mntopts.3

.\" Copyright (c) 2023 Marshall Kirk McKusick
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.Dd January 19, 2023
.Dt MNTOPTS 3
.Os
.Sh NAME
.Nm getmntopts ,
.Nm getmntpoint ,
.Nm chkdoreload ,
.Nm build_iovec ,
.Nm build_iovec_argf ,
.Nm free_iovec ,
.Nm checkpath ,
.Nm rmslashes
.Nd "mount point operations"
.Sh SYNOPSIS
.In mntopts.h
.Ft void
.Fo getmntopts
.Fa "const char *options" "const struct mntopt *mopts"
.Fa "int *flagp" "int *altflagp"
.Fc
.Ft struct statfs *
.Fn getmntpoint "const char *name"
.Ft int
.Fo chkdoreload
.Fa "struct statfs *mntp"
.Fa "void (*prtmsg)(const char *fmt, ...)"
.Fc
.Ft void
.Fo build_iovec
.Fa "struct iovec **iov" "int *iovlen" "const char *name" "void *val"
.Fa "size_t len"
.Fc
.Ft void
.Fo build_iovec_argf
.Fa "struct iovec **iov" "int *iovlen" "const char *name"
.Fa "const char *fmt" "..."
.Fc
.Ft void
.Fn free_iovec "struct iovec **iov" "int *iovlen"
.Ft int
.Fn checkpath "const char *path" "char *resolved"
.Ft void
.Fn rmslashes "char *rrpin" "char *rrpout"
.Sh DESCRIPTION
The
.Nm mntopts
functions support operations associated with a mount point.
For historic reasons are in a file in the sources for the
.Xr mount 8
program.
Thus, to access them the following lines need to be added to the
.Nm Makefile
of the program wanting to use them:
.Bd -literal
SRCS+=	getmntopts.c
MOUNT=  ${SRCTOP}/sbin/mount
CFLAGS+= -I${MOUNT}
\&.PATH: ${MOUNT}
.Ed
.Pp
The
.Fn getmntopts
function takes a comma separated option list and a list
of valid option names, and computes the bitmask
corresponding to the requested set of options.
.Pp
The string
.Fa options
is broken down into a sequence of comma separated tokens.
Each token is looked up in the table described by
.Fa mopts
and the bits in
the word referenced by either
.Fa flagp
or
.Fa altflagp
(depending on the
.Va m_altloc
field of the option's table entry)
are updated.
The flag words are not initialized by
.Fn getmntopts .
The table,
.Fa mopts ,
has the following format:
.Bd -literal
struct mntopt {
	char *m_option;	/* option name */
	int m_inverse;	/* is this a negative option, e.g., "dev" */
	int m_flag;	/* bit to set, e.g., MNT_RDONLY */
	int m_altloc;	/* non-zero to use altflagp rather than flagp */
};
.Ed
.Pp
The members of this structure are:
.Bl -tag -width m_inverse
.It Va m_option
the option name,
for example
.Dq Li suid .
.It Va m_inverse
tells
.Fn getmntopts
that the name has the inverse meaning of the
bit.
For example,
.Dq Li suid
is the string, whereas the
mount flag is
.Dv MNT_NOSUID .
In this case, the sense of the string and the flag
are inverted, so the
.Va m_inverse
flag should be set.
.It Va m_flag
the value of the bit to be set or cleared in
the flag word when the option is recognized.
The bit is set when the option is discovered,
but cleared if the option name was preceded
by the letters
.Dq Li no .
The
.Va m_inverse
flag causes these two operations to be reversed.
.It Va m_altloc
the bit should be set or cleared in
.Fa altflagp
rather than
.Fa flagp .
.El
.Pp
Each of the user visible
.Dv MNT_
flags has a corresponding
.Dv MOPT_
macro which defines an appropriate
.Vt "struct mntopt"
entry.
To simplify the program interface and ensure consistency across all
programs, a general purpose macro,
.Dv MOPT_STDOPTS ,
is defined which
contains an entry for all the generic VFS options.
In addition, the macros
.Dv MOPT_FORCE
and
.Dv MOPT_UPDATE
exist to enable the
.Dv MNT_FORCE
and
.Dv MNT_UPDATE
flags to be set.
Finally, the table must be terminated by an entry with a
.Dv NULL
first element.
.Pp
The
.Fn getmntpoint
function takes the pathname of a possible mount point
or of a device (with or without
.Pa /dev/
prepended to it).
If the pathname is a directory or a file,
.Fn getmntpoint
checks to see if the mount point currently has a filesystem
