Why variables start out with random values in C_问答_开发者

I thin开发者_JS百科k this is wrong, it should start as NULL and not with a random value. In the case that you have a pointer with a random memory address as its default value it could be a very dangerous thing, no?

The variables start out uninitialized because that's the fastest way - why waste the CPU cycles on initialization if you're going to write another value there anyway?

If you want a variable to be initialized after creation, just initialize it. :)

About it being a dangerous thing: Every good compiler will warn you if you try to use a variable without initialization.

No. C is a very efficient language, one that has traditionally been faster that a lot of other languages. One of the reasons for this is that it doesn't do too much on it's own. The programmer controls this.

In the case of initialization, C variables are not initialized to a random value. Rather, they are not initialized and so they contain whatever was at the memory location before.

If you wanted to initialize a variable to, say, 1 in your program, then it would be inefficient if the variable had already been initialized to zero or null. That would mean it was initialized twice.

Execution speed and overhead (or lack thereof) are the main reasons why. C is notorious for letting you walk off the proverbial cliff because it always assumes that the user knows better than it does.

Note that if you declared the variable as static it actually is guaranteed to be initialized to 0.

Variables start out with a random value because you are just handed a block of memory and told to deal with it yourself. It has whatever value that block of memory had before hand. Why should the program waste time setting the value to some arbitrary default when you are likely going to set it yourself later?

The design choice is performance, and it is one of the many reasons why C isn't the preferred language for most projects.

This has nothing to do with "if C were being designed today" or with efficiency of one initialization. Instead think of something like

void foo()
{
    struct bar *ptrs[10000];
    /* do something where only a few indices end up actually getting used */
}

Any language that forces useless initialization on you is doomed to be slow as hell for algorithms that can make use of sparse arrays where you don't care about the majority of the values, and have an easy way of knowing which values you care about.

If you don't like my example with such a large object on the stack, substitute malloc instead. It has the same semantics with regard to initialization.

In either case, if you want zero-initialization, you can get it with {0} or calloc.

It was a design choice made many ears ago, probably for efficiency reasons.

Statically allocated variables (globals and statics) are initialized to 0 if there's no explicit initialization - this could be justified even taking efficiency into account becuase it only occurs once. I'd guess the thinking was that for automatic variables (locals) that are allocated each time a scope is entered, implicit initialization was considered something that might cost too much and therefore should be left to the programmer's responsibility.

If C were being designed today, I wouldn't be surprised if that design decision were changed - especially since compilers are intelligent enough today to be able to optimize away an initialization that gets overwritten before any other use (or potential use).

However, there are so many C compiler toolchains that follow the spec of not initializing automatically, it would be foolish for a compiler to perform implicit initialization to a 'useful' value (like 0 or NULL). That would just encourage people targeting that tool chain to write code that didn't work correctly on other tool chains.

However, compilers can initialize local variables, and they often do. It's just that they initialize the locals to a values that's not generally useful (especially, that doesn't set a pointer to the null pointer). That kind of initialization isn't useful in writing your programming logic against, and it's not intended for that. It's intended to cause deterministic and reproducible errors so that if you erroneously use values that have been set by implicit initialization, you'll be able to find it easily in test/debug.

Usually this compiler behavior is turned on only for debug builds; I could see an argument being made for turning it on in release builds as well - particular if the release build can still optimize it away when the compiler can prove that the implicit initialized value is never used.