Fork system Call

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Example 1: (Independent context)

Click ( here) to collapse or open

2. #include <stdio.h>
4. #include <errno.h>
6. #include <stdlib.h>
8. #include <unistd.h>
10. #include <sys/types.h>
12. #include <fcntl.h>
16. #define ERROR (flag) \
18. if (flag) \
20. {                    \
22. printf ("%d:", __line__); \
24. Fflush (stdout); \
26. Perror ("error"); \
28. Exit (errno); \
30. }
34. int num = 999;
38. int main (int argc,char *argv[])
40. {
42. pid_t pid;
44. char *s = NULL;
48. printf ("Hello, my pid is%d\n\n", getpid ());
52. PID = fork ();
54. ERROR (PID = =-1);
58. if (PID = = 0)
60. {
62. int i = 2;
64. while (i--)
66. {
68. printf ("child process, PID =%d, Ppid =%d\n\n",
70. Getpid (), Getppid ());
72. s = "child process";
74. Sleep (1);
76. }
80. }
82. Else
84. {
86. int i = 2;
88. while (i--)
90. {
92. GetChar ();
94. printf ("parent process, PID =%d, child pid =%d\n\n",
96. Getpid (), PID);
98. s = "parent process";
100. Sleep (1);
102. }
104. }
106. printf ("I ' m%s, Byebye!!! \ n ", s);
110. return 0;
112. }

The compile link runs successfully, the parent process outputs the current and child process numbers, and the child processes output the current and parent process numbers. Results such as:


Example 2: (Independent file reading and writing)
Code:1

Click ( here) to collapse or open

2. #include <stdio.h>
4. #include <errno.h>
6. #include <stdlib.h>
8. #include <unistd.h>
10. #include <sys/types.h>
12. #include <fcntl.h>
16. #define ERROR (flag) \
18. if (flag) \
20. {                    \
22. printf ("%d:", __line__); \
24. Fflush (stdout); \
26. Perror ("error"); \
28. Exit (errno); \
30. }
34. int main (int argc,char *argv[])
36. {
38. #if 0
40. int fd = open (argv[1],o_rdonly);
42. ERROR (FD = =-1);
44. #endif
48. pid_t pid = fork ();
50. ERROR (PID = =-1);
54. #if 1
56. int fd = open (argv[1],o_rdonly);
58. ERROR (FD = =-1);
60. #endif
64. if (PID = = 0)
66. {
68. Char ch;
70. int ret = read (fd,&ch,1);
72. ERROR (Ret < 0);
74. printf ("Child process got%c\n", ch);
76. }
78. Else
80. {
82. Char ch;
84. int ret = read (fd,&ch,1);
86. ERROR (Ret < 0);
88. printf ("Parent process got%c\n", ch);
90. }
94. return 0;
96. }

After the compile link runs successfully, the parent-child process outputs the contents of the file read out from the file. Results such as:


Example 3: Orphan process

Click ( here) to collapse or open

2. #include <stdio.h>
4. #include <errno.h>
6. #include <stdlib.h>
8. #include <unistd.h>
10. #include <sys/types.h>
12. #include <fcntl.h>
16. #define ERROR (flag) \
18. if (flag) \
20. {                    \
22. printf ("%d:", __line__); \
24. Fflush (stdout); \
26. Perror ("error"); \
28. Exit (errno); \
30. }
34. int main (int argc,char *argv[])
36. {
38. pid_t pid;
42. PID = fork ();
44. ERROR (PID = =-1);
46. if (PID = = 0)
48. {
50. int i = 1000;
52. while (i--)
54. {
56. printf ("child process, PID =%d, Ppid =%d\n", Getpid (), Getppid ());
58. Sleep (5);
60. }
62. }
64. Else
66. {
68. printf ("Parent process, Byebye!!! \ n ");
70. Sleep (3);
72. _exit (0);
74. }
78. return 0;
80. }

The compile link runs successfully after the parent-child process outputs the content. Results such as:

The inter-process relationships viewed at another terminal are as follows:


Example 4: Parent process Gets the return value of a child process

Click ( here) to collapse or open

2. #include <stdio.h>
4. #include <errno.h>
6. #include <stdlib.h>
8. #include <unistd.h>
10. #include <sys/types.h>
12. #include <fcntl.h>
14. #include <sys/wait.h>
20. #define ERROR (flag) \
22. if (flag) \
24. {                    \
26. printf ("%d:", __line__); \
28. Fflush (stdout); \
30. Perror ("error"); \
32. Exit (errno); \
34. }
38. int main (int argc,char *argv[])
40. {
42. pid_t pid;
44. PID = fork ();
48. ERROR (PID = =-1);
52. if (PID = = 0)
54. {
56. int i = 3;
58. while (i--)
60. {
62. printf ("child process, PID =%d, Ppid =%d\n\n",
64. Getpid (), Getppid ());
66. Sleep (1);
68. }
72. printf ("Chile process exit, Byebye!!! \ n ");
74. _exit (99);
76. }
80. #if 1
82. int stat;
84. pid_t Child_pid;
88. Child_pid = Wait (&stat);
92. printf ("Child process has exited, PID =%d\n\n", child_pid);
94. if (wifexited (stat))
96. printf ("Child exited with code%d\n\n", Wexitstatus (stat));
98. Else
100. printf ("Child exit abnormally\n\n");
102. #endif
106. printf ("parent process, PID =%d\n", Getpid ());
108. GetChar ();
112. return 0;
114. }

The compile link runs successfully after the parent-child process outputs the content. Results such as:

Example 5: Process exit

Click ( here) to collapse or open

1. #include <stdio.h>
2. #include <stdlib.h>
3. #include <time.h>
4. #include <sys/stat.h>
5. #include <unistd.h>
6. #include <sys/types.h>
7. #include <errno.h>
9. int main (int argc, char *argv[])
10. {
11. printf ("Hello");
13. _exit (0);
15. return 0;
16. }

The compile link runs successfully, with no output content. Results such as:


After commenting out the _exit () function, the following

Click ( here) to collapse or open

1. #include <stdio.h>
2. #include <stdlib.h>
3. #include <time.h>
4. #include <sys/stat.h>
5. #include <unistd.h>
6. #include <sys/types.h>
7. #include <errno.h>
9. int main (int argc, char *argv[])
10. {
11. printf ("Hello");
13. _exit (0);
15. return 0;
16. }

Compile the link to run successfully, output content such as:


_exit () causes the process to exit before character output to the display device.

Fork system Call