Advanced File I/O

Overview

This week extends the foundation of Linux file I/O by exploring advanced file descriptor manipulation and low-level file positioning.
Students will learn how to duplicate file descriptors, control file behavior using fcntl, and manage files beyond simple read/write operations — including seeking, truncation, and sparse file handling.

By the end of this week, students will have a complete understanding of how Linux manages file descriptors and how processes interact with files efficiently and flexibly.

Key Concepts

File Descriptor Duplication

• dup() and dup2() system calls
• File descriptor table and shared open file descriptions
• Standard output redirection example with dup2()
• Relationship between dup2() and shell redirection

File Positioning

• The lseek() system call: repositioning file offsets
• Seeking modes:
  • SEEK_SET – from beginning
  • SEEK_CUR – from current position
  • SEEK_END – from end of file
• Practical uses: appending, measuring file size, random access

Sparse Files and Holes

• Concept of holes (unallocated regions in files)
• Creating and inspecting sparse files
• Comparing allocated and logical sizes
• SEEK_DATA and SEEK_HOLE (modern extensions for efficient traversal)

File Control and Management

• The fcntl() system call:
  • Duplicating descriptors (F_DUPFD)
  • Managing flags (FD_CLOEXEC, O_APPEND, O_NONBLOCK)
• Truncating files with ftruncate()
• Retrieving file information with fstat()

Practice / Lab

Descriptor Duplication

• Redirect program output to files using file descriptor duplication.
• Explore descriptor inheritance and redirection behavior.

File Positioning and Sparse Files

• Create files with gaps using lseek() to demonstrate sparse allocation.
• Compare logical and physical file sizes using shell utilities.

File Control and Metadata

• Use fcntl() to retrieve and modify descriptor properties.
• Truncate and inspect file length using system utilities.

Homework

• FS-4: Redirect standard input
• FS-5: Manaul append
• FS-10: Copy a file accounting holes

Samples

• Redirect standard output and error
• Create a file with holes
• Detect holes in the file

References & Resources

Required

• Kerrisk, The Linux Programming Interface
  • Chapter 5: File I/O – Deeper Concepts
  • Chapter 6: Advanced File I/O

Recommended

• Understanding Sparse Files (Red Hat)
• Beej’s Guide – File Descriptors
• Linux manual page - fcntl(2)
• Linux manual page - dup(2)
• Linux manual page - lseek(3p)
• lseek() in C/C++ to read the alternate nth byte and write it in another file

Quiz (Self-check)

1. What happens to the file offset when a descriptor is duplicated?
2. How does dup2() differ from shell redirection?
3. What is the difference between SEEK_END and SEEK_CUR?
4. How does a sparse file conserve storage space?
5. What are the purposes of SEEK_DATA and SEEK_HOLE?
6. What does FD_CLOEXEC accomplish in practice?

Suggested Tools

• strace – observe system calls like dup2 and lseek
• ls, du, stat – inspect logical and physical file properties
• hexdump – verify data placement within sparse files