#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main(int argc, char const *argv[]) {
    // create copy of process
    if (fork() == 0) {
        // if this is the child (fork returns 0 to child)
        printf("hello from child (%d)\n", getpid());
        // call `execv` to replace all of this process's state with a new program
	    char * args[] = {"/usr/bin/pwd"};
        execv(args[0], args);
        // exec doesn't usually return (unless there's an error)
    } else {
        // this branch represents the parent's continuing execution
        printf("hello from parent (%d)\n", getpid());
        // fork again to make a second child
    	if (fork() == 0) {
            printf("hello from second child.\n");
            exit(0);
            // exit never returns
        }
        int child_status;
        // wait on one of the children to complete
        wait(&child_status);
        printf("child has terminated with status: %d\n", child_status);
        
        // wait for the second child
        wait(&child_status);
        printf("other child has terminated with status: %d\n", child_status);
    }
    // only printed by parent because both children exec/exit before getting to this code
    printf("bye\n");
    return 0;
}

/*
# Demo
## Compile
> gcc -g -o testfork testfork.c
## Run within gdb
> gdb testfork
gdb> break main
gdb> run
# executes first fork, creating child which then execs and becomes `pwd`
gdb> next
## At this point, pstree shows the new pwd child:
bash(63240)---gdb(103877)---testfork(103887)-+-pwd(103893)

# executes second fork, creating second child
gdb> next
## see pstree at this point to see both children, un-reaped:
bash(63240)---gdb(103877)---testfork(103887)-+-pwd(103893)
                                              `-testfork(103904)
# execute the first wait
gdb> next
## pstree at this point should have reaped one of the children (could be either)
bash(63240)---gdb(103877)---testfork(103887)-+-testfork(103904)

#########################################
# In other window:
## List all executing processes
> ps aux

## Show all processes (with their pids) as a tree with `systemd` at the root
## In the output, look for the `bash` process that forked this `pstree` command...
> pstree -p
## Run pstree filtering to just the terminal which is running these 2 bash shells 
## and re-run every second so we can see a "live" view of the process tree
> watch -n 1 pstree -p 61040
## At the point where we've forked both children but haven't executed the wait yet,
## it should look something like this:
tmux(61040)-+-bash(63240)---gdb(103877)---testfork(103887)-+-pwd(103893)
            |                                              `-testfork(103904)
            `-bash(102610)---watch(103762)---watch(103929)---pstree(103930)
*/