fork

 1 #include <unistd.h>
 2 #include <stdlib.h>
 3 #include <stdio.h>
 4 #include <fcntl.h>
 5 #include <error.h>
 6 #include <errno.h>
 7 int glob = 6;
 8 char buf[] = "a write to stdout\n";
 9
10 int main (int argc, char **argv)
11 {
12     int var;
13     pid_t pid;
14     var = 88;
15     if (write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1) {
16         perror("write");
17     }
18
19     if ((pid = fork()) < 0) {
20         perror("fork");
21     } else if(pid == 0){
22         glob++;
23         var++;
24         //printf("child: glob:%d var:%d\n",glob, var);
25     }else {
26         sleep(2);
27     }
28
29     printf("pid:%d,which parent id:%d\n",getpid(), getppid());
30     printf("pid:%d glob:%d var:%d\n",getpid() ,glob, var);
31     return 0;
32 }

fork once call, twice return

output:

a write to stdout
pid:23354,which parent id:23320
pid:23354 glob:7 var:89
pid:23320,which parent id:23317
pid:23320 glob:6 var:88