`umount2` - glibc

Split from the chapter glibc: System Management into the per-function layout; see the method page and the glibc ledger for coverage status.

Field	Value
Header	`sys/mount.h` (`mount` lines 281-282, `umount` line 286, `umount2` line 289 – all unconditional in this header; MS_* flag enum from line 44, MNT_* lines 179-184)
Standards	Linux-specific: “should not be used in programs intended to be portable” (mount.2, umount.2 CONFORMING TO)
Exported	`mount@@GLIBC_2.2.5` (W), `umount@@GLIBC_2.2.5` (W), `umount2@@GLIBC_2.2.5` (W) (nm, host glibc 2.41)
Thread-safety	no ATTRIBUTES tables in mount.2/umount.2; thin syscall wrappers
Ledger ID	GLIBC-mount, GLIBC-umount, GLIBC-umount2

Declaration and gates

#include <sys/mount.h>

int mount(const char *source, const char *target,
          const char *filesystemtype, unsigned long mountflags,
          const void *data);                       /* sys/mount.h:281 */
int umount(const char *target);                    /* sys/mount.h:286 */
int umount2(const char *target, int flags);        /* sys/mount.h:289 */

#define MS_RDONLY   1    /* sys/mount.h enum, line 45 onward */
#define MS_BIND     4096
#define MS_MOVE     8192
#define MS_REC      16384
#define MNT_FORCE   1    /* umount2, sys/mount.h:179 */
#define MNT_DETACH  2    /* sys/mount.h:181 */
#define MNT_EXPIRE  4    /* sys/mount.h:183 */

No feature-test gate: sys/mount.h is a Linux-only header declaring all three unconditionally (next to the new mount API: fsopen, fsmount, move_mount, fsconfig, fspick, open_tree, mount_setattr, lines 293-325).

Semantics

mount attaches source at target for filesystemtype (types listed in /proc/filesystems), with per-filesystem data options and the MS_* bits selecting the operation class (plain mount, MS_BIND, MS_REMOUNT, MS_MOVE, MS_SHARED/SLAVE/PRIVATE/UNBINDABLE, MS_REC to recurse). umount detaches; umount2 refines it: MNT_FORCE aborts pending requests (NFS-class availability limits per filesystem), MNT_DETACH is the lazy unmount that waits for last use, MNT_EXPIRE marks the mount for expiry – first call fails with EAGAIN unless unused, and it conflicts with MNT_DETACH (mount.2, umount.2). All need CAP_SYS_ADMIN. Raw pages: mount, umount2.

Gotchas and portability

MS_MGC_VAL (0xC0ED in the top 16 bits) was mandatory before kernel 2.4 and is now ignored (mount.2 NOTES).
The mount API moved on: file-descriptor-based fsopen/fsmount/ move_mount/mount_setattr live in the same header and have their own syscall pages under fs-mounts-namespace.
No portable equivalent exists on non-Linux; guard with __linux__.
musl 1.2.5: all three present (include/sys/mount.h lines 67-69, src/linux/mount.c).

Errors

EPERM (missing CAP_SYS_ADMIN), EBUSY (target busy / source already mounted), EINVAL (bad flags, MNT_EXPIRE with MNT_DETACH, locked mount), ELOOP, ENAMETOOLONG, ENOTDIR/EISDIR, ENXIO, plus per-filesystem data errors – full lists in mount.2/umount.2.

References

man2: mount.2, umount.2 (covers umount2; umount2.2 is a .so link)
headers: /usr/include/x86_64-linux-gnu/sys/mount.h (lines 21-325)
symbols: nm -D --with-symbol-versions libc.so.6

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