The `fmt` field helps distinguish each subtype, and being able to set it would make it easier for me to instantiate the appropriate type Rust-side (i.e. `FsTab`, `MountInfo`, `UTab`, `Swaps`). https://github.com/util-linux/util-linux/blob/stable/v2.39/libmount/src/mountP.h#L252-L287 ```C /*  * fstab/mountinfo file  */ struct libmnt_table {   int   fmt;     /* MNT_FMT_* file format */   int   nents;    /* number of entries */   int   refcount;  /* reference counter */   int   comms;    /* enable/disable comment parsing */   char     *comm_intro;   /* First comment in file */   char     *comm_tail; /* Last comment in file */   struct libmnt_cache *cache;   /* canonicalized paths/tags cache */   int   (*errcb)(struct libmnt_table *tb,        const char *filename, int line);   int   (*fltrcb)(struct libmnt_fs *fs, void *data);   void     *fltrcb_data;   int   noautofs;  /* ignore autofs mounts */   struct list_head   ents;  /* list of entries (libmnt_fs) */   void     *userdata; }; /*  * Tab file format  */ enum {   MNT_FMT_GUESS,   MNT_FMT_FSTAB,      /* /etc/{fs,m}tab */   MNT_FMT_MTAB = MNT_FMT_FSTAB,  /* alias */   MNT_FMT_MOUNTINFO,    /* /proc/#/mountinfo */   MNT_FMT_UTAB,      /* /run/mount/utab */   MNT_FMT_SWAPS      /* /proc/swaps */ }; ``` M2 - If we were to split `struct libmnt_fs` into one type per file format, would we get data types resembling `struct entry_fstab`, `struct entry_mountinfo`, `struct entry_utab`, `struct_swaps` described below? As far as I can tell, `struct libmnt_fs` is a multi-purpose data type representing a line in either of the following files: - `/etc/fstab` - `/proc/#/mountinfo` - `/run/mount/utab` - `/proc/swaps` Each file has a distinct format. So, some fields present in `struct libmnt_fs` do not apply to certain files. https://github.com/util-linux/util-linux/blob/stable/v2.39/libmount/src/mountP.h#L188-L239 ```C /*  * This struct represents one entry in a fstab/mountinfo file.  * (note that fstab[1] means the first column from fstab, and so on...)  */ struct libmnt_fs {   struct list_head ents;   struct libmnt_table *tab;   int   refcount;  /* reference counter */   unsigned int   opts_age;  /* to sync with optlist */   struct libmnt_optlist *optlist;   int   id;   /* mountinfo[1]: ID */   int   parent;   /* mountinfo[2]: parent */   dev_t    devno;    /* mountinfo[3]: st_dev */   char     *bindsrc;  /* utab, full path from fstab[1] for bind mounts */   char     *source;   /* fstab[1], mountinfo[10], swaps[1]:                      * source dev, file, dir or TAG */   char     *tagname;  /* fstab[1]: tag name - "LABEL", "UUID", ..*/   char     *tagval;   /*      tag value */   char     *root;    /* mountinfo[4]: root of the mount within the FS */   char     *target;   /* mountinfo[5], fstab[2]: mountpoint */   char     *fstype;   /* mountinfo[9], fstab[3]: filesystem type */   char     *optstr;   /* fstab[4], merged options */   char     *vfs_optstr;   /* mountinfo[6]: fs-independent (VFS) options */   char     *opt_fields;   /* mountinfo[7]: optional fields */   char     *fs_optstr; /* mountinfo[11]: fs-dependent options */   char     *user_optstr;  /* userspace mount options */   char     *attrs;   /* mount attributes */   int   freq;    /* fstab[5]: dump frequency in days */   int   passno;   /* fstab[6]: pass number on parallel fsck */   /* /proc/swaps */   char     *swaptype;  /* swaps[2]: device type (partition, file, ...) */   off_t    size;    /* swaps[3]: swaparea size */   off_t    usedsize;  /* swaps[4]: used size */   int   priority;  /* swaps[5]: swap priority */   int   flags;    /* MNT_FS_* flags */   pid_t    tid;     /* /proc/<tid>/mountinfo otherwise zero */   char     *comment;  /* fstab comment */   void     *userdata;  /* library independent data */ }; ``` ### `/etc/fstab` ```C struct entry_fstab {   struct list_head ents;   struct libmnt_table *tab;   int   refcount;  /* reference counter */   unsigned int   opts_age;  /* to sync with optlist */   struct libmnt_optlist *optlist;   char     *bindsrc;  /* utab, full path from fstab[1] for bind mounts */   char     *source;   /* fstab[1], mountinfo[10], swaps[1]:                      * source dev, file, dir or TAG */   char     *tagname;  /* fstab[1]: tag name - "LABEL", "UUID", ..