File Attributes and Operations

A file is the basic unit of data storage, and it carries a rich set of attributes and supports a rich set of operations. Understanding what a file knows about itself, and what you can ask it to do, is essential to understanding how a file system works underneath.

File Attributes

File attributes are the metadata that describe a file’s characteristics and state. This information is stored in the directory entry or the inode of the file system.

Basic Attributes

1. File Name

• Role: the identifier a user uses to recognize and reach the file
• Characteristics: usually chosen by the user, and may contain a wide range of characters
• Restrictions: different file systems impose different limits on name length and permitted characters
• Examples: document.txt, image.jpg, program.exe

2. File Type

• Regular file: stores user data or program code
• Directory file: holds references to other files
• Device file: stands for a hardware device
• Symbolic link: a shortcut that points at another file
• Pipe file: used for inter-process communication

3. File Size

• Definition: the number of bytes of data the file contains
• Units: bytes (B), kilobytes (KB), megabytes (MB), and so on
• Dynamic: the size of a file changes as data is written and deleted

4. Location Information

• Physical location: the physical address of the file on the storage device
• Logical location: the path of the file within the directory structure
• Storage layout: whether the file is stored contiguously or scattered

Time Attributes

1. Creation Time

• Definition: the timestamp of the moment the file was first created
• Use: file management and auditing
• Characteristic: usually cannot be modified

2. Modification Time

• Definition: the time the contents of the file were last modified
• Trigger: updated whenever the contents of the file change
• Use: deciding whether a file needs to be backed up or recompiled

3. Access Time

• Definition: the time the file was last accessed
• Trigger: updated when the file is read, written, or executed
• Use: gathering statistics on how files are used

4. Attribute Change Time

• Definition: the time the attributes of the file, such as permissions or owner, were last changed
• Trigger: updated whenever the metadata of the file changes

Permission Attributes

1. Owner

• Definition: the creator of the file or its current owner
• Rights: holds complete control over the file
• Change: only the superuser or the current owner can change it

2. Group

• Definition: the user group the file belongs to
• Role: grants shared permissions to the members of the group
• Change: the owner or the superuser can change it

3. Permissions

• Read permission (r): allows the contents of the file to be read
• Write permission (w): allows the contents of the file to be modified
• Execute permission (x): allows the file to be executed, for program files

4. Special Permissions

• SUID: the file runs with the identity of its owner when executed
• SGID: the file runs with the identity of its group when executed
• Sticky Bit: only the owner of the file can delete it

Other Attributes

1. File Identifiers

• inode number: the unique identifier of the file within the file system
• device number: the identifier of the storage device the file lives on
• link count: the number of hard links that point at the file

2. Storage Attributes

• block size: the basic unit in which a file system allocates storage space
• blocks occupied: the number of storage blocks the file actually uses
• fragmentation: whether the file is stored contiguously

File Operations

File operations are the basic functions a file system offers to users; together they give complete control over a file.

Basic File Operations

1. Create

// C example
FILE *fp = fopen("newfile.txt", "w");
if (fp != NULL) {
    printf("File created successfully\n");
    fclose(fp);
}

What it does:

• Creates a new file at the given location
• Allocates storage space
• Initializes the file attributes
• Updates the directory structure

2. Delete

// C example
if (remove("file.txt") == 0) {
    printf("File deleted successfully\n");
} else {
    printf("Failed to delete the file\n");
}

What it does:

• Releases the storage space occupied by the file
• Removes the file’s entry from the directory
• Updates the accounting information of the file system

3. Open

// C example
FILE *fp = fopen("file.txt", "r");
if (fp != NULL) {
    // The file was opened successfully; read and write operations are now allowed
    fclose(fp);
}

What it does:

• Verifies that the file exists and that access is permitted
• Creates a file descriptor
• Initializes the file pointer
• Loads the metadata of the file

4. Close

// C example
fclose(fp);  // close the file

What it does:

