Undefined behaviour is not as prevalent in real world code as reading articles from HN might make you think.
You think so? Every company I've worked for, or that I've known people that worked there, always enabled -Wall for their C and C++ code. Most OSS software compiles with all warnings enabled.
I think the issue with undefined behavior in C/C++ is extremely overblown, aside from fun academic examples like 'what does i++++i++ evaluate to' there isn't actually all that much undefined behavior or gotchas in C/C++. I would say there are less, compared to other languages I know.
You need to know that the problem exists in order to know that you have a problem. There are many C programmers who learned C back in the 1980s who don't even realize these are issues.
I'm still adding the compiler specific annotations to add format string checking to custom variadic logging functions in codebases I inherit, and finding multiple bugs.
Of course you want -Wall -Wextra -Werror -pedantic. ;)
There's nothing like the experience of trying to fix somebody else's code which compiled fine on gcc version 8.97 but which now fails to compile on gcc version 8.98 because the new compiler has some new warnings, which it's now treating as errors, and now fails to compile.
...and you've got stuff to do, and the program isn't even broken.
Well, it may be — that's one of the problems with C: you never really know for sure if a warning really matters or not. But man, there sure are a lot of them!
Last I checked he has a couple hundred points on the hacker news internet forums.
Also just last week I found and reported some undefined behavior in a major c++ package that's used by almost every player in as many as several industries. I don't expect it will ever make any difference in production, but it still snuck in.
I'm sure you could say that about pretty much any programming language: "The amount of terrible X in the real world is enormous". There are also plenty of clean, nice, safe C code around (and any other language), there's no need to over-generalize ("Almost no one else does").
But the damage is far greater in C. In other languages you won't have arbitrary code execution or privilege escalation just because the programmer is not careful. Nor will there be, in other languages, so many nondeterministic bugs that show up once in a blue moon.
Sure you do. Remember the YAML fiasco with Ruby? How about the thousand-and-one RCE issues with PHP? eval isn't evil for no reason.
No, it's possible to make system insecure with pretty much any language if programmer is not careful. SQL injection, cross-site scripting, cross-site request forgery and the list goes on..
There's bad code everywhere. Some languages make it a bit easier, but it's really not the languages fault.
If your best defense of a language is "well, at least people use it", that's a bad sign.
void func(void *ptr)
{
uint32_t *ip = ptr;
ptr[0] = 123;
}Otherwise most every assignment after call to malloc would be undefined.
From C1X, section 6.3.2.3:
"A pointer to an object type may be converted to a pointer to a different object type. If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined."
Though that is quite odd, since any pointer can be converted to void* , which only needs alignment to the char type. So converting from x* -> y* is undefined, but x* -> void* -> y* is defined.
>> uint8_t x[100];
>> uint32_t *y = &x[1];
And then dereference y, most RISC architectures will trap on the unaligned access. It doesn't matter if there is an intermediate void pointer or not.
According to my reading an intermediate void pointer allows the pointer casting to stay well defined. However this seems unsafe, even without getting into dereferencing, because implementations are allowed to store omit bits if they assume aligned pointers.
"A pointer to an object type may be converted to a pointer to a different object type. If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined."
The resulting uint32_t pointer in my example is not correctly aligned for the reference type, so undefined behavior (e.g., a trap on RISC) occurs. What's an example of a statement in a "non-trivial" C program that is in common use but you think is undefined?
(1) I didn't say your example didn't demonstrate a violation, but it misses the point, because it doesn't invoke an intermediate void pointer:
"A pointer to void may be converted to or from a pointer to any object type. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer."
(2) That was my attempt at coming up with a good example, but it seems, due to the above clause, the casting between incompatible pointers via void * is technically "legal".
Undefined:
int64_t a = 42;
void* p = &a;
int32_t* i = p;
printf("%i", *i);
int64_t a = 42;
void* p = &a;
char* ch = p;
printf("%c", *ch);
An object shall have its stored value accessed only by an lvalue expression that has one of the following types 73) or 88):
* a type compatible with the effective type of the object,
* a qualified version of a type compatible with the effective type of the object,
* a type that is the signed or unsigned type corresponding to the effective type of the object,
* a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
* an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or
* a character type.
73) or 88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.
I still wonder if my snippet counts as undefined behavior, since it does dereference an "unknown" void pointer, which may have come from an incompatible object type.
BTW, the latest C1X draft is only at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
It counts as potentially undefined behavior - depends on what you pass in. NULL? UB. Pointer-to-uint64_t? UB. Pointer-to-uint32_t? Perfectly defined behavior! ...well, assuming we use ip[0] = 123; instead of ptr[0] = 123;, which won't compile as I've just noticed.
That said, there are some ways to construct pointers which are in and of themselves undefined behavior for merely constructing the pointer:
http://stackoverflow.com/questions/23683029/is-gccs-option-o...
More samples:
int a[] = { 1, 2, 3 };
int* b = a+0; // Perfectly defined/legal/normal
int* c = a+3; // Perfectly defined/legal/normal, just don't deference it (as it points past the end of the array)
int* d = a+4; // Undefined behavior. HAIL SATAN!
int* e = a-1; // Undefined behavior. Also apparently potentially caused optimization induced breakage in practice. HAIL GCC!