open函数中O_CLOEXEC标志的开关

最新推荐文章于 2024-09-04 12:03:34 发布

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最新推荐文章于 2024-09-04 12:03:34 发布

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man open里有这么一个flag:

O_CLOEXEC (Since Linux 2.6.23)
Enable the close-on-exec flag for the new file descriptor. Specifying this flag permits a program to avoid additional fcntl(2) F_SETFD
operations to set the FD_CLOEXEC flag. Additionally, use of this flag is essential in some multithreaded programs since using a separate fcntl(2) F_SETFD operation to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one
thread opens a file descriptor at the same time as another thread does a fork(2) plus execve(2).

　　意思就是新的内核里的这个选项是把fcntl的这个设置放在open里原子操作,以免在多线程程序里有可能会出现fcntl在设置的同时其它线程在fork+execve,虽然在线程里fork比较罕见.这个选项的意思就是子进程默认是继承父进程打开的所有fd,如果句柄加入了这个设置,在execve替换进程时就会关闭设置这个选项的所有fd.即当调用exec（）函数成功后，文件描述符会自动关闭。在以往的内核版本（2.6.23以前）中，需要调用
fcntl(fd, F_SETFD, FD_CLOEXEC) 来设置这个属性。而新版本（2.6.23开始）中，可以在调用open函数的时候，通过 flags 参数设置 CLOEXEC 功能，如 open(filename, O_CLOEXEC)。

虽然新版本支持在open时设置CLOEXEC，但是在编译的时候还是会提示错误 - error: ‘O_CLOEXEC’ undeclared (first use in this function)。原来这个新功能要求我们手动去打开，需要设置一个宏(_GNU_SOURCE)。可通过以下两种方法来设置这个宏以打开新功能：

1. 在源代码中加入 #define _GNU_SOURCE
2. 在编译参数中加入 -D_GNU_SOURCE

#include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #include <pthread.h>
 
 int flag=0;
 void* threadFunc(void* arg)
 {
 #ifdef _CLOEXEC_
     int fd = open("mm.log", O_CREAT|O_WRONLY|O_TRUNC|O_CLOEXEC, 0644);
 #else
     int fd = open("mm.log", O_CREAT|O_WRONLY|O_TRUNC, 0644);
 #endif
     printf("fd for mm.log is : %d\n", fd);
     if(fd<0) { perror("open mm.log failed"); exit(-1);}
 
     char s[32]="";
     sprintf(s, "%u", getpid());
     write(fd, s, strlen(s));
     flag=1;
     sleep(1000);
 }
 
 int main(int argc, char* argv[])
 {
     pthread_t thd;
     pthread_create(&thd, NULL, threadFunc, NULL);
 
     while(!flag);
 
     pid_t pid;
     pid=fork();
     if(pid<0) { perror("fork failed"); exit(-1);}
     else if(pid==0)
     {
         //child
         if(execl("/bin/sleep", "sleep", "100")== -1) { perror("exit failed"); exit(-1);}
 
         _exit(0);
     }
     printf("child pid: %u\n", pid);
     exit(0);
 }

使用两个选项编译两个程序:

[libgcc@Heineken ~/cpp]$ gcc -U_CLOEXEC_ -o nocloexec o_cloexec.c -lpthread
[libgcc@Heineken ~/cpp]$ gcc -D_CLOEXEC_ -o cloexec o_cloexec.c -lpthread
[libgcc@Heineken ~/cpp]$ ./nocloexec
fd for mm.log is : 3
child pid: 2201
[libgcc@Heineken ~/cpp]$ ps -ef|grep 220[1]
libgcc    2201     1 0 03:23 pts/0    00:00:00 sleep 100
[libgcc@Heineken ~/cpp]$ lsof -p 2201
COMMAND PID   USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
sleep   2201 libgcc cwd    DIR 253,0     4096 319493 /home/libgcc/cpp
sleep   2201 libgcc rtd    DIR 253,0     4096      2 /
sleep   2201 libgcc txt    REG 253,0    25688 262402 /bin/sleep
sleep   2201 libgcc mem    REG 253,0   151500 677607 /lib/ld-2.12.90.so
sleep   2201 libgcc mem    REG 253,0 1889628 677608 /lib/libc-2.12.90.so
sleep   2201 libgcc mem    REG 253,0 99158720 399306 /usr/lib/locale/locale-archive
sleep   2201 libgcc    0u   CHR 136,0      0t0      3 /dev/pts/0
sleep   2201 libgcc    1u   CHR 136,0      0t0      3 /dev/pts/0
sleep   2201 libgcc    2u   CHR 136,0      0t0      3 /dev/pts/0
sleep   2201 libgcc    3w   REG 253,0        4 336396 /home/libgcc/cpp/mm.log
[libgcc@Heineken ~/cpp]$ ./cloexec
fd for mm.log is : 3
child pid: 2216
[libgcc@Heineken ~/cpp]$ ps -ef|grep 221[6]
libgcc    2216     1 0 03:24 pts/0    00:00:00 sleep 100
[libgcc@Heineken ~/cpp]$ lsof -p 2216
COMMAND PID   USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
sleep   2216 libgcc cwd    DIR 253,0     4096 319493 /home/libgcc/cpp
sleep   2216 libgcc rtd    DIR 253,0     4096      2 /
sleep   2216 libgcc txt    REG 253,0    25688 262402 /bin/sleep
sleep   2216 libgcc mem    REG 253,0   151500 677607 /lib/ld-2.12.90.so
sleep   2216 libgcc mem    REG 253,0 1889628 677608 /lib/libc-2.12.90.so
sleep   2216 libgcc mem    REG 253,0 99158720 399306 /usr/lib/locale/locale-archive
sleep   2216 libgcc    0u   CHR 136,0      0t0      3 /dev/pts/0
sleep   2216 libgcc    1u   CHR 136,0      0t0      3 /dev/pts/0
sleep   2216 libgcc    2u   CHR 136,0      0t0      3 /dev/pts/0