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C errno全局变量是否是线程安全的 #138

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zhangyachen opened this issue Dec 2, 2018 · 0 comments

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commented Dec 2, 2018

在使用多线程时,遇到了一个问题:线程例程中如果需要使用errno全局变量,如何保证errno的线程安全性?例如一个简单的线程池代码:

for(int i=0;i<THREADNUM;i++){
    pthread_create(&pid,NULL,start_routine,NULL);
}

while(1){
    connfd = accept(listenfd,(struct sockaddr *)&clientaddr,&clientlen);
    sbuf_insert(&buf,connfd);      //put connfd into pool
}

void *start_routine(void *argv){
    int connfd;
    int p = pthread_detach(pthread_self());
    while(1){
        connfd = sbuf_remove(&buf);        //thread get connfd
        doit(connfd);         //what if doit set global variable errno 
        close(connfd);
    }

    return NULL;
}

关于C中错误处理的问题,可以参考Error Handling in C programs,简单的说很多系统调用只会返回成功或者失败,具体失败的原因会设置全局变量errno供调用方自己读取,所以引发了多线程里errno线程安全的问题。
如何解决这个问题?毕竟设置errno的过程我们不能干预。上网搜了才发现,在POSIX标准中,重定义了errno,使之为线程安全的变量:

Redefinition of errno
In POSIX.1, errno is defined as an external global variable. But this definition is unacceptable in a multithreaded environment, because its use can result in nondeterministic results. The problem is that two or more threads can encounter errors, all causing the same errno to be set. Under these circumstances, a thread might end up checking errno after it has already been updated by another thread.
To circumvent the resulting nondeterminism, POSIX.1c redefines errno as a service that can access the per-thread error number as follows (ISO/IEC 9945:1-1996, §2.4):
Some functions may provide the error number in a variable accessed through the symbol errno. The symbol errno is defined by including the header <errno.h>, as specified by the C Standard ... For each thread of a process, the value of errno shall not be affected by function calls or assignments to errno by other threads.

颠覆了我的世界观呀,那这怎么实现的全局变量能够线程安全呢?

在error.h中(vim下按gf跳到库文件),看到如下定义:

/* Declare the `errno' variable, unless it's defined as a macro by
   bits/errno.h.  This is the case in GNU, where it is a per-thread
   variable.  This redeclaration using the macro still works, but it
   will be a function declaration without a prototype and may trigger
   a -Wstrict-prototypes warning.  */
#ifndef errno
extern int errno;
#endif

如果bits/error.h中没有定义errno,才会定义errno。

在bits/errno.h中,找到关于errno定义部分:

extern int *__errno_location (void) __THROW __attribute__ ((__const__));

#  if !defined _LIBC || defined _LIBC_REENTRANT
/* When using threads, errno is a per-thread value.  */
#   define errno (*__errno_location ())
#  endif

可以清晰的看到,bits/errno.h对errno进行了重定义。从 __attribute__ ((__const__))推测出__errno_location ()会返回与参数无关的与线程绑定的一个特定地址,应用层直接从该地址取出errno的。(关于__attribute__用法可以参考Using GNU C attribute)。但是上面使用了条件编译,也就是有两种方法可以使得gcc重定义errno:

  • 不定义宏_LIBC
  • 定义宏_LIBC_REENTRANT

但是很有意思的是,我们在编译时,压根不能设置_LIBC,感兴趣的可以自己试一下:

gcc -D_LIBC a.c
In file included from /usr/include/gnu/stubs.h:9:0,
                 from /usr/include/features.h:385,
                 from /usr/include/stdio.h:28,
                 from a.c:1:
/usr/include/gnu/stubs-64.h:7:3: error: #error Applications may not define the macro _LIBC
  #error Applications may not define the macro _LIBC
   ^

我们在编译时设置了宏_LIBC,但是编译失败,原来在gnu/stubs-64.h中会检测,如果有_LIBC宏定义,直接报错终止预编译:

#ifdef _LIBC
 #error Applications may not define the macro _LIBC
#endif

也就是在正常情况下,我们使用gcc编译的程序,全局变量errno一定是线程安全的。现在只剩下了一个问题,__errno_location是怎么实现的。遗憾的是,我并没有找到这个函数的实现,我们可以写个小程序汇编看一下在哪实现的:

#include <stdio.h>
#include <errno.h>

int main(){

    int i = errno;
    printf("%d",i);

    return 0;
}

查看汇编代码:

0x0000000000400588 in main ()
=> 0x0000000000400588 <main+8>: e8 7b fe ff ff  callq  0x400408 <__errno_location@plt>

@plt看出应该是动态库延迟绑定,进去看看:

0x0000003dc8e148c0 in _dl_runtime_resolve () from /lib64/ld-linux-x86-64.so.2
=> 0x0000003dc8e148c0 <_dl_runtime_resolve+0>:  48 83 ec 38     sub    $0x38,%rsp

呃呃,难道__errno_location定义在linux代码中?算了不看了,到这已经基本解决了我的问题:多线程如何保证errno全局变量的线程安全性,哈哈。猜测实现方式应该跟thread-local有关。

最后,虽然在多线程中我们不用保证errno的线程安全,但是如果需要编写信号处理函数时,我们仍然要保证errno的安全性,因为操作系统可能不会新创建一个线程来处理信号处理函数:

void handle_signal(int sig){
    int savedErrno;
    savedErrno = errno;
    /* Do something when recevied this sig */

    errno = savedErrno;
}

参考资料:

@zhangyachen zhangyachen added the C label Dec 2, 2018

@zhangyachen zhangyachen changed the title C errno是否是线程安全的 C errno全局变量是否是线程安全的 Dec 2, 2018

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