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MOUNT(2) System Calls Manual MOUNT(2)

mount, unmount — mount or dismount a filesystem

#include <sys/types.h>
#include <sys/mount.h>

int
mount(const char *type, const char *dir, int flags, void *data);

int
unmount(const char *dir, int flags);

The () function grafts a filesystem object onto the system file tree at the point dir. The argument data describes the filesystem object to be mounted. The argument type tells the kernel how to interpret data (see type below). The contents of the filesystem become available through the new mount point dir. Any files in dir at the time of a successful mount are swept under the carpet, so to speak, and are unavailable until the filesystem is unmounted.

The following flags may be specified to suppress default semantics which affect filesystem access.

The filesystem should be treated as read-only: even the superuser may not write to it.
Do not update the access time on files in the filesystem unless the modification or status change times are also being updated.
Do not allow files to be executed from the filesystem.
Do not honor setuid or setgid bits on files when executing them.
Do not interpret special files on the filesystem.
All I/O to the filesystem should be done synchronously.
All I/O to the filesystem should be done asynchronously.
Use soft dependencies on an FFS filesystem. This flag is provided for compatibility only and has no effect on OpenBSD.
Processes that ask for memory to be made writeable plus executable using the mmap(2) and mprotect(2) system calls are killed by default. This option allows those processes to continue operation. The option is typically used on the /usr/local filesystem.

The flag MNT_UPDATE indicates that the mount command is being applied to an already mounted filesystem. This allows the mount flags to be changed without requiring that the filesystem be unmounted and remounted. Some filesystems may not allow all flags to be changed. For example, most filesystems will not allow a change from read-write to read-only.

The type argument defines the type of the filesystem. The types of filesystems known to the system are defined in <sys/mount.h>. data is a pointer to a structure that contains the type specific arguments to mount. The currently supported types of filesystems and their type specific data are:

MOUNT_CD9660

struct iso_args {
    char	*fspec;	    /* block special device to mount */
    struct	export_args export_info;
    			    /* network export info */
    int	flags;		    /* mounting flags, see below */
};
#define	ISOFSMNT_NORRIP	  0x00000001 /* disable Rock Ridge Ext.*/
#define	ISOFSMNT_GENS	  0x00000002 /* enable generation numbers */
#define	ISOFSMNT_EXTATT	  0x00000004 /* enable extended attributes */
#define	ISOFSMNT_NOJOLIET 0x00000008 /* disable Joliet Ext.*/
#define	ISOFSMNT_SESS	  0x00000010 /* use iso_args.sess */

MOUNT_FFS

struct ufs_args {
      char      *fspec;             /* block special file to mount */
      struct    export_args export_info;
      				    /* network export information */
};

MOUNT_MFS

struct mfs_args {
      char	*fspec;             /* name to export for statfs */
      struct	export_args export_info;
      				    /* if we can export an MFS */
      caddr_t	base;               /* base of filesystem in mem */
      u_long	size;               /* size of filesystem */
};

MOUNT_MSDOS

struct msdosfs_args {
	char	*fspec;	   /* blocks special holding fs to mount */
	struct	export_args export_info;
			   /* network export information */
	uid_t	uid;	   /* uid that owns msdosfs files */
	gid_t	gid;	   /* gid that owns msdosfs files */
	mode_t  mask;	   /* mask to be applied for msdosfs perms */
	int	flags;	   /* see below */
};

/*
 * Msdosfs mount options:
 */
#define	MSDOSFSMNT_SHORTNAME 1  /* Force old DOS short names only */
#define	MSDOSFSMNT_LONGNAME  2  /* Force Win'95 long names */
#define	MSDOSFSMNT_NOWIN95   4  /* Completely ignore Win95 entries */

