Kevin Sooter - Processes

README.md

@@ -3,19 +3,22 @@
 Now that we've talked a bit about how processes and system calls work at a high level, it's time to apply these concepts by doing some exercises related to process creation and making system calls. We'll be utilizing the `fork()`, `exec()`, `wait()`, and `pipe()` system calls in order to create processes and even have them pass messages to each other.
 
 ## Objective
-To introduce and familiarize yourself with some basic system calls pertaining to process 
+
+To introduce and familiarize yourself with some basic system calls pertaining to process
 creation, to spawn some new processes, and to practice writing more C!
 
 ## `fork()`
-The `fork()` system call is used by a parent process to create a new child process. Its 
-actual implementation isn't as intuitive as it could be, though. When a parent process 
-executes `fork()`, the new child process that is created is an almost exact copy of the 
-calling process from the operating system's perspective. We say _almost_ an exact copy 
-to delineate the fact that while the new child process has the same instruction set 
-(i.e. code) as its parent process, the child process starts executing at the point right 
-after `fork()` is called in the parent process. 
+
+The `fork()` system call is used by a parent process to create a new child process. Its
+actual implementation isn't as intuitive as it could be, though. When a parent process
+executes `fork()`, the new child process that is created is an almost exact copy of the
+calling process from the operating system's perspective. We say _almost_ an exact copy
+to delineate the fact that while the new child process has the same instruction set
+(i.e. code) as its parent process, the child process starts executing at the point right
+after `fork()` is called in the parent process.
 
 Let's look at a program that calls `fork()` to try to give an example of what this means:
+
 ```c
 // p1.c
 #include <stdio.h>
@@ -39,7 +42,9 @@ int main(int argc, char *argv[])
     return 0;
 }
 ```
+
 Running this program, one of the possible outputs could be something like this:
+
 ```
 prompt> ./p1
 hello world (pid: 29146)
@@ -48,22 +53,24 @@ hello, child here (pid: 29147)
 prompt>
 ```
 
-So as we can see, the child process doesn't return the exact same output as its parent process, which 
-is good, because otherwise spawning child processes wouldn't be nearly as useful. Notice that the child 
-process has its own process identifier (PID); it also has its own address space, program counter, and 
-execution context that are not the same as its parent's. 
+So as we can see, the child process doesn't return the exact same output as its parent process, which
+is good, because otherwise spawning child processes wouldn't be nearly as useful. Notice that the child
+process has its own process identifier (PID); it also has its own address space, program counter, and
+execution context that are not the same as its parent's.
 
-More specifically, if we look at the `int rc = fork();` line, which both parent and chlid execute, they 
-receive different results here. The parent process receives the PID of the child process it just spawned, 
-while the child process receives 0 if the `fork()` call was successful. 
+More specifically, if we look at the `int rc = fork();` line, which both parent and chlid execute, they
+receive different results here. The parent process receives the PID of the child process it just spawned,
+while the child process receives 0 if the `fork()` call was successful.
 
 ## `wait()`
-So `fork()` allows us to spawn new child processes, but the thing with having multiple processes is that 
-the order in which they execute is not deterministic, i.e., the parent may execute before the child or 
-the child may execute before the parent; we can't say for certain. The `wait()` system call (along with 
-the more complete `waitpid()` system call) allows us to get around this non-determinism if that is something 
-that needs to be accounted for. Let's add a call to `waitpid()` in our previous example program to ensure that 
+
+So `fork()` allows us to spawn new child processes, but the thing with having multiple processes is that
+the order in which they execute is not deterministic, i.e., the parent may execute before the child or
+the child may execute before the parent; we can't say for certain. The `wait()` system call (along with
+the more complete `waitpid()` system call) allows us to get around this non-determinism if that is something
+that needs to be accounted for. Let's add a call to `waitpid()` in our previous example program to ensure that
 the child always runs before its parent:
+
 ```c
 // p2.c
 #include <stdio.h>
@@ -89,11 +96,14 @@ int main(int argc, char *argv[])
     return 0;
 }
 ```
-Here, the `waitpid()` function suspends the parent process until the child process pointed at by `rc` terminates. Thus, we ensure that the parent process only runs after the child process has finished its execution. 
+
+Here, the `waitpid()` function suspends the parent process until the child process pointed at by `rc` terminates. Thus, we ensure that the parent process only runs after the child process has finished its execution.
 
