'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH statmount 2 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
statmount \- get a mount status
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/mount.h>" "  /* Definition of STATMOUNT_* constants */"
.B #include <unistd.h>
.P
.BI "int syscall(SYS_statmount, struct mnt_id_req *" req ,
.BI "            struct statmount *" smbuf ", size_t " bufsize ,
.BI "            unsigned long " flags );
.P
.B #include <linux/mount.h>
.fi
.P
.EX
.B struct mnt_id_req {
.BR "    __u32  size;" "    /* sizeof(struct mnt_id_req) */"
.BR "    __u64  mnt_id;" "  /* The mnt_id being queried */"
.BR "    __u64  param;" "   /* An ORed combination of the STATMOUNT_ constants */"
.B };
.P
.B struct statmount {
.B "    __u32  size;"
.B "    __u64  mask;"
.B "    __u32  sb_dev_major;"
.B "    __u32  sb_dev_minor;"
.B "    __u64  sb_magic;"
.B "    __u32  sb_flags;"
.B "    __u32  fs_type;"
.B "    __u64  mnt_id;"
.B "    __u64  mnt_parent_id;"
.B "    __u32  mnt_id_old;"
.B "    __u32  mnt_parent_id_old;"
.B "    __u64  mnt_attr;"
.B "    __u64  mnt_propagation;"
.B "    __u64  mnt_peer_group;"
.B "    __u64  mnt_master;"
.B "    __u64  propagate_from;"
.B "    __u32  mnt_root;"
.B "    __u32  mnt_point;"
.B "    char   str[];"
.B };
.EE
.P
.IR Note :
glibc provides no wrapper for
.BR statmount (),
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
To access a mount's status,
the caller must have CAP_SYS_ADMIN in the user namespace.
.P
This function returns information about a mount,
storing it in the buffer pointed to by
.IR smbuf .
The returned buffer is a
.I struct statmount
which is of size
.I bufsize
with the fields filled in as described below.
.P
(Note that reserved space and padding is omitted.)
.SS The mnt_id_req structure
.I req.size
is used by the kernel to determine which
.I struct\~mnt_id_req
is being passed in;
it should always be set to
.IR sizeof(struct\~mnt_id_req) .
.P
.I req.mnt_id
can be obtained from either
.BR statx (2)
using
.B STATX_MNT_ID_UNIQUE
or from
.BR listmount (2)
and is used as the identifier to query the status of the desired mount point.
.P
.I req.param
is used to tell the kernel which fields the caller is interested in.
It is an ORed combination of the following constants
.P
.in +4n
.TS
lB l.
STATMOUNT_SB_BASIC	/* Want/got sb_* */
STATMOUNT_MNT_BASIC	/* Want/got mnt_* */
STATMOUNT_PROPAGATE_FROM	/* Want/got propagate_from */
STATMOUNT_MNT_ROOT	/* Want/got mnt_root  */
STATMOUNT_MNT_POINT	/* Want/got mnt_point */
STATMOUNT_FS_TYPE	/* Want/got fs_type */
.TE
.in
.P
In general,
the kernel does
.I not
reject values in
.I req.param
other than the above.
(For an exception,
see
.B EINVAL
in errors.)
Instead,
it simply informs the caller which values are supported
by this kernel and filesystem via the
.I statmount.mask
field.
Therefore,
.I do not
simply set
.I req.param
to
.B UINT_MAX
(all bits set),
as one or more bits may,
in the future,
be used to specify an extension to the buffer.
.SS The returned information
The status information for the target mount is returned in the
.I statmount
structure pointed to by
.IR smbuf .
.P
The fields in the
.I statmount
structure are:
.TP
.I smbuf.size
The size of the returned
.I smbuf
structure,
including any of the strings fields that were filled.
.TP
.I smbuf.mask
The ORed combination of
.BI STATMOUNT_ *
flags indicating which fields were filled in and thus valid.
The kernel may return fields that weren't requested,
and may fail to return fields that were requested,
depending on what the backing file system and kernel supports.
In either case,
.I req.param
will not be equal to
.IR mask .
.TP
.I smbuf.sb_dev_major
.TQ
.I smbuf.sb_dev_minor
The device that is mounted at this mount point.
.TP
.I smbuf.sb_magic
The file system specific super block magic.
.TP
.I smbuf.sb_flags
The flags that are set on the super block,
an ORed combination of
.BR SB_RDONLY ,
.BR SB_SYNCHRONOUS ,
.BR SB_DIRSYNC ,
.BR SB_LAZYTIME .
.TP
.I smbuf.fs_type
The offset to the location in the
.I smbuf.str
buffer that contains the string representation of the mounted file system.
It is a null-terminated string.
.TP
.I smbuf.mnt_id
The unique mount ID of the mount.
.TP
.I smbuf.mnt_parent_id
The unique mount ID of the parent mount point of this mount.
If this is the root mount point then
.IR smbuf.mnt_id\~==\~smbuf.parent_mount_id .
.TP
.I smbuf.mnt_id_old
This corresponds to the mount ID that is exported by
.IR /proc/ pid /mountinfo .
.TP
.I smbuf.mnt_parent_id_old
This corresponds to the parent mount ID that is exported by
.IR /proc/ pid /mountinfo .
.TP
.I smbuf.mnt_attr
The
.BI MOUNT_ATTR_ *
flags set on this mount point.
.TP
.I smbuf.mnt_propagation
The mount propagation flags,
which can be one of
.BR MS_SHARED ,
.BR MS_SLAVE ,
.BR MS_PRIVATE ,
.BR MS_UNBINDABLE .
.TP
.I smbuf.mnt_peer_group
The ID of the shared peer group.
.TP
.I smbuf.mnt_master
The mount point receives its propagation from this mount ID.
.TP
.I smbuf.propagate_from
The ID from the namespace we propagated from.
.TP
.I smbuf.mnt_root
The offset to the location in the
.I smbuf.str
buffer that contains the string representation of the mount
relative to the root of the file system.
It is a null-terminated string.
.TP
.I smbuf.mnt_point
The offset to the location in the
.I smbuf.str
buffer that contains the string representation of the mount
relative to the current root (ie if you are in a
.BR chroot ).
It is a null-terminated string.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EPERM
Permission is denied for accessing this mount.
.TP
.B EFAULT
.I req
or
.I smbuf
points to a location outside the process's accessible
address space.
.TP
.B EINVAL
Invalid flag specified in
.IR flags .
.TP
.B EINVAL
.I req
is of insufficient size to be utilized.
.TP
.B E2BIG
.I req
is too large.
.TP
.B EOVERFLOW
The size of
.I smbuf
is too small to contain either the
.IR smbuf.fs_type ,
.IR smbuf.mnt_root ,
or
.IR smbuf.mnt_point .
Allocate a larger buffer and retry the call.
.TP
.B ENOENT
The specified
.I req.mnt_id
doesn't exist.
.TP
.B ENOMEM
Out of memory (i.e., kernel memory).
.SH STANDARDS
Linux.
.SH SEE ALSO
.BR listmount (2),
.BR statx (2)