ex1 - Readiness

1. Explain why the output of the following code snippet is "char=1, int=4, long=8" in x86 (64-bit).

   printf("char=%d, int=%d, long=%d", \
   sizeof(char), sizeof(int), sizeof(long));
   

2. Explain why the output of the following code snippet is "st0 = 8, st1 = 8".

   struct st0 {
     int x;
     char y;
   };
   struct st1 {
     int x;
     char y;
     char z;
   };
   int main()
   {
     printf("st0 = %d, st1 = %d\n",
     sizeof(struct st0), sizeof(struct st1));
   }
   

3. Explain why the output of the following code snippet is "i=5, j=10".

   int main()
   {
     int i, j, *p, *q;
     p = &i;
     q = &j;
     *p = 5;
     *q = *p + i;
     printf("i = %d, j = %d\n", i, j);
     return 0;
   }
   

4. Explain why the output of the following code is 0x124000.

   #define PGSIZE 4096
   #define CONVERT(sz) (((sz)+PGSIZE-1) & ~(PGSIZE-1))
   
   printf("0x%x", CONVERT(0x123456));
   

5. Assuming the first printf results "1 = 0x7fffdfbf7f00", explain why the rest of the output is as follows:

   2 = 0x7fffdfbf7f04
   3 = 0x7fffdfbf7f00
   4 = 0x7fffdfbf7f14
   main() {
   int x[5];
     printf("1 = %p\n", x);
     printf("2 = %p\n", x+1);
     printf("3 = %p\n", &x);
     printf("4 = %p\n", &x+1);
     return 0;
   }
   

6. In GDB, what does the following sequence of commands do?

   (gdb) break main
   (gdb) run
   (gdb) next
   (gdb) print argc
   

7. Briefly explain what the following Makefile snippet does:

   KERNOBJS = bio.o console.o exec.o file.o fs.o ide.o ioapic.o
   ...
   kernel: $(KERNOBJS) entry.o kernel.ld
       $(LD) $(LDFLAGS) -T kernel.ld -o kernel entry.o $(KERNOBJS)
   

8. Consider the following C program:

   #include <stdio.h>
   #include <unistd.h>
   #include <sys/types.h>
   #include <sys/wait.h>
   
   int main(int argc, char *argv[]) {
     printf("parent: pid = %d\n", getpid());
   
     pid_t pid = fork();
   
     if (pid == 0) {
       // child process
       printf("child: pid = %d, parent pid = %d\n", getpid(), getppid());
       char *args[] = {"/bin/echo", "hello from exec", NULL};
       execv(args[0], args);
       printf("this line should not be printed\n");
     } else {
       // parent process
       wait(NULL);
       printf("parent: child has exited\n");
     }
   
     return 0;
   }
   

• 8.1 Explain why "this line should not be printed" is not printed?

• 8.2 What happens to the child process after execv is called?

• 8.3 What is the role of wait(NULL) in the parent process?

9. In a tmux session, open two terminal panes side-by-side.

Use one pane to run xv6 with make qemu-nox, and use the other pane to locate the source code of the xv6 kernel's main() function using cscope.

Take a screenshot showing both the running xv6 kernel and cscope side by side, and upload it.