open函数中O_CLOEXEC标志的开关
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