• Flushes buffered data
• Releases the file descriptor
• Updates the file attributes, such as the access time
• Frees system resources

Data Read and Write Operations

1. Read

// C example
char buffer[1024];
size_t bytes_read = fread(buffer, 1, sizeof(buffer), fp);
if (bytes_read > 0) {
    printf("Read %zu bytes\n", bytes_read);
}

What it does:

• Reads a given amount of data from the current position in the file
• Advances the file pointer
• Updates the access time of the file
• Handles read errors

2. Write

// C example
char data[] = "Hello, World!";
size_t bytes_written = fwrite(data, 1, strlen(data), fp);
if (bytes_written > 0) {
    printf("Wrote %zu bytes\n", bytes_written);
}

What it does:

• Writes data at the current position in the file
• Allocates the storage space that is needed
• Updates the file size and the modification time
• Handles write errors

3. Seek

// C example
fseek(fp, 100, SEEK_SET);  // seek to byte 100
fseek(fp, -50, SEEK_CUR);  // move 50 bytes back from the current position
fseek(fp, 0, SEEK_END);    // seek to the end of the file

What it does:

• Moves the file pointer
• Supports both relative and absolute positioning
• Prepares the file for subsequent read and write operations

Advanced File Operations

1. Rename

// C example
if (rename("oldname.txt", "newname.txt") == 0) {
    printf("File renamed successfully\n");
}

What it does:

• Changes the name of the file
• Leaves the contents and attributes of the file untouched
• Updates the directory structure

2. Copy

// C example
FILE *source = fopen("source.txt", "r");
FILE *dest = fopen("dest.txt", "w");
char buffer[1024];
size_t bytes;

while ((bytes = fread(buffer, 1, sizeof(buffer), source)) > 0) {
    fwrite(buffer, 1, bytes, dest);
}

fclose(source);
fclose(dest);

What it does:

• Creates a complete copy of the file
• Leaves the original file unchanged
• Allocates storage space for the new file

3. Move

// C example
// Move within the same file system
if (rename("file.txt", "newdir/file.txt") == 0) {
    printf("File moved successfully\n");
}

What it does:

• Changes the place of the file within the directory structure
• Within a single file system, usually only the directory entry has to be updated
• Across file systems, the file must be copied and the original deleted

File Attribute Operations

1. Changing Permissions

// C example
#include <sys/stat.h>
chmod("file.txt", S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);

What it does:

• Changes the access permissions of the file
• Only the owner of the file or the superuser may do this
• Takes effect immediately

2. Changing the Owner

// C example
#include <unistd.h>
chown("file.txt", new_uid, new_gid);

What it does:

• Changes the owner and the group of the file
• Only the superuser may do this
• Affects the access permissions of the file

3. Changing Timestamps

// C example
#include <utime.h>
struct utimbuf times;
times.actime = time(NULL);   // access time
times.modtime = time(NULL);  // modification time
utime("file.txt", &times);

What it does:

• Changes the access and modification times of the file
• Is used for file synchronization and backup

Error Handling for File Operations

Common Error Types

1. Permission Errors

• EACCES: insufficient permissions
• EPERM: the operation was not permitted

2. File Does Not Exist

• ENOENT: the file or directory does not exist
• ENOTDIR: a component of the path is not a directory

3. Out of Storage Space

• ENOSPC: the device has no space left
• EDQUOT: the user’s quota has been reached

4. File Already Exists

• EEXIST: the file already exists
• EISDIR: a file operation was attempted on a directory

An Example of Error Handling

// C example
FILE *fp = fopen("file.txt", "r");
if (fp == NULL) {
    perror("Failed to open the file");
    switch (errno) {
        case EACCES:
            printf("Insufficient permissions\n");
            break;
        case ENOENT:
            printf("The file does not exist\n");
            break;
        default:
            printf("Other error\n");
    }
    return -1;
}

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Glossary