text: #include "types.h"
  #include "param.h"
  #include "memlayout.h"
  //#include "mmu.h"
  #include "proc.h"
  #include "defs.h"
  #include "x86.h"
  #include "elf.h"
  
  int
  exec(char *path, char **argv)
  {
    char *s, *last;
    int i, off;
    uint argc, sz, sp, ustack[3+MAXARG+1];
    struct elfhdr elf;
    struct inode *ip;
    struct proghdr ph;
    pde_t *pgdir, *oldpgdir;
  
    begin_op();
  
    if((ip = namei(path)) == 0){
      end_op();
      return -1;
    }
    ilock(ip);
    pgdir = 0;
  
    // Check ELF header
    if(readi(ip, (char*)&elf, 0, sizeof(elf)) != sizeof(elf))
      goto bad;
    if(elf.magic != ELF_MAGIC)
      goto bad;
  
    if((pgdir = setupkvm()) == 0)
      goto bad;
  
    // Load program into memory.
    sz = 0;
    for(i=0, off=elf.phoff; i<elf.phnum; i++, off+=sizeof(ph)){
      if(readi(ip, (char*)&ph, off, sizeof(ph)) != sizeof(ph))
        goto bad;
      if(ph.type != ELF_PROG_LOAD)
        continue;
      if(ph.memsz < ph.filesz)
        goto bad;
      if(ph.vaddr + ph.memsz < ph.vaddr)
        goto bad;
      if((sz = allocuvm(pgdir, sz, ph.vaddr + ph.memsz)) == 0)
        goto bad;
      if(ph.vaddr % PGSIZE != 0)
        goto bad;
      if(loaduvm(pgdir, (char*)ph.vaddr, ip, ph.off, ph.filesz) < 0)
        goto bad;
    }
    iunlockput(ip);
    end_op();
    ip = 0;
  
    // Allocate two pages at the next page boundary.
    // Make the first inaccessible.  Use the second as the user stack.
    sz = PGROUNDUP(sz);
    if((sz = allocuvm(pgdir, sz, sz + 2*PGSIZE)) == 0)
      goto bad;
    clearpteu(pgdir, (char*)(sz - 2*PGSIZE));
    sp = sz;
  
    // Push argument strings, prepare rest of stack in ustack.
    for(argc = 0; argv[argc]; argc++) {
      if(argc >= MAXARG)
        goto bad;
      sp = (sp - (strlen(argv[argc]) + 1)) & ~3;
      if(copyout(pgdir, sp, argv[argc], strlen(argv[argc]) + 1) < 0)
        goto bad;
      ustack[3+argc] = sp;
    }
    ustack[3+argc] = 0;
  
    ustack[0] = 0xffffffff;  // fake return PC
    ustack[1] = argc;
    ustack[2] = sp - (argc+1)*4;  // argv pointer
  
    sp -= (3+argc+1) * 4;
    if(copyout(pgdir, sp, ustack, (3+argc+1)*4) < 0)
      goto bad;
  
    // Save program name for debugging.
    for(last=s=path; *s; s++)
      if(*s == '/')
        last = s+1;
    safestrcpy(proc->name, last, sizeof(proc->name));
  
    // Commit to the user image.
    oldpgdir = proc->pgdir;
    proc->pgdir = pgdir;
    proc->sz = sz;
    proc->tf->eip = elf.entry;  // main
    proc->tf->esp = sp;
    switchuvm(proc);
    freevm(oldpgdir);
    return 0;
  
   bad:
    if(pgdir)
      freevm(pgdir);
    if(ip){
      iunlockput(ip);
      end_op();
    }
    return -1;
  }
  