fork和僵尸进程
1. 关于fork
fork()函数：
用于创建一个进程，所创建的进程复制父进程的代码段/数据段/BSS段/堆/栈等所有用户空间信息；在内核中操作系统重新为其申请了一个PCB，并使用父进程的PCB进行初始化；
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
|
#include <iostream>
#include <unistd.h>
using
namespace
std;
int
val = 10;
int
main(
int
argc,
char
*argv[])
{
pid_t pid;
int
lval = 20;
pid = fork();
if
(pid == 0){
val += 2;
lval += 5;
}
else
{
val -= 2;
lval += 5;
}
if
(pid == 0){
cout <<
"val:"
<< val <<
", lval = "
<< lval << endl;
}
else
{
cout <<
"val:"
<< val <<
", lval = "
<< lval << endl;
}
return
0;
}
|
对于父进程而言，fork()函数返回子进程的ID（子进程的PID）；而对于子进程而言，fork函数返回0。
僵尸进程
父进程创建子进程后，子进程运行到终止时刻（例如，调用
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
int
main(){
pid_t pid = fork();
if
(pid == 0){
cout <<
"I am a Child Process."
<<endl;
}
else
{
cout <<
"I am a Father Process and Child Process is "
<< pid << endl;
sleep(30);
//让父进程休眠30秒，此时便于观察子进程的状态
}
if
(pid == 0){
cout <<
" Child Process exits "
<< endl;
}
else
{
cout <<
"Father Process exits "
<< endl;
}
return
0;
}
|
此时，运行该程序，查看后台进程可知（test16是该测试程序的名称，defunct表示僵尸进程）：
gqx@gqx-Lenovo-Product:~$ ps -au USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 923 0.6 0.9 480840 159824 tty7 Ssl+ 4月09 36:07 /usr/lib/xorg/ root 1351 0.0 0.0 17676 1768 tty1 Ss+ 4月09 0:00 /sbin/agetty - ... gqx 24856 0.0 0.0 0 0 pts/11 Z+ 11:03 0:00 [tes16] <defunct> gqx 24859 0.0 0.0 39104 3300 pts/3 R+ 11:03 0:00 ps -au
僵尸进程的消除
方法一：调用wait()函数：
/* Wait for a child to die. When one does, put its status in *STAT_LOC and return its process ID. For errors, return (pid_t) -1. This function is a cancellation point and therefore not marked with __THROW. */ extern __pid_t wait (__WAIT_STATUS __stat_loc);
成功返回终止的进程ID，失败返回-1；子进程的最终返回值将指向该函数参数所指向的内存空间，但函数所指向的内存单元总还含有其他的信息，需要使用宏进行分离。
# define WIFEXITED(status) __WIFEXITED (__WAIT_INT (status)) //子进程正常终止返回"true" # define WEXITSTATUS(status) __WEXITSTATUS (__WAIT_INT (status)) //返回子进程的返回值
要注意的是：如果没有已终止的子进程，那么程序将被阻塞，直到有子进程终止。
方法二：调用waitpid()函数
/* Wait for a child matching PID to die. If PID is greater than 0, match any process whose process ID is PID. If PID is (pid_t) -1, match any process. If PID is (pid_t) 0, match any process with the same process group as the current process. If PID is less than -1, match any process whose process group is the absolute value of PID. If the WNOHANG bit is set in OPTIONS, and that child is not already dead, return (pid_t) 0. If successful, return PID and store the dead child's status in STAT_LOC. Return (pid_t) -1 for errors. If the WUNTRACED bit is set in OPTIONS, return status for stopped children; otherwise don't. This function is a cancellation point and therefore not marked with __THROW. */ extern __pid_t waitpid (__pid_t __pid, int *__stat_loc, int __options);
第一个参数：如果__pid的值是-1，则与wait()函数相同，可以等待任意的子程序终止；如果是0，则等待进程组识别码与目前进程相同的任何子进程；如果pid>0，则等待任何子进程识别码为 pid 的子进程。
第二个参数：与前一个函数wait()的参数意义相同。
第三个参数：常用WNOHANG——若pid指定的子进程没有结束，则waitpid()函数返回0，不予以等待。若结束，则返回该子进程的ID。
示例程序如下：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
|
#include <iostream>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
using
namespace
std;
void
read_childproc(
int
sig){
int
status;
pid_t id = waitpid(-1, &status, WNOHANG);
if
(WIFEXITED(status)){
printf
(
"Remove proc id: %d \n"
, id);
printf
(
"Child send: %d \n"
, WEXITSTATUS(status));
}
}
int
main(){
pid_t pid;
struct
sigaction act;
act.sa_handler = read_childproc;
sigemptyset(&act.sa_mask);
act.sa_flags = 0;
sigaction(SIGCHLD, &act, 0);
pid = fork();
if
(pid == 0){
puts
(
"Hi, I am a child process!"
);
sleep(6);
return
12;
}
else
{
printf
(
"Child proc id: %d \n"
, pid);
pid = fork();
if
(pid == 0){
puts
(
"Hi, I am a child process!"
);
sleep(13);
exit
(24);
}
else
{
int
i;
printf
(
"Child proc id: %d \n"
, pid);
for
(i = 0; i < 4; i++){
puts
(
"wait..."
