Directory Operations in Operating Systems

Directories are special structures used to organize files. Unlike regular files, you cannot directly write to directories because they store file system metadata.

👉 Instead, directories are modified indirectly through system calls.

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1. Creating Directories

 System Call: mkdir()

• Used to create a new directory

• Syntax:

  mkdir("dirname", permissions);
  

 Example

mkdir foo

Internally:

mkdir("foo", 0777);

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What Happens When a Directory is Created?

Even an “empty” directory contains:

Entry	Meaning
.	Refers to itself
..	Refers to parent directory

👉 These ensure:

• Navigation in file system

• Hierarchical structure

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2. Reading Directories

Directories cannot be read using normal read() calls. Instead, special APIs are used.

 System Calls:

• opendir()

• readdir()

• closedir()

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Basic Workflow

DIR *dp = opendir(".");
struct dirent *d;

while ((d = readdir(dp)) != NULL) {
    printf("%d %s\n", d->d_ino, d->d_name);
}

closedir(dp);

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Directory Entry Structure

struct dirent {
    char d_name[256];   // filename
    ino_t d_ino;        // inode number
    off_t d_off;        // offset
    unsigned short d_reclen;
    unsigned char d_type;
};

Key Idea

👉 Directory only stores:

• Filename

• Inode number

For more details (size, permissions), use:

• stat() system call

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 Real-world Example

Command:

ls

• Uses readdir() internally

• ls -l additionally uses stat() for metadata

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3. Deleting Directories

 System Call: rmdir()

• Used to remove a directory

• Condition: Directory must be empty

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 Example

rmdir foo

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 Important Constraint

👉 Directory must contain only:

• .

• ..

Otherwise:

• ❌ rmdir() fails

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 Why Restriction Exists?

• Prevents accidental deletion of large data

• Safer than file deletion

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Important Warning (Real System Behavior)

Command:

rm -rf *

👉 Recursively deletes:

• All files

• All directories

⚠️ If run at root (/):

• Entire system data can be erased

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 Summary Table

Operation	System Call	Key Idea
Create directory	mkdir()	Creates with . and ..
Read directory	opendir(), readdir()	Iterates entries
Delete directory	rmdir()	Only if empty

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Key Concept for Students

👉 Directories are not normal files:

• Cannot be directly modified

• Managed only via OS-controlled operations

👉 They act as:

filename → inode mapping