The following functions and data structure access the mtab file.
— Data Type: struct mntent
This structure is used with the getmntent, getmntent_t,
addmntent, and hasmntopt functions.
This 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.
This element points to a string describing the mount point of the
filesystem. It corresponds to the fs_file element in
mnt_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.
This symbol expands to "ignore". The value is sometime used in
fstab files to make sure entries are not used without removing them.
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.
This symbol expands to "swap". It names the special fstab
entry which names one of the possibly multiple swap partitions.
The 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 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:
Expands to "defaults". This option should be used alone since it
indicates all values for the customizable values are chosen to be the
Expands to "ro". See the FSTAB_RO value, it means the
filesystem is mounted read-only.
Expand to "rw". See the FSTAB_RW value, it means the
filesystem is mounted with read and write permissions.
Expands to "suid". This means that the SUID bit (see How Change Persona) is respected when a program from the filesystem is
Expands to "nosuid". This is the opposite of MNTOPT_SUID,
the SUID bit for all files from the filesystem is ignored.
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.
This elements corresponds to fs_freq and also specifies the
frequency in days in which dumps are made.
This 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
libc contains functions to alter the file and test for specific options.
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.
— Function: int endmntent (FILE *stream)
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
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
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, '\012' to
encode a tab character and '\\' to encode a backslash.
If there was an error or the end of the file is reached the return value
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.
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:
error while reading the file,
end of file reached,
bufsize is too small for reading a complete new entry.
— Function: int addmntent (FILE *stream, const struct mntent *mnt)
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
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
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.
Published under the terms of the GNU General Public License