MOUNT_NFS

struct nfs_args {
      int	version;	/* args structure version */
      struct sockaddr *addr;	/* file server address */
      int	addrlen;	/* length of address */
      int	sotype;       	/* Socket type */
      int	proto;        	/* and Protocol */
      u_char	*fh;          	/* File handle to be mounted */
      int	fhsize;       	/* Size, in bytes, of fh */
      int	flags;        	/* flags */
      int	wsize;        	/* write size in bytes */
      int	rsize;        	/* read size in bytes */
      int	readdirsize;  	/* readdir size in bytes */
      int	timeo;        	/* initial timeout in .1 secs */
      int	retrans;      	/* times to retry send */
      int	maxgrouplist; 	/* Max. size of group list */
      int	readahead;    	/* # of blocks to readahead */
      int	leaseterm;    	/* Term (sec) of lease */
      int	deadthresh;   	/* Retrans threshold */
      char	*hostname;    	/* server's name */
      int	acregmin;     /* Attr cache file recently modified */
      int	acregmax;     	/* ac file not recently modified */
      int	acdirmin;     	/* ac for dir recently modified */
      int	acdirmax;     /* ac for dir not recently modified */
};

MOUNT_NTFS

struct ntfs_args {
        char    *fspec;	/* block special device to mount */
        struct  export_args export_info;
			/* network export information */
        uid_t   uid;	/* uid that owns ntfs files */
        gid_t   gid;	/* gid that owns ntfs files */
        mode_t  mode;	/* mask to be applied for ntfs perms */
        u_long  flag;	/* additional flags */
};

/*
 * ntfs mount options:
 */
#define     NTFS_MFLAG_CASEINS      0x00000001
#define     NTFS_MFLAG_ALLNAMES     0x00000002

MOUNT_UDF

struct udf_args {
	char	*fspec;	/* block special device to mount */
};

The () function call disassociates the filesystem from the specified mount point dir.

The flags argument may specify MNT_FORCE to specify that the filesystem should be forcibly unmounted even if files are still active. Active special devices continue to work, but any further accesses to any other active files result in errors even if the filesystem is later remounted.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

mount() will fail when one of the following occurs:

[]
The caller is not the superuser.
[]
The pathname exceeded MNAMELEN characters.
[]
Too many symbolic links were encountered in translating a pathname.
[]
A component of dir does not exist.
[]
A component of name is not a directory, or a path prefix of special is not a directory.
[]
An argument given was invalid.
[]
Another process currently holds a reference to dir.
[]
dir points outside the process's allocated address space.
[]
type is not supported by the kernel.

The following errors can occur for a “ufs” filesystem mount:

[]
A component of ufs_args fspec does not exist.
[]
fspec is not a block device.
[]
The major device number of fspec is out of range (this indicates no device driver exists for the associated hardware).
[]
fspec is already mounted.
[]
The super block for the filesystem had a bad magic number, an out of range block size, or an invalid combination of flags.
[]
Not enough memory was available to read the cylinder group information for the filesystem.
[]
An I/O error occurred while reading the super block or cylinder group information.
[]
fspec points outside the process's allocated address space.
[]
The filesystem was not unmounted cleanly and MNT_FORCE was not specified.
[]
An attempt was made to mount a 4.2BSD filesystem without the MNT_RDONLY flag.

The following errors can occur for an NFS filesystem mount:

[]
timed out trying to contact the server.
[]
Some part of the information described by nfs_args points outside the process's allocated address space.

The following errors can occur for a filesystem mount:

[]
No space remains in the mount table.
[]
The super block for the filesystem had a bad magic number or an out of range block size.
[]
Not enough memory was available to read the cylinder group information for the filesystem.
[]
A paging error occurred while reading the super block or cylinder group information.
[]
points outside the process's allocated address space.

unmount() may fail with one of the following errors:

[]
The caller is not the superuser.
[]
A component of the path is not a directory.
[]
An argument given was invalid.
[]
A component of a pathname exceeded NAME_MAX characters, or an entire pathname (including the terminating NUL) exceeded PATH_MAX bytes.
[]
Too many symbolic links were encountered in translating the pathname.
[]
The requested directory is not in the mount table.
[]
A process is holding a reference to a file located on the filesystem.
[]
An I/O error occurred while writing cached filesystem information.
[]
dir points outside the process's allocated address space.

statfs(2), mount(8), mount_mfs(8), umount(8)

The mount() and umount() system calls first appeared in Version 1 AT&T UNIX; umount() was renamed to unmount() in 4.3BSD-Reno.

The flags argument is supported by mount() since Version 5 AT&T UNIX and by unmount() since 4.3BSD-Reno. The current calling convention involving type and data arguments was introduced by 4.3BSD-Reno as well.

Some of the error codes need translation to more obvious messages.

November 10, 2023 OpenBSD-7.6