Computer Systems II
Crea ng and Execu ng Processes
1
Unix System Calls
2
1 How To Create New Processes?
Underlying mechanism - A process runs fork to create a child process - Parent and children execute concurrently - Child process is a duplicate of the parent process
parent
fork()
child
3
Process Crea on
A er a fork, both parent and child keep running, and each can fork off other processes.
A process tree results. The root of the tree is a special process created by the OS during startup.
A process can choose to wait for children to terminate. For example, if C issued a wait() system call, it would block un l G finished.
4
2 Bootstrapping
When a computer is switched on or reset, there must be an ini al program that gets the system running
This is the bootstrap program - Ini alize CPU registers, device controllers, memory - Load the OS into memory - Start the OS running
OS starts the first process (such as “init”)
OS waits for some event to occur - Hardware interrupts or so ware interrupts (traps)
5
Fork System Call
Current process split into two: parent, child
fork() Returns -1 if unsuccessful
Returns 0 in the child Stack Stack Returns the child’s iden fier in the parent ret = 0 Data Data
Text ret = xxx Text
6
3 Fork System Call
The child process inherits from parent - iden cal copy of memory - CPU registers - all files that have been opened by the parent
Execu on proceeds concurrently with the instruc on following the fork system call
The execu on context (PCB) for the child process is a copy of the parent’s context at the me of the call
7
How fork Works (1)
pid = 25
Data Resources Text Stack PCB File
ret = fork(); switch(ret) { case -1: perror(“fork”); exit(1); case 0: // I am the child exit(0); default: // I am parent ... UNIX wait(0); } 8
4 How fork Works (2)
pid = 25 pid = 26
Data Resources Data Text Text Stack Stack PCB File PCB
ret = 0 ret = fork(); ret = 26 ret = fork(); switch(ret) switch(ret) { { case -1: case -1: perror(“fork”); perror(“fork”); exit(1); exit(1); case 0: // I am the child case 0: // I am the child exit(0); exit(0); default: // I am parent ... default: // I am parent ... UNIX wait(0); wait(0); } }
9
How fork Works (3)
pid = 25 pid = 26
Data Resources Data Text Text Stack Stack PCB File PCB
ret = 0 ret = fork(); ret = 26 ret = fork(); switch(ret) switch(ret) { { case -1: case -1: perror(“fork”); perror(“fork”); exit(1); exit(1); case 0: // I am the child case 0: // I am the child exit(0); exit(0); default: // I am parent ... default: // I am parent ... UNIX wait(0); wait(0); } }
10
5 How fork Works (4)
pid = 25 pid = 26
Data Resources Data Text Text Stack Stack PCB File PCB
ret = 0 ret = fork(); ret = 26 ret = fork(); switch(ret) switch(ret) { { case -1: case -1: perror(“fork”); perror(“fork”); exit(1); exit(1); case 0: // I am the child case 0: // I am the child exit(0); exit(0); default: // I am parent ... default: // I am parent ... UNIX wait(0); wait(0); } }
11
How fork Works (5)
pid = 25 pid = 26
Data Resources Data Text Text Stack Stack PCB File PCB
ret = 0 ret = fork(); ret = 26 ret = fork(); switch(ret) switch(ret) { { case -1: case -1: perror(“fork”); perror(“fork”); exit(1); exit(1); case 0: // I am the child case 0: // I am the child exit(0); exit(0); default: // I am parent ... default: // I am parent ... UNIX wait(0); wait(0); } }
12
6 How fork Works (6)
pid = 25
Data Resources Text Stack Process Status File
ret = fork(); ret = 26 switch(ret) { case -1: perror(“fork”); exit(1); case 0: // I am the child exit(0); default: // I am parent ... UNIX wait(0); < … >
13
Orderly Termina on: exit()
To finish execu on, a child may call exit(number)
This system call: - Saves result = argument of exit - Closes all open files, connec ons - Deallocates memory - Checks if parent is alive - If parent is alive, holds the result value un l the parent requests it (with wait); in this case, the child process does not really die, but it enters a zombie/defunct state - If parent is not alive, the child terminates (dies)
14
7 Wai ng for the Child to Finish
Parent may want to wait for children to finish - Example: a shell wai ng for opera ons to complete
Wai ng for any some child to terminate: wait() - Blocks un l some child terminates - Returns the process ID of the child process - Or returns -1 if no children exist (i.e., already exited)
Wai ng for a specific child to terminate: waitpid() - Blocks ll a child with par cular process ID terminates
#include
pid_t wait(int *status); pid_t waitpid(pid_t pid, int *status, int options);
15
State Transi on on wait and exit Calls
16
8 Other useful system calls: getpid, getppid
getpid returns the iden fier of the calling process. Example call (pid is an integer): pid = getpid();
getppid returns the iden fier of the parent.
Note: - Zombies can be no ced by running the 'ps' command (shows the process list); you will see the string "
ps -ef ps –ef | grep mdamian
17
Fork Example 1: What Output?
int main() { pid_t pid; int x = 1;
pid = fork(); if (pid != 0) { printf(“parent: x = %d\n”, --x); exit(0); } else { printf(“child: x = %d\n”, ++x); exit(0); } }
18
9 Fork Example 2
Key Points - Both parent and child can con nue forking
void fork2() { printf("L0\n"); Bye fork(); L1 Bye printf("L1\n"); fork(); Bye printf("Bye\n"); L0 L1 Bye }
19
Fork Example 3
void fork3() Bye { L2 Bye printf("L0\n"); fork(); Bye printf("L1\n"); L1 L2 Bye fork(); Bye printf("L2\n"); L2 Bye fork(); printf("Bye\n"); Bye } L0 L1 L2 Bye
20
10 Fork Example 4
void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }
21
Fork Example 5
void fork5() { printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }
22
11 Summary
Fork - Creates a duplicate of the calling process - The result is two processes: parent and child - Both con nue execu ng from the same point on
Exit - Orderly program termina on - Unblocks wai ng parent
Wait - Used by parent - Waits for child to finish execu on
23
12