For some programs it is desirable and necessary to access information about whether a certain filesystem is mounted and, if it is, where, or simply to get lists of all the available filesystems. The GNU C Library provides some functions to retrieve this information portably.
Traditionally Unix systems have a file named /etc/fstab which
describes all possibly mounted filesystems. The mount program
uses this file to mount at startup time of the system all the
necessary filesystems. The information about all the filesystems
actually mounted is normally kept in a file named either
/var/run/mtab or /etc/mtab. Both files share the same
syntax and it is crucial that this syntax is followed all the time.
Therefore it is best to never directly write the files. The functions
described in this section can do this and they also provide the
functionality to convert the external textual representation to the
internal representation.
Note that the fstab and mtab files are maintained on a system by convention. It is possible for the files not to exist or not to be consistent with what is really mounted or available to mount, if the system's administration policy allows it. But programs that mount and unmount filesystems typically maintain and use these files as described herein.
The filenames given above should never be used directly. The portable
way to handle these file is to use the macro _PATH_FSTAB,
defined in fstab.h, or _PATH_MNTTAB, defined in
mntent.h and paths.h, for fstab; and the macro
_PATH_MOUNTED, also defined in mntent.h and
paths.h, for mtab. There are also two alternate macro
names FSTAB, MNTTAB, and MOUNTED defined but
these names are deprecated and kept only for backward compatibility.
The names _PATH_MNTTAB and _PATH_MOUNTED should always be used.