> For the complete documentation index, see [llms.txt](https://xiayingp.gitbook.io/build_a_os/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://xiayingp.gitbook.io/build_a_os/file-system/directory-layer.md).

# Directory Layer

## Directory Layer

```c
struct dirent {
  ushort inum;
  char name[DIRSIZ];
};
```

### Where does directory inode store?

It is just an inode. Allocated by `ialloc`, the same on-disk inode allocation method. Grab an empty for use. Its data are all `dirent`s.

### Lookup entry in directory

```c
// Look for a directory entry in a directory.
// If found, set *poff to byte offset of entry.
struct inode*
dirlookup(struct inode *dp, char *name, uint *poff)
{
  uint off, inum;
  struct dirent de;

  if(dp->type != T_DIR)
    panic(“dirlookup not DIR”);

  for(off = 0; off < dp->size; off += sizeof(de)){
    if(readi(dp, 0, (uint64)&de, off, sizeof(de)) != sizeof(de))
      panic(“dirlookup read”);
    if(de.inum == 0)
      continue;
    if(namecmp(name, de.name) == 0){
      // entry matches path element
      if(poff)
        *poff = off;
      inum = de.inum;
      return iget(dp->dev, inum);
    }
  }

  return 0;
}
```

### Create new directory entry

```c
// Write a new directory entry (name, inum) into the directory dp.
int
dirlink(struct inode *dp, char *name, uint inum)
{
  int off;
  struct dirent de;
  struct inode *ip;

  // Check that name is not present.
  if((ip = dirlookup(dp, name, 0)) != 0){
    iput(ip);
    return -1;
  }

  // Look for an empty dirent.
  for(off = 0; off < dp->size; off += sizeof(de)){
    if(readi(dp, 0, (uint64)&de, off, sizeof(de)) != sizeof(de))
      panic(“dirlink read”);
    if(de.inum == 0)
      break;
  }

  strncpy(de.name, name, DIRSIZ);
  de.inum = inum;
  if(writei(dp, 0, (uint64)&de, off, sizeof(de)) != sizeof(de))
    panic(“dirlink”);

  return 0;
}
```


---

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