c - 线程安全与可重入

Question

最近，我问了一个问题，标题是"Is malloc thread safe?" ，在里面我问，“malloc 是可重入的吗？”

我的印象是所有可重入都是线程安全的。

这个假设是错误的吗？

Answer 1

TL;DR:一个函数可以是可重入的、线程安全的，或者两者兼而有之。

thread-safety 的维基百科文章和 reentrancy非常值得一读。以下是一些引用:

一个函数是线程安全的如果:

it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time.

一个函数是可重入如果:

it can be interrupted at any point during its execution and then safely called again ("re-entered") before its previous invocations complete execution.

作为可能重入的例子，维基百科给出了一个被系统中断调用的函数的例子:假设当另一个中断发生时它已经在运行。但是不要仅仅因为您没有使用系统中断进行编码就认为您是安全的:如果您使用回调或递归函数，您可能会在单线程程序中遇到重入问题。

The key for avoiding confusion is that reentrant refers to only one thread executing. It is a concept from the time when no multitasking operating systems existed.

示例

(根据维基百科文章略作修改)

例子1:非线程安全，不可重入

/* As this function uses a non-const global variable without
   any precaution, it is neither reentrant nor thread-safe. */

int t;

void swap(int *x, int *y)
{
    t = *x;
    *x = *y;
    *y = t;
}

例子2:线程安全，不可重入

/* We use a thread local variable: the function is now
   thread-safe but still not reentrant (within the
   same thread). */

__thread int t;

void swap(int *x, int *y)
{
    t = *x;
    *x = *y;
    *y = t;
}

例子3:非线程安全，可重入

/* We save the global state in a local variable and we restore
   it at the end of the function.  The function is now reentrant
   but it is not thread safe. */

int t;

void swap(int *x, int *y)
{
    int s;
    s = t;
    t = *x;
    *x = *y;
    *y = t;
    t = s;
}

例子4:线程安全，可重入

/* We use a local variable: the function is now
   thread-safe and reentrant, we have ascended to
   higher plane of existence.  */

void swap(int *x, int *y)
{
    int t;
    t = *x;
    *x = *y;
    *y = t;
}