英文:
Why does the parent process always executes later than the child process it creates without wait() in UNIX?
问题
根据您提供的信息，父进程将"a"写入myFile.txt，而子进程将"b"写入该文件。您多次执行此程序，发现文本文件一直是"ba"，这意味着父进程始终在子进程之后执行。但是，您注意到父进程没有调用wait()函数。有人能解释为什么父进程在子进程之后执行吗？
根据代码和描述，这是因为操作系统的调度机制。在fork()之后，操作系统决定哪个进程首先运行。通常情况下，父进程和子进程的执行顺序是不确定的，取决于操作系统的调度策略。父进程和子进程之间的执行顺序可能会因为各种因素而变化，包括系统负载、进程优先级等。
在您的情况下，父进程和子进程都尝试写入文件myFile.txt。由于文件操作是一种共享资源，操作系统会根据其调度策略来决定哪个进程首先获得写入权限。因此，即使父进程在代码中出现在子进程之前，实际执行顺序可能会有所不同。
要实现明确定义的执行顺序，您可以使用进程同步机制，如信号量或互斥锁，以确保父进程在子进程之后执行。如果不使用这些机制，您不能依赖于操作系统的调度来保证执行顺序。
英文:
I have a homework question:
> Q7: After executing the following code, a new file named myFile.txt is generated. Is the content in myFile.txt will be consistent? Why?
And here is the code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/wait.h>
int main(int argc, char *argv[]){
printf("hello world (pid:%d)\n", (int)getpid());
int fd = open("myFile.txt", O_CREAT|O_RDWR);
if(fd == -1 ) {
printf("Unable to open the file\n exiting....\n");
return 0;
}
int rc = fork();
if (rc < 0) {
fprintf(stderr, "fork failed\n");
exit(1);
}
else if (rc == 0) {
printf("hello, I am child (pid:%d)\n", (int)getpid());
char myChar='a';
write(fd, &myChar,1);
printf("writing a character to the file from child\n");
}
else {
printf("hello, I am parent of %d (pid:%d)\n",
rc, (int)getpid());
char myChar='b';
write(fd, &myChar,1);
printf("writing a character to the file from parent\n");
}
return 0;
}
The parent will write "a" into myFile.txt while the child writes "b" into that file. I executed this program several times, finding the txt file consistent being "ba", meaning that the parent always comes after the child. However, I noticed that there's no wait() function called by the parent process. Can someone explain why the parent comes after the child?
答案1
得分: 1
并发任务的顺序由操作系统定义。如果需要确保操作的明确定义顺序，您需要使用同步原语（文件锁、管道、互斥锁、条件变量、信号量等）。
英文:
The order of concurrent tasks are implementation defined by the OS. You need to use synchronization primitives to ensure a well defined order of actions if required (file locks, pipes, mutex, condition variables, semaphores etc).
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