swapper 进程(系统进程)：ID = 0，只负责调度进程，并不执行磁盘上的任何程序
init 进程：ID = 1，负责读取与系统有关的初始化文件。init进程不会终止。它是一个普通的用户进程，以超级用户特权运行。
/sbin/init
init进程收养僵死进程-->只要有一个子进程终止，init就会调用一个wait函数取得其终止状态，防止了系统中塞满僵死进程。
进程正常或者异常终止--->内核就会向其父进程发送SIGCHLD信号，这种信号市内核向父进程发送的异步通知，父进程可以选择处理或者忽略。
exec 函数：
#include "apue.h"
#include <sys/wait.h>
//char* env_init[] = {"USER=unknown","PATH=/tmp",NULL};
int main()
{
pid_t pid;
if((pid = fork())<0){
printf("fork err!");
}else if(pid == 0){
if(execle("/bin/echo","echo","dd","MY arg2",(char*)0)<0)
printf("execle err\n");
}
if(waitpid(pid,NULL,0)<0)
printf("wait err!\n");
if((pid = fork())<0){
printf("fork error!\n");
}
else if(pid ==0){
if(execlp("echo","echo","only 1 arg",(char*)0)<0)
printf("execlp err\n");
}
exit(0);
}输出：
kali@kali:~/Desktop/Linux Study/Hellos/Chapter8$ ./Exec_8_16
dd MY arg2
kali@kali:~/Desktop/Linux Study/Hellos/Chapter8$ only 1 arg通过nice()改变进程调度优先级
#include "apue.h"
#include <errno.h>
#include <sys/time.h>
#ifdef MACOS
#include <sys/syslimits.h>
#elif defined(BSD)
#include <sys/param.h>
#endif
unsigned long long count;
struct timeval end;
void checktime(char* str){
struct timeval tv;
gettimeofday(&tv,NULL);
if(tv.tv_sec>= end.tv_sec && tv.tv_usec >= end.tv_usec){
printf("%s count = %lld\n",str,count);
exit(0);
}
}
int main(int argc,char* argv[]){
pid_t pid;
char *s;
int nzero,ret;
int adj = 0;
setbuf(stdout,NULL);
#ifdef NZERO
nzero = NZERO;
#elif defined _SC_NZERO
nzero = sysconf(_SC_NZERO);
#else
#error NZERO undifined
#endif
printf("NZERO = %d\n",nzero);
if(argc ==2)
adj = strtol(argv[1],NULL,10);/* Convert a string to a long integer. */
gettimeofday(&end,NULL);
end.tv_sec+=10;
if((pid = fork())<0){
printf("fork err\n");
}else if(pid ==0){
s = "child";
printf("current nice value is %d,adjusting by %d\n",nice(0)+nzero,adj);
errno = 0;
if((ret = nice(adj))==-1 &&errno!=0)
printf("child set scheduling priority");
printf("now the child nice value is %d\n",ret+nzero);
}
else{
s = "parent";
printf("current parent process nice value is %d\n",nice(0)+nzero);
}
for(;;){
if(++count == 0)
printf("%s counter warp",s);
checktime(s);
}
}输出：
kali@kali:~/Desktop/Linux Study/Hellos/Chapter8$ ./Nice_8_30 10
NZERO = 20
current parent process nice value is 20
current nice value is 20,adjusting by 10
now the child nice value is 30
child count = 545334793
parent count = 540767853通过times()函数获取自己以及已经终止子进程的用户CPU时间和系统CPU时间
clock_t times(struct tms *buf);
/* Structure describing CPU time used by a process and its children. */
struct tms
{
clock_t tms_utime; /* User CPU time. */
clock_t tms_stime; /* System CPU time. */
clock_t tms_cutime; /* User CPU time of dead children. */
clock_t tms_cstime; /* System CPU time of dead children. */
};
#include "apue.h"
#include <sys/times.h>
static void pr_time(clock_t,struct tms*,struct tms*);
static void do_cmd(char*);
int main(int argc,char* argv[]){
int i;
setbuf(stdout,NULL);
for (int i = 1; i < argc; i++)
{
do_cmd(argv[i]);
}
exit(0);
}
static void do_cmd(char* cmd){
struct tms tmsstart,tmsend;
clock_t start,end;
int status;
printf("\n command:%s \n",cmd);
if((start = times(&tmsstart)) == -1)
printf("time err\n");
if((status = system(cmd))<0)
printf("syetem err\n");
if((end = times(&tmsend))==-1)
printf("time err\n");
pr_time(end-start,&tmsstart,&tmsend);
pr_exit(status);
}
static void pr_time(clock_t real,struct tms*tmsstart,struct tms* tmsend){
static long clktick = 0;
if(clktick ==0){
if((clktick = sysconf(_SC_CLK_TCK))<0) /*fetch clock ticks per second first time */
printf("sysconf err\n!");
}
printf(" real: %7.2f\n",real/(double)clktick);
printf(" user: %7.2f\n",(tmsend->tms_utime -tmsstart->tms_utime)/(double)clktick);
printf(" sys: %7.2f\n",(tmsend->tms_stime -tmsstart->tms_stime)/(double)clktick);
printf(" child user: %7.2f\n",(tmsend->tms_cutime -tmsstart->tms_cutime)/(double)clktick);
printf(" child sys: %7.2f\n",(tmsend->tms_cstime -tmsstart->tms_cstime)/(double)clktick);
}输出：
kali@kali:~/Desktop/Linux Study/Hellos/Chapter8$ ./Times_8_31 "sleep 5" "date" "man bash >/dev/null"
command:sleep 5
real: 5.01
user: 0.00
sys: 0.00
child user: 0.00
child sys: 0.00
normal terminnation,exit status = 0
command:date
Wed 10 Jun 2020 10:50:38 PM EDT
real: 0.00
user: 0.00
sys: 0.00
child user: 0.00
child sys: 0.00
normal terminnation,exit status = 0
command:man bash >/dev/null
real: 0.26
user: 0.00
sys: 0.00
child user: 0.14
child sys: 0.20
normal terminnation,exit status = 0