 ## `exec()`
-The `exec()` system call is used in order to run a program that is different from the calling program (since `fork` only executes copies of the program that called it). 
+
+The `exec()` system call is used in order to run a program that is different from the calling program (since `fork` only executes copies of the program that called it).
 Let's say we wanted to spin up a child process to execute a word count program. Here's how what a program that does that might look like:
+
 ```c
 // p3.c
 #include <stdio.h>
@@ -113,8 +123,8 @@ int main(int argc, char *argv[])
     } else if (rc == 0) {    // child process satisfies this branch
         printf("hello, child here (pid: %d) \n", (int) getpid());
         char *myargs[3];    // allocate an array of chars to hold 3 bytes
-        // `strdup` duplicates the given input string 
-        myargs[0] = strdup("wc");      // pass the name of the program we want to run as the first array element 
+        // `strdup` duplicates the given input string
+        myargs[0] = strdup("wc");      // pass the name of the program we want to run as the first array element
         myargs[1] = strdup("p3.c");    // argument: the file to count
         myargs[2] = NULL;              // marks the end of the array
         execvp(myargs[0], myargs);     // runs the word count program, passing in the `myargs` array to the word count program as arguments
@@ -127,16 +137,19 @@ int main(int argc, char *argv[])
     return 0;
 }
 ```
-Here, we're doing the same thing as before, forking a new child process from a parent process. Then, inside that 
-child process, we're calling `execvp()` with the arguments it needs to run the word count program. Note that `exec` 
-does not spin up _another_ child process from the child process in which we called it. It transforms the process 
-that called it into the new program that was passed to `exec`, in this case, `wc`, the word count process. That's 
-why we still had to have the parent process `fork` a new child process. 
+
+Here, we're doing the same thing as before, forking a new child process from a parent process. Then, inside that
+child process, we're calling `execvp()` with the arguments it needs to run the word count program. Note that `exec`
+does not spin up _another_ child process from the child process in which we called it. It transforms the process
+that called it into the new program that was passed to `exec`, in this case, `wc`, the word count process. That's
+why we still had to have the parent process `fork` a new child process.
 
 ## `pipe()`
-Conceptually, a pipe is a uni-directional channel between two processes that would otherwise be isolated from each other. When a pipe is established between two processes, one process has access to the write end of the pipe, while the other has read access to the other end of the pipe. Thus, if you want two-way communication between two processes, two pipes will have to be created between both processes, one in each direction. 
 
-Some things to keep in mind when working with pipes is that when a process writes to a pipe, the other process receives the data in FIFO order (which makes sense when you think about the pipe analogy in real life). Additionally, if the process with read access tries to read from the pipe before anything has been written to it, the reading process is suspended until there is some data to read. 
+Conceptually, a pipe is a uni-directional channel between two processes that would otherwise be isolated from each other. When a pipe is established between two processes, one process has access to the write end of the pipe, while the other has read access to the other end of the pipe. Thus, if you want two-way communication between two processes, two pipes will have to be created between both processes, one in each direction.
+
+Some things to keep in mind when working with pipes is that when a process writes to a pipe, the other process receives the data in FIFO order (which makes sense when you think about the pipe analogy in real life). Additionally, if the process with read access tries to read from the pipe before anything has been written to it, the reading process is suspended until there is some data to read.
+
 ```c
 #include <stdio.h>
 #include <stdlib.h>
@@ -151,7 +164,7 @@ char* msg3 = "hello world #3";
 int main()
 {
     char inbuf[MSGSIZE];    // a buffer that will hold the incoming data that is being written
-    int p[2];               // a two-element array to hold the read and write file descriptors that are used by the pipe   
+    int p[2];               // a two-element array to hold the read and write file descriptors that are used by the pipe
 
     // establish our pipe, passing it the p array so that it gets populated by the read and write file descriptors
     if (pipe(p) < 0) {
@@ -165,28 +178,33 @@ int main()
     write(p[1], msg3, MSGSIZE);
 
     for (int i = 0; i < 3; i++) {
-        // read 16 bytes of data from the read file descriptor 
+        // read 16 bytes of data from the read file descriptor
         read(p[0], inbuf, MSGSIZE);
         printf("% s\n", inbuf);
     }
 
     return 0;
 }
 ```
-This program isn't actually sending data between two different processes. The process that is doing the writing is also doing the reading. This process is essentially just talking to itself. But at least it gives you an idea of how to create pipes and send data between both ends. 
+
+This program isn't actually sending data between two different processes. The process that is doing the writing is also doing the reading. This process is essentially just talking to itself. But at least it gives you an idea of how to create pipes and send data between both ends.
 
