Documentation/userspace-api/ioctl/ioctl-decoding.rst
Source file repositories/reference/linux-study-clean/Documentation/userspace-api/ioctl/ioctl-decoding.rst
File Facts
- System
- Linux kernel
- Corpus path
Documentation/userspace-api/ioctl/ioctl-decoding.rst- Extension
.rst- Size
- 830 bytes
- Lines
- 32
- Domain
- Support Tooling And Documentation
- Bucket
- Documentation
- Inferred role
- Support Tooling And Documentation: documentation
- Status
- atlas-only
Why This File Exists
Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
- Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
- Defines or uses C structs; map object ownership, embedded links, reference counts, and lock ownership.
Dependency Surface
- No C-style include directives detected by the generator.
Detected Declarations
- No top-level syscall, struct, function, initcall, or export declaration detected by the generator.
Annotated Snippet
==============================
Decoding an IOCTL Magic Number
==============================
To decode a hex IOCTL code:
Most architectures use this generic format, but check
include/ARCH/ioctl.h for specifics, e.g. powerpc
uses 3 bits to encode read/write and 13 bits for size.
====== ==================================
bits meaning
====== ==================================
31-30 00 - no parameters: uses _IO macro
10 - read: _IOR
01 - write: _IOW
11 - read/write: _IOWR
29-16 size of arguments
15-8 ascii character supposedly
unique to each driver
7-0 function #
====== ==================================
So for example 0x82187201 is a read with arg length of 0x218,
character 'r' function 1. Grepping the source reveals this is::
#define VFAT_IOCTL_READDIR_BOTH _IOR('r', 1, struct dirent [2])Annotation
- Atlas domain: Support Tooling And Documentation / Documentation.
- Implementation status: atlas-only.
Implementation Notes
- This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
- Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
- Driver-family pages are intentionally pattern-oriented unless they are part of the selected PCIe/NVMe representative device path.