);
sleep(5);
}
}
}
return
0;
}
|
1. 关于fork
fork()函数：
用于创建一个进程，所创建的进程复制父进程的代码段/数据段/BSS段/堆/栈等所有用户空间信息；在内核中操作系统重新为其申请了一个PCB，并使用父进程的PCB进行初始化；
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
|
#include <iostream>
#include <unistd.h>
using
namespace
std;
int
val = 10;
int
main(
int
argc,
char
*argv[])
{
pid_t pid;
int
lval = 20;
pid = fork();
if
(pid == 0){
val += 2;
lval += 5;
}
else
{
val -= 2;
lval += 5;
}
if
(pid == 0){
cout <<
"val:"
<< val <<
", lval = "
<< lval << endl;
}
else
{
cout <<
"val:"
<< val <<
", lval = "
<< lval << endl;
}
return
0;
}
|
对于父进程而言，fork()函数返回子进程的ID（子进程的PID）；而对于子进程而言，fork函数返回0。
僵尸进程
父进程创建子进程后，子进程运行到终止时刻（例如，调用
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
int
main(){
pid_t pid = fork();
if
(pid == 0){
cout <<
"I am a Child Process."
<<endl;
}
else
{
cout <<
"I am a Father Process and Child Process is "
<< pid << endl;
sleep(30);
//让父进程休眠30秒，此时便于观察子进程的状态
}
if
(pid == 0){
cout <<
" Child Process exits "
<< endl;
}
else
{
cout <<
"Father Process exits "
<< endl;
}
return
0;
}
|
此时，运行该程序，查看后台进程可知（test16是该测试程序的名称，defunct表示僵尸进程）：
gqx@gqx-Lenovo-Product:~$ ps -au USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 923 0.6 0.9 480840 159824 tty7 Ssl+ 4月09 36:07 /usr/lib/xorg/ root 1351 0.0 0.0 17676 1768 tty1 Ss+ 4月09 0:00 /sbin/agetty - ... gqx 24856 0.0 0.0 0 0 pts/11 Z+ 11:03 0:00 [tes16] <defunct> gqx 24859 0.0 0.0 39104 3300 pts/3 R+ 11:03 0:00 ps -au
僵尸进程的消除
方法一：调用wait()函数：
/* Wait for a child to die. When one does, put its status in *STAT_LOC and return its process ID. For errors, return (pid_t) -1. This function is a cancellation point and therefore not marked with __THROW. */ extern __pid_t wait (__WAIT_STATUS __stat_loc);
成功返回终止的进程ID，失败返回-1；子进程的最终返回值将指向该函数参数所指向的内存空间，但函数所指向的内存单元总还含有其他的信息，需要使用宏进行分离。
# define WIFEXITED(status) __WIFEXITED (__WAIT_INT (status)) //子进程正常终止返回"true" # define WEXITSTATUS(status) __WEXITSTATUS (__WAIT_INT (status)) //返回子进程的返回值
要注意的是：如果没有已终止的子进程，那么程序将被阻塞，直到有子进程终止。
方法二：调用waitpid()函数
/* Wait for a child matching PID to die. If PID is greater than 0, match any process whose process ID is PID. If PID is (pid_t) -1, match any process. If PID is (pid_t) 0, match any process with the same process group as the current process. If PID is less than -1, match any process whose process group is the absolute value of PID. If the WNOHANG bit is set in OPTIONS, and that child is not already dead, return (pid_t) 0. If successful, return PID and store the dead child's status in STAT_LOC. Return (pid_t) -1 for errors. If the WUNTRACED bit is set in OPTIONS, return status for stopped children; otherwise don't. This function is a cancellation point and therefore not marked with __THROW. */ extern __pid_t waitpid (__pid_t __pid, int *__stat_loc, int __options);
第一个参数：如果__pid的值是-1，则与wait()函数相同，可以等待任意的子程序终止；如果是0，则等待进程组识别码与目前进程相同的任何子进程；如果pid>0，则等待任何子进程识别码为 pid 的子进程。
第二个参数：与前一个函数wait()的参数意义相同。
第三个参数：常用WNOHANG——若pid指定的子进程没有结束，则waitpid()函数返回0，不予以等待。若结束，则返回该子进程的ID。
示例程序如下：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
|
#include <iostream>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
using
namespace
std;
void
read_childproc(
int
sig){
int
status;
pid_t id = waitpid(-1, &status, WNOHANG);
if
(WIFEXITED(status)){
printf
(
"Remove proc id: %d \n"
, id);
printf
(
"Child send: %d \n"
, WEXITSTATUS(status));
}
}
int
main(){
pid_t pid;
struct
sigaction act;
act.sa_handler = read_childproc;
sigemptyset(&act.sa_mask);
act.sa_flags = 0;
sigaction(SIGCHLD, &act, 0);
pid = fork();
if
(pid == 0){
puts
(
"Hi, I am a child process!"
);
sleep(6);
return
12;
}
else
{
printf
(
"Child proc id: %d \n"
, pid);
pid = fork();
if
(pid == 0){
puts
(
"Hi, I am a child process!"
);
sleep(13);
exit
(24);
}
else
{
int
i;
printf
(
"Child proc id: %d \n"
, pid);
for
(i = 0; i < 4; i++){
puts
(
"wait..."
);
sleep(5);
}
}
}
return
0;
}
|