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The following functions and data structure access the mtab file.
This structure is used with the getmntent, getmntent_t,
addmntent, and hasmntopt functions.
char *mnt_fsnameThis element contains a pointer to a string describing the name of the
special device from which the filesystem is mounted. It corresponds to
the fs_spec element in struct fstab.
char *mnt_dirThis element points to a string describing the mount point of the
filesystem. It corresponds to the fs_file element in
struct fstab.
char *mnt_typemnt_type describes the filesystem type and is therefore
equivalent to fs_vfstype in struct fstab. mntent.h
defines a few symbolic names for some of the values this string can have.
But since the kernel can support arbitrary filesystems it does not
make much sense to give them symbolic names. If one knows the symbol
name one also knows the filesystem name. Nevertheless here follows the
list of the symbols provided in mntent.h.
MNTTYPE_IGNORE ¶This symbol expands to "ignore". The value is sometime used in
fstab files to make sure entries are not used without removing them.
MNTTYPE_NFS ¶Expands to "nfs". Using this macro sometimes could make sense
since it names the default NFS implementation, in case both version 2
and 3 are supported.
MNTTYPE_SWAP ¶This symbol expands to "swap". It names the special fstab
entry which names one of the possibly multiple swap partitions.
char *mnt_optsThe element contains a string describing the options used while mounting
the filesystem. As for the equivalent element fs_mntops of
struct fstab it is best to use the function getsubopt
(see Parsing of Suboptions) to access the parts of this string.
The mntent.h file defines a number of macros with string values which correspond to some of the options understood by the kernel. There might be many more options which are possible so it doesn’t make much sense to rely on these macros but to be consistent here is the list:
MNTOPT_DEFAULTS ¶Expands to "defaults". This option should be used alone since it
indicates all values for the customizable values are chosen to be the
default.
MNTOPT_RO ¶Expands to "ro". See the FSTAB_RO value, it means the
filesystem is mounted read-only.
MNTOPT_RW ¶Expand to "rw". See the FSTAB_RW value, it means the
filesystem is mounted with read and write permissions.
MNTOPT_SUID ¶Expands to "suid". This means that the SUID bit (see How an Application Can Change Persona) is respected when a program from the filesystem is
started.
MNTOPT_NOSUID ¶Expands to "nosuid". This is the opposite of MNTOPT_SUID,
the SUID bit for all files from the filesystem is ignored.
MNTOPT_NOAUTO ¶Expands to "noauto". At startup time the mount program
will ignore this entry if it is started with the -a option to
mount all filesystems mentioned in the fstab file.
As for the FSTAB_* entries introduced above it is important to
use strcmp to check for equality.
mnt_freqThis elements corresponds to fs_freq and also specifies the
frequency in days in which dumps are made.
mnt_passnoThis element is equivalent to fs_passno with the same meaning
which is uninteresting for all programs beside dump.
For accessing the mtab file there is again a set of three functions to access all entries in a row. Unlike the functions to handle fstab these functions do not access a fixed file and there is even a thread safe variant of the get function. Beside this the GNU C Library contains functions to alter the file and test for specific options.
Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd lock | See POSIX Safety Concepts.
The setmntent function prepares the file named FILE which
must be in the format of a fstab and mtab file for the
upcoming processing through the other functions of the family. The
mode parameter can be chosen in the way the opentype
parameter for fopen (see Opening Streams) can be chosen. If
the file is opened for writing the file is also allowed to be empty.
If the file was successfully opened setmntent returns a file
descriptor for future use. Otherwise the return value is NULL
and errno is set accordingly.
Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.
This function takes for the stream parameter a file handle which
previously was returned from the setmntent call.
endmntent closes the stream and frees all resources.
The return value is 1 unless an error occurred in which case it is 0.
Preliminary: | MT-Unsafe race:mntentbuf locale | AS-Unsafe corrupt heap init | AC-Unsafe init corrupt lock mem | See POSIX Safety Concepts.
The getmntent function takes as the parameter a file handle
previously returned by successful call to setmntent. It returns
a pointer to a static variable of type struct mntent which is
filled with the information from the next entry from the file currently
read.
The file format used prescribes the use of spaces or tab characters to
separate the fields. This makes it harder to use name containing one
of these characters (e.g., mount points using spaces). Therefore
these characters are encoded in the files and the getmntent
function takes care of the decoding while reading the entries back in.
'\040' is used to encode a space character, '\011' to
encode a tab character, '\012' to encode a newline character,
and '\\' to encode a backslash.
If there was an error or the end of the file is reached the return value
is NULL.
This function is not thread-safe since all calls to this function return
a pointer to the same static variable. getmntent_r should be
used in situations where multiple threads access the file.
Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.
The getmntent_r function is the reentrant variant of
getmntent. It also returns the next entry from the file and
returns a pointer. The actual variable the values are stored in is not
static, though. Instead the function stores the values in the variable
pointed to by the result parameter. Additional information (e.g.,
the strings pointed to by the elements of the result) are kept in the
buffer of size bufsize pointed to by buffer.
Escaped characters (space, tab, backslash) are converted back in the
same way as it happens for getmentent.
The function returns a NULL pointer in error cases. Errors could be:
Preliminary: | MT-Safe race:stream locale | AS-Unsafe corrupt | AC-Unsafe corrupt | See POSIX Safety Concepts.
The addmntent function allows adding a new entry to the file
previously opened with setmntent. The new entries are always
appended. I.e., even if the position of the file descriptor is not at
the end of the file this function does not overwrite an existing entry
following the current position.
The implication of this is that to remove an entry from a file one has to create a new file while leaving out the entry to be removed and after closing the file remove the old one and rename the new file to the chosen name.
This function takes care of spaces and tab characters in the names to be
written to the file. It converts them and the backslash character into
the format describe in the getmntent description above.
This function returns 0 in case the operation was successful.
Otherwise the return value is 1 and errno is set
appropriately.
Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This function can be used to check whether the string pointed to by the
mnt_opts element of the variable pointed to by mnt contains
the option opt. If this is true a pointer to the beginning of the
option in the mnt_opts element is returned. If no such option
exists the function returns NULL.
This function is useful to test whether a specific option is present but
when all options have to be processed one is better off with using the
getsubopt function to iterate over all options in the string.
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