 ## What You'll be Doing for this Lab
+
 This was your first introduction to making system calls to an operating system kernel. Obviously, there are many more system calls that we'll talk about in a future lesson, but for now we'll just practice with these. Once you've cloned this repo, go into each directory, open up the exercise file, read the prompt, and write some C code! Compile your code with `gcc [NAME OF FILE].c -o [NAME OF EXECUTABLE]`, then run the executable with `./[NAME OF EXECUTABLE]`.
 
 ## Further Reading
+
 If you would like to read more on this topic, these chapters from _Operating Systems: Three Easy Pieces_ heavily informed this particular topic and material: [http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf](http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf)
 [http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-mechanisms.pdf](http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-mechanisms.pdf)
 
 Additionally, it's always a good idea to read the man pages for any system calls you're trying to use:
- - The man page for `fork`: [https://linux.die.net/man/2/fork](https://linux.die.net/man/2/fork)
- - The man page for `wait` and `waitpid`: [https://linux.die.net/man/3/waitpid](https://linux.die.net/man/3/waitpid)
- - The man page for the `exec` family: [https://linux.die.net/man/3/exec](https://linux.die.net/man/3/exec)
- - The man page for the `pipe` system call: [https://linux.die.net/man/2/pipe](https://linux.die.net/man/2/pipe)
+
+- The man page for `fork`: [https://linux.die.net/man/2/fork](https://linux.die.net/man/2/fork)
+- The man page for `wait` and `waitpid`: [https://linux.die.net/man/3/waitpid](https://linux.die.net/man/3/waitpid)
+- The man page for the `exec` family: [https://linux.die.net/man/3/exec](https://linux.die.net/man/3/exec)
+- The man page for the `pipe` system call: [https://linux.die.net/man/2/pipe](https://linux.die.net/man/2/pipe)
 
 ## Stretch Goal
-Open up the `/stretch` directory. In there, you'll find an involved exercise pertaining to file locking and concurrency. Read the README included in that directory for instructions on what to do. 
+
+Open up the `/stretch` directory. In there, you'll find an involved exercise pertaining to file locking and concurrency. Read the README included in that directory for instructions on what to do.

ex1/ex1.c

@@ -2,13 +2,42 @@
 // (e.g., x) and set its value to something (e.g., 100). What value is the variable in the child process?
 // What happens to the variable when both the child and parent change the value of x?
 
+// 1. What value is the variable in the child process?
+// The value is also the same. It looks like the x variable is in a global scope.
+
+// 2.What happens to the variable when both the child and parent change the value of x?
+// The x variable changes due to the scope of being inside the conditional statements.
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 
 int main(void)
 {
     // Your code here
+    int x = 100;
+    printf("x value before calling fork: %d \n", x);
+
+    int rc = fork();
+
+    // child process starts execution here
+    if (rc < 0)
+    { // fork failed; exit
+        fprintf(stderr, "fork failed\n");
+        exit(1);
+    }
+    else if (rc == 0)
+    { // child process satisfies this branch
+        x = 200;
+        printf("hello, child here (pid: %d) \n", (int)getpid());
+        printf("x value in child when changed: %d \n", x);
+    }
+    else
+    {
+        x = 300;
+        printf("hello, parent here (pid: %d) of child %d \n", (int)getpid(), rc);
+        printf("x value in parent when changed: %d \n", x);
+    }
 
     return 0;
 }