Daniel P. Berrangé <berra...@redhat.com> writes: > On Tue, Feb 14, 2023 at 05:07:38PM +0100, Juan Quintela wrote: >> Hi >> >> I want to enter a discussion about changing the default of the style >> guide. >> >> There are several reasons for that: >> - they exist since C99 (i.e. all supported compilers support them) >> - they eliminate the posibility of an unitialized variable. > > Actually they don't do that reliably. In fact, when combined > with usage of 'goto', they introduce uninitialized variables, > despite the declaration having an initialization present, and > thus actively mislead reviewers into thinking their code is > safe. > > Consider this example:
[...] > What happens is that when you 'goto $LABEL' across a variable > declaration, the variable is in scope at your target label, but > its declared initializers never get run :-( > > Luckily you can protect against that with gcc: > > $ gcc -Wjump-misses-init -Wall -o mixed mixed.c > mixed.c: In function ‘foo’: > mixed.c:7:12: warning: jump skips variable initialization [-Wjump-misses-init] > 7 | goto cleanup; > | ^~~~ > mixed.c:15:5: note: label ‘cleanup’ defined here > 15 | cleanup: > | ^~~~~~~ > mixed.c:11:13: note: ‘items’ declared here > 11 | int *items = malloc(sizeof(int) *nitems); > | ^~~~~ > mixed.c:7:12: warning: jump skips variable initialization [-Wjump-misses-init] > 7 | goto cleanup; > | ^~~~ > mixed.c:15:5: note: label ‘cleanup’ defined here > 15 | cleanup: > | ^~~~~~~ > mixed.c:10:12: note: ‘nitems’ declared here > 10 | int nitems = 3; > | ^~~~~~ > > > however that will warn about *all* cases where we jump over a > declared variable, even if the variable we're jumping over is > not used at the target label location. IOW, it has significant > false positive rates. There are quite a few triggers for this > in the QEMU code already if we turn on this warning. > > It also doesn't alter that the code initialization is misleading > to read. Yup. Strong dislike. >> - (at least for me), declaring the index inside the for make clear >> that index is not used outside the for. > > I'll admit that declaring loop indexes in the for() is a nice > bit, but I'm not a fan in general of mixing the declarations > in the middle of code for projects that use the 'goto cleanup' > pattern. A declaration in a for statement's first operand is effectively at the beginning of a block. Therefore, use of this feature is already sanctioned by the QEMU Coding Style. The proposed patch at most clarifies this. >> - Current documentation already declares that they are allowed in some >> cases. >> - Lots of places already use them. >> >> We can change the text to whatever you want, just wondering if it is >> valib to change the standard. >> >> Doing a trivial grep through my local qemu messages (around 100k) it >> shows that some people are complaining that they are not allowed, and >> other saying that they are used all over the place. > > IMHO the status quo is bad because it is actively dangerous when > combined with goto and we aren't using any compiler warnings to > help us. > > Either we allow it, but use -Wjump-misses-init to prevent mixing > delayed declarations with gotos, and just avoid this when it triggers > a false positive. > > Or we forbid it, rewrite current cases that use it, and then add > -Wdeclaration-after-statement to enforce it. I'm in favour of -Wdeclaration-after-statement. > IMHO if we are concerned about uninitialized variables then I think > a better approach is to add -ftrivial-auto-var-init=zero, which will > make the compiler initialize all variables to 0 if they lack an > explicit initializer. How often do we get bitten by uninitialized variables despite -Wmaybe-uninitialized? Honest question! >> Discuss.