Fork system call
From Teknologisk videncenter
fork() is one of several functions to "spawn" a process
Use of fork
fork() creates a new process by duplicating the calling process. The new process is referred to as the child process. The calling process is referred to as the parent process.
- PID - Process ID - the ID of a running process
- PPID - Parent Process ID - default the ID of the process that created this process or 1 if the parent died.
1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <unistd.h>
4
5
6 int main(void) {
7 printf("I am parent - my PPID is %d and my PID is %d\n", getppid(), getpid() );
8
9 if (fork() == 0) {
10 printf("I am child - my PPID is %d and my PID is %d\n", getppid(), getpid() );
11 printf("Child dying\n");
12 return(0);
13 }
14 printf("parent dying\n");
15 return(0);
16 }
Example of creating a daemon in Linux
Example of creating a daemon in Linux: (See systemd debian for systemd daemons)
- setsid() - damon will be the session leader. Sets Session ID (SID) and Group ID (GID) to Process ID (PID)
- Reopening stdin, stdout and stderr to /dev/null detaches the process from the controlling terminal and Parent PID is set to 1 (init or systemd - depending on Linux version/distribution)
1 #include <stdio.h>
2 #include <unistd.h>
3 #include <stdlib.h>
4 #include <syslog.h>
5 void daemon_create(void);
6
7 int main(void) {
8 printf("Starting my_daemon\n");
9 openlog("my_daemon", LOG_INFO, LOG_DAEMON);
10 daemon_create();
11 for (int i=0; i < 100; i++) {
12 syslog(LOG_INFO,"Countet to %d", i);
13 sleep(2);
14 }
15 return(0);
16 }
17
18 void daemon_create(void) {
19 pid_t pid;
20
21 pid = fork();
22 if(pid < 0){
23 syslog(LOG_ERR, "Error in fork: %m");
24 exit(1);
25 }
26 if(pid > 0) {
27 syslog(LOG_INFO, "Parent dying");
28 exit(0); // Parent die
29 }
30 if(setsid() < 0){ // Make child session leader
31 syslog(LOG_ERR, "Error in setsid: %m");
32 exit(2);
33 }
34 if (freopen("/dev/null", "r", stdin) == NULL
35 || freopen("/dev/null", "w", stdout) == NULL
36 || freopen("/dev/null", "w", stderr) == NULL ) {
37 syslog(LOG_ERR, "When creating daemon freopen from STDIN/OUT/ERR failed: %m");
38 exit(3);
39 }
40 }
Checking the daemon
- Daemon detached from TTY - PPID is 1
- Session and session group leader - SID and PGID same as PID
$ ps xo uname,pid,ppid,pgid,sid,tty,comm | grep -P '(my_daemon|COMMAND)'
USER PID PPID PGID SID TTY COMMAND
heth 778254 1 778254 778254 ? my_daemon
$ tail -f /var/log/sysg
Dec 20 12:37:59 emb3 my_daemon: Countet to 2
Dec 20 12:38:01 emb3 my_daemon: Countet to 3
Dec 20 12:38:03 emb3 my_daemon: Countet to 4
Dec 20 12:38:05 emb3 my_daemon: Countet to 5
...
Example of creating several processes
See same example in bash
1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <unistd.h>
4 #include <wait.h>
5
6 void childprocess(char *name, int count_to) {
7 int counter = 0;
8
9 printf("My name is %s and my PID is %d\n", name, getpid());
10
11 while (counter < count_to) {
12 sleep(1);
13 counter = counter +1;
14 printf("%s counted to %i\n", name, counter);
15 }
16 printf("%s signing off\n", name);
17 }
18
19
20 int main(void) {
21 int wstatus; // Used by c wait function
22
23 printf("I am the main function and my PID is %i\n", getpid());
24
25 if (fork() == 0) { // fork returns 0 for child
26 childprocess("Hans", 5);
27 return(0); // Child process finished
28 }
29
30 if (fork() == 0) {
31 childprocess("Ulla", 3);
32 return(0);
33 }
34
35 while(wait(&wstatus) > 0);
36 printf("Main signing off\n");
37 return(0);
38 }
Parent and child with different code
If we want a parent and a child to execute different code and share a resource, for example a server that send information to a client.
Three files:
- start.c - forks and starts the server code as parent and the client code as child.
- server.c"' - contains the server code
- client.c - contains the client code
start.c
1 #include <stdio.h>
2 #include <unistd.h>
3 #include <sys/types.h>
4
5 int main()
6 {
7 int arr[2];
8 char argv[50];
9 pipe(arr);
10 printf("Hello world\n");
11 if(fork())
12 {
13 printf("Starting server as parent - my PID is %d\n", getpid());
14 fflush(stdout);
15 close(arr[0]);
16 sprintf(argv,"%d",arr[1]);
17 execlp("./server","server",argv,NULL);
18 }
19 else
20 {
21 printf("Starting client as child - my PID is %d\n", getpid());
22 fflush(stdout);
23 close(arr[1]);
24 sprintf(argv,"%d",arr[0]);
25 execlp("./client","client",argv,NULL);
26 }
27 return 1;
28 }
server.c
1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <unistd.h>
4 #include <stdlib.h>
5 #include <string.h>
6
7 int main(int argc,char *argv[])
8 {
9 char buf[40]="hello";
10 int fd,i=0;
11
12 fd = atoi(argv[1]);
13 printf("Server started my PID is %d\n", getpid());
14 while(i < 5)
15 {
16 i++;
17 sprintf(buf,"hello: %d",i);
18 write(fd,buf,strlen(buf)+1);
19 sleep(1);
20 }
21 buf[0] == 0;
22 write(fd,buf,1);
23 puts("Server signing out");
24
25 return 0;
26 }
client.c
1 #include <stdio.h>
2 #include <unistd.h>
3 #include <sys/types.h>
4 #include <stdlib.h>
5
6 int main(int argc,char *argv[])
7 {
8 char buf[11];
9 int fd;
10
11 printf("Client started my PID is %d\n", getpid());
12 fd = atoi(argv[1]);
13 while(read(fd,buf,10) > 0)
14 {
15 printf("Client received: %s\n",buf);
16 }
17 printf("Client signing out");
18 return 0;
19 }