Working with the Unix OS

Processes I
process is the child process.
main()
{
int childpid;
if ((childpid = fork()) == -1) {
fprintf(stderr, "can't fork\n"); exit(1);
} else if (childpid == 0) { /* child process */
printf("child: childpid=%d, parentpid=%d\n",
getpid(), getppid()); exit(0);
} else { /* parent process */
printf("parent: childpid=%d, parentpid=%d\n",
childpid, getpid()); exit(0);}
}
fork operation:
parent process
fork
child process
child process
- Text segment can be shared.
- Child's copy of the data segment is a copy of the parent's data segment, not the program's disk file
1. Process makes a copy of itself
- one copy can handle an operation while other copy does another task
- typical of network servers
2. Process executes another program
- fork to make a copy of itself
- issue exec to execute new program
exit System Call
- Process terminates
- exit status 0 to 255 (nonzero indicates error)
- _exit function avoids any standard I/O cleanup
exec System Call
- Replaces current process with new program .
- There are 6 version of exec
int execlp(char*file, char *arg, ..., NULL);
int execvp(char *file, char **argv);
int execl(char *path, char *arg, ..., NULL);
int execv{char *path, char **argv);
int execle{char *path, char *arg, ..., NULL, char **envp);
int execve{char *path, char **argv, char **envp);
- Exec process inherits attributes: process ID, parent process ID, process group ID, terminal group ID, time left
until an alarm clock signal, root directory, current working directory, first mode creation mask, file locks, real
user ID, real group ID
- Attributes that can change: effective user ID, effective group ID
- If the set-user-ID bit is set then effective user ID is changed to the user ID of the owner of the program
wait System Call
- A process can wait for a child process to finish
- Wait returns a process ID when a child process
- calls exit or
- is terminated by a signal or
- is being traced and the process stops
- Steps taken by kernel when a child process exits
if parent process has called a wait, then the parent is notified else the terminating process is marked as a
zombie process (kernel releases resources but keeps its exit status)
- If parent process terminates before child process then parent process ID is set to 1.
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