user/dumbfork.c

   1 // Ping-pong a counter between two processes.
   2 // Only need to start one of these -- splits into two, crudely.
   3
   4 #include <inc/string.h>
   5 #include <inc/lib.h>
   6
   7 envid_t dumbfork(void);
   8
   9 void
  10 umain(void)
  11 {
  12         envid_t who;
  13         int i;
  14
  15         // fork a child process
  16         who = dumbfork();
  17
  18         // print a message and yield to the other a few times
  19         for (i = 0; i < (who ? 10 : 20); i++) {
  20                 cprintf("%d: I am the %s!\n", i, who ? "parent" : "child");
  21                 sys_yield();
  22         }
  23 }
  24
  25 void
  26 duppage(envid_t dstenv, void *addr)
  27 {
  28         int r;
  29
  30         // This is NOT what you should do in your fork.
  31         if ((r = sys_page_alloc(dstenv, addr, PTE_P|PTE_U|PTE_W)) < 0)
  32                 panic("sys_page_alloc: %e", r);
  33         if ((r = sys_page_map(dstenv, addr, 0, UTEMP, PTE_P|PTE_U|PTE_W)) < 0)
  34                 panic("sys_page_map: %e", r);
  35         memmove(UTEMP, addr, PGSIZE);
  36         if ((r = sys_page_unmap(0, UTEMP)) < 0)
  37                 panic("sys_page_unmap: %e", r);
  38 }
  39
  40 envid_t
  41 dumbfork(void)
  42 {
  43         envid_t envid;
  44         uint8_t *addr;
  45         int r;
  46         extern unsigned char end[];
  47
  48         // Allocate a new child environment.
  49         // The kernel will initialize it with a copy of our register state,
  50         // so that the child will appear to have called sys_exofork() too -
  51         // except that in the child, this "fake" call to sys_exofork()
  52         // will return 0 instead of the envid of the child.
  53         envid = sys_exofork();
  54         if (envid < 0)
  55                 panic("sys_exofork: %e", envid);
  56         if (envid == 0) {
  57                 // We're the child.
  58                 // The copied value of the global variable 'env'
  59                 // is no longer valid (it refers to the parent!).
  60                 // Fix it and return 0.
  61                 env = &envs[ENVX(sys_getenvid())];
  62                 return 0;
  63         }
  64
  65         // We're the parent.
  66         // Eagerly copy our entire address space into the child.
  67         // This is NOT what you should do in your fork implementation.
  68         for (addr = (uint8_t*) UTEXT; addr < end; addr += PGSIZE)
  69                 duppage(envid, addr);
  70
  71         // Also copy the stack we are currently running on.
  72         duppage(envid, ROUNDDOWN(&addr, PGSIZE));
  73
  74         // Start the child environment running
  75         if ((r = sys_env_set_status(envid, ENV_RUNNABLE)) < 0)
  76                 panic("sys_env_set_status: %e", r);
  77
  78         return envid;
  79 }
  80