process.md

Process

1. Private Address Space
2. user mode & kernel mode
3. context switch

context switch will make cache cold.

user mode and kernel mode can switch by setting EFLAGS bits.

当system call发生错误时,典型的会返回-1，并设置全局整数变量errno来表示出了什么错误.

###Obtaining Process IDs

#include <sys/types.h> 
#include <unistd.h>

pid_t getpid(void); 
pid_t getppid(void);

                    Returns: PID of either the caller or the parent

###Creating and Terminating Processes #include <stdlib.h>

void exit(int status) 
                    This function does not return

#include #include

pid_t fork(void)
                    Return 0 to child, return child pid to parent

新创建的child process几乎和parent process一模一样, child process得到与parent process虚拟地址空间相同的（但是独立的）一份拷贝，包括文本,数据,bss段,堆以及用户栈,还有文件描述符.

注意, fork是COW(copy on write)的.

###Reaping Child Processes A terminated process that has not yet been reaped is called a zombie.

If the parent process terminates without reaping its zombie children, the kernel arranges for the init process to reap them. The init process has a PID of 1 and is created by the kernel during system initialization.

A process waits for its children to terminate or stop by calling the waitpid function.

#include <sys/types.h> 
#include <sys/wait.h>
pid_t waitpid(pid_t pid, int *status, int options);

                    Returns: PID of child if OK, 0 (if WNOHANG) or −1 on error

如何pid > 0,等待特定的process. 如果pid = -1,等待集合就是所有子进程集合.

wait(&status) === waitpid(-1, &status, 0)

###Putting Processes to Sleep #include <unistd.h> unsigned int sleep(unsigned int secs);

                    Returns: seconds left to sleep

pause which puts the calling function to sleep until a signal is received by the process. #include <unistd.h> int pause(void);

                    Always returns -1

###Loading and Running Programs #include <unistd.h> int execve(const char *filename, const char *argv[], const char *envp[]); Does not return if OK, returns −1 on error execve调用一次从不返回，并没有创造出新的子进程，它只是在当前进程的上下文中加载并运行一个新的程序.