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c_pitfalls.md
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@ -10,7 +10,7 @@ Unless specified otherwise, this article supposes the C99 standard of the C lang
## Undefined/Unspecified Behavior
Undefined (completely unpredictable), unspecified (safe but potentially differing) and implementation-defined (consistent within implementation but potentially differing between them) behavior poses a kind of unpredictability and sometimes non-intuitive, tricky behavior of certain operations that may differ between compilers, platforms or runs because they are not exactly described by the language specification; this is mostly done on purpose so as to allow some implementation freedom which allows implementing the language in a way that is most efficient on given platform. One has to be very careful about not letting such behavior break the program on platforms different from the one the program is developed on. Note that tools such as [cppcheck](cppcheck.md) can help find undefined behavior in code. Description of some such behavior follows.
Undefined (completely unpredictable), unspecified (safe but potentially differing) and implementation-defined (consistent within implementation but potentially differing between them) behavior poses a kind of unpredictability and sometimes non-intuitive, tricky behavior of certain operations that may differ between compilers, platforms or runs because they are not exactly described by the language specification; this is mostly done on purpose so as to allow some implementation freedom which allows implementing the language in a way that is most efficient on given platform. One must be very cautious about not letting such behavior break the program on platforms different from the one the program is developed on. Note that tools such as [cppcheck](cppcheck.md) can help find undefined behavior in code. Description of some such behavior follows.
There are tools for detecting undefined behavior, see e.g. [clang](clang.md)'s UBSan (https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html).
@ -103,7 +103,7 @@ And so on.
## Compiler Optimizations
C compilers perform automatic optimizations and other transformations of the code, especially when you tell them to optimize aggressively (`-O3`) which is a standard practice to make programs run faster. However this makes compilers perform a lot of [magic](magic.md) and may lead to unexpected and unintuitive undesired behavior such as bugs or even the "unoptimization of code". { I've seen a code I've written have bigger size when I set the `-Os` flag (optimize for smaller size). ~drummyfish }
C compilers perform automatic [optimizations](optimization.md) and other transformations of the code, especially when you tell them to optimize aggressively (`-O3`) which is a standard practice to make programs run faster. However this makes compilers perform a lot of [magic](magic.md) and may lead to unexpected and unintuitive undesired behavior such as bugs or even the "unoptimization of code". { I've seen a code I've written have bigger size when I set the `-Os` flag (optimize for smaller size). ~drummyfish }
Aggressive optimization may firstly lead to tiny bugs in your code manifesting in very weird ways, it may happen that a line of code somewhere which may somehow trigger some tricky [undefined behavior](undefined_behavior.md) may cause your program to crash in some completely different place. Compilers exploit undefined behavior to make all kinds of big brain reasoning and when they see code that MAY lead to undefined behavior a lot of chain reasoning may lead to very weird compiled results. Remember that undefined behavior, such as overflow when adding signed integers, doesn't mean the result is undefined, it means that ANYTHING CAN HAPPEN, the program may just start printing nonsensical stuff on its own or your computer may explode. So it may happen that the line with undefined behavior will behave as you expect but somewhere later on the program will just shit itself. For these reasons if you encounter a very weird bug, try to disable optimizations and see if it goes away -- if it does, you may be dealing with this kind of stuff. Also check your program with tools like [cppcheck](cppcheck.md).
@ -124,7 +124,7 @@ if (x)
As in typical code this works the same and is faster. However if the variable *x* is part of shared memory and can be changed by an outside process during the execution of the loop, this optimization can no longer be done as it results in different behavior. This can be prevented with the `volatile` keyword which tells the compiler to not perform such optimizations.
Of course this applies to other languages as well, but C is especially known for having a lot of undefined behavior, so be careful.
Of course this applies to other languages as well, but C is especially known for having a lot of undefined behavior, so we ought to be cautious.
## Other