IOCTL_IFLAGS

Section: Linux Programmer's Manual (2)
Updated: 2019-11-19
Page Index

NAME

ioctl_iflags - ioctl() operations for inode flags

DESCRIPTION

Various Linux filesystems support the notion of inode flags---attributes that modify the semantics of files and directories. These flags can be retrieved and modified using two ioctl(2) operations:

int attr; fd = open("pathname", ...);

ioctl(fd, FS_IOC_GETFLAGS, &attr); /* Place current flags
in 'attr' */ attr |= FS_NOATIME_FL; /* Tweak returned bit mask */ ioctl(fd, FS_IOC_SETFLAGS, &attr); /* Update flags for inode
referred to by 'fd' */

The lsattr(1) and chattr(1) shell commands provide interfaces to these two operations, allowing a user to view and modify the inode flags associated with a file.

The following flags are supported (shown along with the corresponding letter used to indicate the flag by lsattr(1) and chattr(1)):

FS_APPEND_FL 'a': The file can be opened only with the O_APPEND flag. (This restriction applies even to the superuser.) Only a privileged process (CAP_LINUX_IMMUTABLE) can set or clear this attribute.
FS_COMPR_FL 'c': Store the file in a compressed format on disk. This flag is not supported by most of the mainstream filesystem implementations; one exception is btrfs(5).
FS_DIRSYNC_FL 'D' (since Linux 2.6.0): Write directory changes synchronously to disk. This flag provides semantics equivalent to the mount(2) MS_DIRSYNC option, but on a per-directory basis. This flag can be applied only to directories.
FS_IMMUTABLE_FL 'i': The file is immutable: no changes are permitted to the file contents or metadata (permissions, timestamps, ownership, link count and so on). (This restriction applies even to the superuser.) Only a privileged process (CAP_LINUX_IMMUTABLE) can set or clear this attribute.
FS_JOURNAL_DATA_FL 'j': Enable journaling of file data on ext3(5) and ext4(5) filesystems. On a filesystem that is journaling in ordered or writeback mode, a privileged (CAP_SYS_RESOURCE) process can set this flag to enable journaling of data updates on a per-file basis.
FS_NOATIME_FL 'A': Don't update the file last access time when the file is accessed. This can provide I/O performance benefits for applications that do not care about the accuracy of this timestamp. This flag provides functionality similar to the mount(2) MS_NOATIME flag, but on a per-file basis.
FS_NOCOW_FL 'C' (since Linux 2.6.39): The file will not be subject to copy-on-write updates. This flag has an effect only on filesystems that support copy-on-write semantics, such as Btrfs. See chattr(1) and btrfs(5).
FS_NODUMP_FL 'd': Don't include this file in backups made using dump(8).
FS_NOTAIL_FL 't': This flag is supported only on Reiserfs. It disables the Reiserfs tail-packing feature, which tries to pack small files (and the final fragment of larger files) into the same disk block as the file metadata.
FS_PROJINHERIT_FL 'P' (since Linux 4.5): Inherit the quota project ID. Files and subdirectories will inherit the project ID of the directory. This flag can be applied only to directories.
FS_SECRM_FL 's': Mark the file for secure deletion. This feature is not implemented by any filesystem, since the task of securely erasing a file from a recording medium is surprisingly difficult.
FS_SYNC_FL 'S': Make file updates synchronous. For files, this makes all writes synchronous (as though all opens of the file were with the O_SYNC flag). For directories, this has the same effect as the FS_DIRSYNC_FL flag.
FS_TOPDIR_FL 'T': Mark a directory for special treatment under the Orlov block-allocation strategy. See chattr(1) for details. This flag can be applied only to directories and has an effect only for ext2, ext3, and ext4.
FS_UNRM_FL 'u': Allow the file to be undeleted if it is deleted. This feature is not implemented by any filesystem, since it is possible to implement file-recovery mechanisms outside the kernel.

In most cases, when any of the above flags is set on a directory, the flag is inherited by files and subdirectories created inside that directory. Exceptions include FS_TOPDIR_FL, which is not inheritable, and FS_DIRSYNC_FL, which is inherited only by subdirectories.

CONFORMING TO

Inode flags are a nonstandard Linux extension.

NOTES

In order to change the inode flags of a file using the FS_IOC_SETFLAGS operation, the effective user ID of the caller must match the owner of the file, or the caller must have the CAP_FOWNER capability.

The type of the argument given to the FS_IOC_GETFLAGS and FS_IOC_SETFLAGS operations is int *, notwithstanding the implication in the kernel source file include/uapi/linux/fs.h that the argument is long *.

COLOPHON

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