Linux EBADF (errno 9) — Bad File Descriptor

Linux EBADF error occurs when a program tries to perform I/O on a file descriptor that is not open or not valid for the attempted operation. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: EBADF (errno 9) means the file descriptor number is invalid or not open. Common when reading from a closed file descriptor or writing to a read-only descriptor. Can also occur when using select()/poll()/epoll() with invalid or closed descriptors. In shell scripts: EBADF from redirection to a closed file descriptor. Thread safety: one thread closing an FD while another is using it. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Using a file descriptor after it has been closed. Writing to a file descriptor opened as read-only (or vice versa). Race condition: one thread closes FD while another reads/writes. File descriptor reuse after close — new FD may have different semantics. Shell script using a file descriptor number that was never opened. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Use strace to trace file operations: strace -e openat,close,read,write ./program. Check for double-close bugs: closing the same FD twice (second close returns EBADF). Verify open mode matches usage: O_RDONLY cannot write, O_WRONLY cannot read. For multi-threaded code: ensure FD lifecycle is protected by locks or atomic operations. Use Valgrind with --track-fds=yes to detect FD leaks and invalid FD usage. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

This article is part of our Linux Error Codes collection on Error Codes Wiki. We provide comprehensive, up-to-date information to help you find solutions quickly.