blocks|key|1198299|text|我认为答案是“视情况而定”。如果使用正确的标志构建线程代码，线程安全的C运行时库通常会将errno实现为函数调用(宏扩展为函数)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1198300|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I think the answer is "it depends". Thread-safe C runtime libraries usually implement errno as a function call (macro expanding to a function) if you're building threaded code with the correct flags.

blocks|key|1198369|text|在许多Unix系统上，使用-D_REENTRANT编译可确保errno是线程安全的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1198370|例如：|1198371|#if+defined(_REENTRANT)+%7C%7C+_POSIX_C_SOURCE+-+0+>=+199506L
extern+int+*___errno();
#define+errno+(*(___errno()))
#else
extern+int+errno;
/*+ANSI+C%2B%2B+requires+that+errno+be+a+macro+*/
#if+__cplusplus+>=+199711L
#define+errno+errno
#endif
#endif++/*+defined(_REENTRANT)+*/|code-block|syntax|javascript|1198372|entityMap^0|D|C|U|5|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|U|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

On many Unix systems, compiling with <code>-D_REENTRANT</code> ensures that <code>errno</code> is thread-safe.

For example:

<pre><code>#if defined(_REENTRANT) || _POSIX_C_SOURCE - 0 &gt;= 199506L
extern int *___errno();
#define errno (*(___errno()))
#else
extern int errno;
/* ANSI C++ requires that errno be a macro */
#if __cplusplus &gt;= 199711L
#define errno errno
#endif
#endif /* defined(_REENTRANT) */
</code></pre>

blocks|key|1198410|text|这是来自我的苹果电脑上的<sys/errno.h>：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1198411|#include+<sys/cdefs.h>
__BEGIN_DECLS
extern+int+*+__error(void);
#define+errno+(*__error())
__END_DECLS|code-block|syntax|javascript|1198412|因此，errno现在是一个函数__error()。该函数的实现是线程安全的。|1198413|entityMap^0|C|D|0|0|3|5|F|9|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

This is from <code>&lt;sys/errno.h&gt;</code> on my Mac:

<pre><code>#include &lt;sys/cdefs.h&gt;
__BEGIN_DECLS
extern int * __error(void);
#define errno (*__error())
__END_DECLS
</code></pre>

So <code>errno</code> is now a function <code>__error()</code>. The function is implemented so as to be thread-safe.

blocks|key|1198466|text|我们可以通过在机器上运行一个简单的程序来检查。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1198467|#include+<stdio.h>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#include+<pthread.h>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#include+<errno.h>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#define+NTHREADS+5+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
void+*thread_function(void+*);+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

int++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
main()+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
{++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++pthread_t+thread_id[NTHREADS];++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++int+i,+j;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++for(i=0;+i+<+NTHREADS;+i%2B%2B)+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++{
++++++pthread_create(+&thread_id[i],+NULL,+thread_function,+NULL+);++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++}+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++for(j=0;+j+<+NTHREADS;+j%2B%2B)+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++{+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
++++++pthread_join(+thread_id[j],+NULL);+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++}+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++return+0;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
}++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

void+*thread_function(void+*dummyPtr)++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
{++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++printf("Thread+number+%25ld+addr(errno):%25p\n",+pthread_self(),+&errno);+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
}|code-block|syntax|javascript|1198468|运行此程序，您可以在每个线程中看到errno的不同地址。在我的机器上运行的输出如下所示：|1198469|Thread+number+140672336922368+addr(errno):0x7ff0d4ac0698+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Thread+number+140672345315072+addr(errno):0x7ff0d52c1698+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Thread+number+140672328529664+addr(errno):0x7ff0d42bf698+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Thread+number+140672320136960+addr(errno):0x7ff0d3abe698+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Thread+number+140672311744256+addr(errno):0x7ff0d32bd698+|1198470|请注意，所有线程的地址都是不同的。|1198471|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

We can check by running a simple program on a machine.

<pre><code>#include &lt;stdio.h&gt; 
#include &lt;pthread.h&gt; 
#include &lt;errno.h&gt; 
#define NTHREADS 5 
void *thread_function(void *); 

int 
main() 
{ 
 pthread_t thread_id[NTHREADS]; 
 int i, j; 

 for(i=0; i &lt; NTHREADS; i++) 
 {
 pthread_create( &amp;thread_id[i], NULL, thread_function, NULL ); 
 } 

 for(j=0; j &lt; NTHREADS; j++) 
 { 
 pthread_join( thread_id[j], NULL); 
 } 
 return 0; 
} 

void *thread_function(void *dummyPtr) 
{ 
 printf("Thread number %ld addr(errno):%p\n", pthread_self(), &amp;errno); 
}
</code></pre>

Running this program and you can see different addresses for errno in each thread. The output of a run on my machine looked like:-

<pre><code>Thread number 140672336922368 addr(errno):0x7ff0d4ac0698 
Thread number 140672345315072 addr(errno):0x7ff0d52c1698 
Thread number 140672328529664 addr(errno):0x7ff0d42bf698 
Thread number 140672320136960 addr(errno):0x7ff0d3abe698 
Thread number 140672311744256 addr(errno):0x7ff0d32bd698 
</code></pre>

Notice that address is different for all threads.

In <code>errno.h</code>, this variable is declared as <code>extern int errno;</code> so my question is, is it safe to check <code>errno</code> value after some calls or use perror() in multi-threaded code. Is this a thread safe variable? If not, then whats the alternative ? 

I am using linux with gcc on x86 architecture.

Is errno thread-safe?

Unix

在errno.h中，这个变量被声明为extern int errno;，所以我的问题是，在一些调用之后检查perror值或在多线程代码中使用errno ()是否安全。这是一个线程安全变量吗？如果不是，那么还有什么选择呢？我和gcc在x86架构上使用linux。

问errno是线程安全的吗？
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