning. They remove a bug. This code: if (buf = malloc(buflen) == NULL) goto outofmemory; is actually equivalent to that code: if (buf = (malloc(buflen) == NULL)) goto outofmemory; So, malloc gives you a pointer, which is compared to the null pointer, giving you either 0 or 1. And that is assigned to buf. Hopefully your compiler will warn you about the type error that spawns from such dark magic.
  - deleted 12y ago
    [deleted]
  - cperciva 12y ago
    Oops, quite right. I was originally thinking of if ((rc = pthread_mutex_lock(mtx))) err("mutex_lock failed: %s", strerror(rc)); but decided to switch to a better-known function at the last minute and completely lost the point.
jleader 12y ago
Assignment-during-test is flagged because it's common for a typo to conflate assignment and equality-testing. Equality-testing is very common in an if or while, so an assignment inside an if or while has a fairly high likelihood of actually being a mis-typed equality-test. Switch, on the other hand, is very rarely used with an equality-test (since equality-test only returns true or false, why would you use a switch when an if would suffice?). So an assignment inside a switch is much less likely to be a mis-typed equality-test.

by rosser 12y ago

I'm kinda surprised that the assignment-during-test thing doesn't generate a warning in the case of a switch statement. It does in other cases (if and while do under gcc and clang, at least).

cperciva 12y ago

If it creates warnings, you can silence them by adding extra parentheses, e.g., if ((buf = malloc(buflen)) == NULL) goto outofmemory;

rosser 12y ago
I was originally going to comment saying exactly that — under gcc and clang, at least, just the extra parens will disable the warning; no comparison necessary — but thought to test that the warning is generated in case of a switch statement (and is then disabled by the extra set of parens), which led to realizing the behavior is different. Just surprised by the inconsistency.
loup-vaillant 12y ago
Careful with that example. The parentheses here don't just remove the warning. They remove a bug. This code: if (buf = malloc(buflen) == NULL) goto outofmemory; is actually equivalent to that code: if (buf = (malloc(buflen) == NULL)) goto outofmemory; So, malloc gives you a pointer, which is compared to the null pointer, giving you either 0 or 1. And that is assigned to buf. Hopefully your compiler will warn you about the type error that spawns from such dark magic.
- deleted 12y ago
  [deleted]
- cperciva 12y ago
  Oops, quite right. I was originally thinking of if ((rc = pthread_mutex_lock(mtx))) err("mutex_lock failed: %s", strerror(rc)); but decided to switch to a better-known function at the last minute and completely lost the point.

jleader 12y ago

Assignment-during-test is flagged because it's common for a typo to conflate assignment and equality-testing. Equality-testing is very common in an if or while, so an assignment inside an if or while has a fairly high likelihood of actually being a mis-typed equality-test. Switch, on the other hand, is very rarely used with an equality-test (since equality-test only returns true or false, why would you use a switch when an if would suffice?). So an assignment inside a switch is much less likely to be a mis-typed equality-test.