mounted on it.
If the pathname is a device,
.Fn getmntpoint
checks to see if it is currently mounted.
If there is an associated mount, a pointer to a
.Vt "struct statfs"
is returned.
The returned result is stored in a static buffer that is over-written
each time the
.Fn getmntpoint
function or the
.Xr getmntinfo 3
library routine is called.
If no mount is found, NULL is returned.
.Pp
The
.Fn chkdoreload
function takes a pointer to a 
.Vt "struct statfs" .
If the filesystem associated with the mount point is mounted read-only,
.Fn chkdoreload
requests the filesystem to reload all of its metadata from its backing store.
The second parameter is the function to call to print an error message
if the reload fails.
If no error message is desired, a
.Dv NULL
can be passed as the second argument.
The
.Fn chkdoreload
function returns zero on success or non-zero on failure.
.Pp
The
.Fn build_iovec
function adds a parameter to a list of parameters to be passed to the
.Xr nmount 2
system call.
The parameter list is built up in
.Va iov
and its length is kept in
.Va iovlen .
Before the first call to
.Fn build_iovec ,
.Va iov
should be set to
.Dv NULL
and
.Va iovlen
should be set to 0.
The parameter name is passed in
.Va name .
The value of the parameter name is pointed to by
.Va val .
The size of the value is passed in
.Va len .
If the value is a string, a
.Va len
of -1 is passed to indicate that the length should be determined using
.Xr strlen 3 .
If the parameter has no value,
.Va name
should be
.Dv NULL
and
.Va len
should be 0.
.Pp
The
.Fn build_iovec_argf
function adds a formatted parameter to a list of parameters to be passed
to the
.Xr nmount 2
system call.
The parameter list is built up in
.Va iov
and its length is kept in
.Va iovlen .
Before the first call to
.Fn build_iovec_argf ,
.Va iov
should be set to
.Dv NULL
and
.Va iovlen
should be set to 0.
The parameter name is passed in
.Va name .
The value of the parameter name is described by a format string pointed to by
.Va fmt .
If the parameter has no value,
.Va name
should be
.Dv NULL .
.Pp
The
.Fn free_iovec
function frees the memory in the
.Va iov
vector of the length specified in
.Va iovlen
that was previously allocated by the
.Fn build_iovec
and / or
.Fn build_iovec_argf
functions.
The
.Va iov
is set to
.Dv NULL
and the
.Va iovlen
is set to 0 to indicate that the space has been freed.
.Pp
The
.Fn checkpath
function uses
.Xr realpath 3
to verify that its
.Va path
argument is valid and references a directory.
The
.Fn checkpath
function returns zero on success or non-zero on failure.
.Pp
The
.Fn rmslashes
function removes all double slashes and trailing slashes from its
.Va rrpin
pathname parameter and returns the resulting pathname in its
.Va rrpout
parameter.
.Sh EXAMPLES
Most commands will use the standard option set.
Local file systems which support the
.Dv MNT_UPDATE
flag, would also have an
.Dv MOPT_UPDATE
entry.
This can be declared and used as follows:
.Bd -literal
#include "mntopts.h"

struct mntopt mopts[] = {
	MOPT_STDOPTS,
	MOPT_UPDATE,
	{ NULL }
};

	...
	mntflags = mntaltflags = 0;
	...
	getmntopts(options, mopts, &mntflags, &mntaltflags);
	...
.Ed
.Sh DIAGNOSTICS
If the external integer variable
.Va getmnt_silent
is zero, then the
.Fn getmntopts
function displays an error message and exits if an
unrecognized option is encountered.
Otherwise unrecognized options are silently ignored.
By default
.Va getmnt_silent
is zero.
.Sh SEE ALSO
.Xr err 3 ,
.Xr mount 8 ,
.Xr nmount 8
.Sh HISTORY
The
.Fn getmntopts
function appeared in
.Bx 4.4 .
The
.Fn build_iovec ,
.Fn build_iovec_argf ,
.Fn free_iovec ,
.Fn checkpath ,
and
.Fn rmslashes
functions were added with
.Xr nmount 8
in
.Fx 5.0 .
The
.Fn getmntpoint
and
.Fn chkdoreload
functions were added in
.Fx 13.2 .