alloca() is very useful if you can't use a standard local variable because its size would need to be determined at runtime and you can absolutely guarantee that the pointer you get from alloca() will NEVER be used after this function returns. You can be fairly safe if you do not return the pointer, or anything that contains it. do not store the pointer in any structure allocated on the heap do not let any other thread use the pointer The real danger comes from the chance that someone else will violate these conditions sometime later. With that in mind it's great for passing buffers to functions that format text into them :)

这里的大多数回答都忽略了一点:使用_alloca()可能比仅仅在堆栈中存储大对象更糟糕，这是有原因的。

自动存储和_alloca()之间的主要区别是，后者有一个额外的(严重的)问题:分配的块不受编译器控制，因此编译器无法优化或回收它。

比较:

while (condition) {
    char buffer[0x100]; // Chill.
    /* ... */
}

:

while (condition) {
    char* buffer = _alloca(0x100); // Bad!
    /* ... */
}

后者的问题应该是显而易见的。

Alloca()很好，很有效……但它也被深深打破了。

broken scope behavior (function scope instead of block scope) use inconsistant with malloc (alloca()-ted pointer shouldn't be freed, henceforth you have to track where you pointers are coming from to free() only those you got with malloc()) bad behavior when you also use inlining (scope sometimes goes to the caller function depending if callee is inlined or not). no stack boundary check undefined behavior in case of failure (does not return NULL like malloc... and what does failure means as it does not check stack boundaries anyway...) not ansi standard

在大多数情况下，您可以使用局部变量和主要大小来替换它。如果它用于大型对象，将它们放在堆上通常是一个更安全的想法。

如果你真的需要它，你可以使用VLA(在c++中没有VLA，太糟糕了)。在作用域行为和一致性方面，它们比alloca()要好得多。在我看来，VLA是一种正确的分配。

当然，使用所需空间的主要部分的本地结构或数组仍然更好，如果没有这样的主要堆分配，则使用普通malloc()可能是明智的。 我没有看到你真的真的需要alloca()或VLA的用例。

答案就在手册页中(至少在Linux上):

返回值 函数的作用是:返回一个指向对象开头的指针 分配空间。如果 分配的原因 堆栈溢出，程序行为未定义。

这并不是说它永远不应该被使用。我工作的一个OSS项目广泛使用它，只要你不滥用它(分配巨大的值)，它是好的。一旦超过了“几百字节”的标记，就应该转而使用malloc和friends。您可能仍然会遇到分配失败，但至少您会得到一些失败的指示，而不是仅仅耗尽堆栈。

其他答案都是正确的。但是，如果使用alloca()要分配的对象相当小，我认为这是一种比使用malloc()或其他方法更快、更方便的好技术。

换句话说，alloca(0x00ffffff)是危险的，可能会导致溢出，就像char hugeArray[0x00ffffff];是多少。小心谨慎，通情达理，你会没事的。

alloca() is very useful if you can't use a standard local variable because its size would need to be determined at runtime and you can absolutely guarantee that the pointer you get from alloca() will NEVER be used after this function returns. You can be fairly safe if you do not return the pointer, or anything that contains it. do not store the pointer in any structure allocated on the heap do not let any other thread use the pointer The real danger comes from the chance that someone else will violate these conditions sometime later. With that in mind it's great for passing buffers to functions that format text into them :)