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KERNEL_MOUNT(9) Kernel Developer's Manual KERNEL_MOUNT(9)


free_mntarg, kernel_mount, kernel_vmount, mount_arg, mount_argb, mount_argf, mount_argsufunctions provided as part of the kernel mount interface


free_mntarg(struct mntarg *ma);

kernel_mount(struct mntarg *ma, int flags);

kernel_vmount(int flags, ...);

struct mntarg *
mount_arg(struct mntarg *ma, const char *name, const void *val, int len);

struct mntarg *
mount_argb(struct mntarg *ma, int flag, const char *name);

struct mntarg *
mount_argf(struct mntarg *ma, const char *name, const char *fmt, ...);

struct mntarg *
mount_argsu(struct mntarg *ma, const char *name, const void *val, int len);


The () family of functions are provided as an API for building a list of mount arguments which will be used to mount file systems from inside the kernel. By accumulating a list of arguments, the API takes shape and provides the information necessary for the kernel to control the mount(8) utility. When an error occurs, the process will stop. This will not cause a panic(9).

The header of the structure is stored in src/sys/kern/vfs_mount.c which permits automatic structure creation to ease the mount process. Memory allocation must always be freed when the entire process is complete, it is an error otherwise.

The () function is used to free or clear the mntarg structure.

The () function pulls information from the structure to perform the mount request on a given file system. Additionally, the kernel_mount() function always calls the free_mntarg() function. If ma contains any error code generated during the construction, that code will be called and the file system mount will not be attempted.

The () is a function similar to printf(9) which is used to mount a file system.

The () function takes a plain argument and crafts parts of the structure with regards to various mount options. If the length is a value less than 0, strlen(3) is used. This argument will be referenced until either free_mntarg() or kernel_mount() is called.

The () function is used to add boolean arguments to the structure. The flag is the boolean value and name must start with "no", otherwise a panic will occur.

The () function adds printf(9) style arguments to the current structure.

The () function will add arguments to the structure from a userland string.


An example of the *_cmount() function:

static int
msdosfs_cmount(struct mntarg *ma, void *data, int flags, struct thread *td)
	struct msdosfs_args args;
	int error;

	if (data == NULL)
		return (EINVAL);
	error = copyin(data, &args, sizeof(args));
	if (error)
		return (error);

	ma = mount_argsu(ma, "from", args.fspec, MAXPATHLEN);
	ma = mount_arg(ma, "export", &args.export, sizeof(args.export));
	ma = mount_argf(ma, "uid", "%d", args.uid);
	ma = mount_argf(ma, "gid", "%d", args.gid);
	ma = mount_argf(ma, "mask", "%d", args.mask);
	ma = mount_argf(ma, "dirmask", "%d", args.dirmask);

	ma = mount_argb(ma, args.flags & MSDOSFSMNT_SHORTNAME, "noshortname");
	ma = mount_argb(ma, args.flags & MSDOSFSMNT_LONGNAME, "nolongname");
	ma = mount_argb(ma, !(args.flags & MSDOSFSMNT_NOWIN95), "nowin95");
	ma = mount_argb(ma, args.flags & MSDOSFSMNT_KICONV, "nokiconv");

	ma = mount_argsu(ma, "cs_win", args.cs_win, MAXCSLEN);
	ma = mount_argsu(ma, "cs_dos", args.cs_dos, MAXCSLEN);
	ma = mount_argsu(ma, "cs_local", args.cs_local, MAXCSLEN);

	error = kernel_mount(ma, flags);

	return (error);

When working with kernel_vmount(), varargs must come in pairs, e.g., {name, value}.

	error = kernel_vmount(
	    "fstype", vfsname,
	    "fspath", "/",
	    "from", path,




The kernel_mount() family of functions and this manual page first appeared in FreeBSD 6.0.


The kernel_mount() family of functions and API was developed by Poul-Henning Kamp <>. This manual page was written by Tom Rhodes <>.

December 13, 2004 Debian