*/   char     *tagval;   /*      tag value */   char     *target;   /* mountinfo[5], fstab[2]: mountpoint */   char     *fstype;   /* mountinfo[9], fstab[3]: filesystem type */   char     *optstr;   /* fstab[4], merged options */   char     *user_optstr;  /* userspace mount options */   char     *attrs;   /* mount attributes */   int   freq;    /* fstab[5]: dump frequency in days */   int   passno;   /* fstab[6]: pass number on parallel fsck */   char     *comment;  /* fstab comment */   void     *userdata;  /* library independent data */ }; ``` ### `/proc/#/mountinfo` ```C struct entry_mountinfo {   struct list_head ents;   struct libmnt_table *tab;   int   refcount;  /* reference counter */   unsigned int   opts_age;  /* to sync with optlist */   struct libmnt_optlist *optlist;   int   id;   /* mountinfo[1]: ID */   int   parent;   /* mountinfo[2]: parent */   dev_t    devno;    /* mountinfo[3]: st_dev */   char     *source;   /* fstab[1], mountinfo[10], swaps[1]:                      * source dev, file, dir or TAG */   char     *root;    /* mountinfo[4]: root of the mount within the FS */   char     *target;   /* mountinfo[5], fstab[2]: mountpoint */   char     *fstype;   /* mountinfo[9], fstab[3]: filesystem type */   char     *vfs_optstr;   /* mountinfo[6]: fs-independent (VFS) options */   char     *opt_fields;   /* mountinfo[7]: optional fields */   char     *fs_optstr; /* mountinfo[11]: fs-dependent options */   char     *user_optstr;  /* userspace mount options */   pid_t    tid;     /* /proc/<tid>/mountinfo otherwise zero */ }; ``` ### `/run/mount/utab` ```C struct entry_utab {   struct list_head ents;   struct libmnt_table *tab;   int   refcount;  /* reference counter */   unsigned int   opts_age;  /* to sync with optlist */   struct libmnt_optlist *optlist;   int   id;   /* mountinfo[1]: ID */   int   parent;   /* mountinfo[2]: parent */   dev_t    devno;    /* mountinfo[3]: st_dev */   char     *bindsrc;  /* utab, full path from fstab[1] for bind mounts */   char     *source;   /* fstab[1], mountinfo[10], swaps[1]:                      * source dev, file, dir or TAG */   char     *root;    /* mountinfo[4]: root of the mount within the FS */   char     *target;   /* mountinfo[5], fstab[2]: mountpoint */   char     *user_optstr;  /* userspace mount options */   char     *attrs;   /* mount attributes */   void     *userdata;  /* library independent data */ }; ``` ### `/proc/swaps` ```C struct entry_swaps {   struct list_head ents;   struct libmnt_table *tab;   int   refcount;  /* reference counter */   unsigned int   opts_age;  /* to sync with optlist */   struct libmnt_optlist *optlist;   char     *source;   /* fstab[1], mountinfo[10], swaps[1]:                      * source dev, file, dir or TAG */   /* /proc/swaps */   char     *swaptype;  /* swaps[2]: device type (partition, file, ...) */   off_t    size;    /* swaps[3]: swaparea size */   off_t    usedsize;  /* swaps[4]: used size */   int   priority;  /* swaps[5]: swap priority */ }; ``` M3 - About `struct libmnt_fs` mentioned in question `M2`, is the following a good partition of which subsets of functions, acting on the struct, are specific/exclusive to each type of file entry?  (We assume that items not in a union of the subsets below are shared between line types) ### Functions specific/exclusive to `/etc/fstab` - `mnt_fs_append_comment` - `mnt_fs_append_options ` - `mnt_fs_get_comment ` - `mnt_fs_get_freq ` - `mnt_fs_get_fstype ` - `mnt_fs_get_option ` - `mnt_fs_get_options ` - `mnt_fs_get_passno ` - `mnt_fs_get_source ` - `mnt_fs_get_tag ` - `mnt_fs_get_target ` - `mnt_fs_match_fstype ` - `mnt_fs_match_options ` - `mnt_fs_match_target ` - `mnt_fs_prepend_options ` - `mnt_fs_print_debug ` - `mnt_fs_set_comment ` - `mnt_fs_set_freq ` - `mnt_fs_set_fstype ` - `mnt_fs_set_options ` - `mnt_fs_set_passno ` - `mnt_fs_set_source ` - `mnt_fs_set_target ` - `mnt_fs_streq_target ` - `mnt_fs_to_mntent ` - `mnt_new_fs ` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-append-comment ```text int mnt_fs_append_comment (struct libmnt_fs *fs,             const char *comm); See also mnt_fs_set_comment(). Parameters fs fstab entry pointer comm comment string Returns 0 on success or <0 in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-append-options ```text int mnt_fs_append_options (struct libmnt_fs *fs,             const char *optstr); Parses (splits) optstr and appends results to VFS, FS and userspace lists of options. If optstr is NULL, then fs is not modified and 0 is returned. Parameters fs fstab/mtab/mountinfo entry optstr mount options Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-comment ```text const char * mnt_fs_get_comment (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns 0 on success, 1 when not found the name or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-freq ```text int mnt_fs_get_freq (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns dump frequency in days. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-fstype ```text const char * mnt_fs_get_fstype (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns pointer to filesystem type. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-option ```text int mnt_fs_get_option (struct libmnt_fs *fs,           const char *name,           char **value,           size_t *valsz); Parameters fs fstab/mtab/mountinfo entry pointer name option name value returns pointer to the beginning of the value (e.g. name=VALUE) or NULL valsz returns size of options value or 0 Returns 0 on success, 1 when name not found or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-options ```text const char * mnt_fs_get_options (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns pointer to string or NULL in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-passno ```text int mnt_fs_get_passno (struct libmnt_fs *fs); Parameters fs fstab/mtab entry pointer Returns "pass number on parallel fsck". ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-source ```text const char * mnt_fs_get_source (struct libmnt_fs *fs); Parameters fs struct libmnt_file (fstab/mtab/mountinfo) fs Returns mount source. Note that the source could be unparsed TAG (LABEL/UUID). See also mnt_fs_get_srcpath() and mnt_fs_get_tag(). ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-tag ```text int mnt_fs_get_tag (struct libmnt_fs *fs,         const char **name,         const char **value); "TAG" is NAME=VALUE (e.g. LABEL=foo) The TAG is the first column in the fstab file. The TAG or "srcpath" always has to be set for all entries. See also mnt_fs_get_source(). char *src; struct libmnt_fs *fs = mnt_table_find_target(tb, "/home", MNT_ITER_FORWARD); if (!fs) goto err; src = mnt_fs_get_srcpath(fs); if (!src) { char *tag, *val; if (mnt_fs_get_tag(fs, &tag, &val) == 0) printf("%s: %s\n", tag, val); // LABEL or UUID } else printf("device: %s\n", src); // device or bind path Parameters fs fs name returns pointer to NAME string value returns pointer to VALUE string Returns 0 on success or negative number in case a TAG is not defined. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-target ```text const char * mnt_fs_get_target (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns pointer to mountpoint path or NULL ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-match-fstype ```text int mnt_fs_match_fstype (struct libmnt_fs *fs,            const char *types); For more details see mnt_match_fstype(). Parameters fs filesystem types filesystem name or comma delimited list of filesystems Returns 1 if fs type is matching to types , else 0. The function returns 0 when types is NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-match-options ```text int mnt_fs_match_options (struct libmnt_fs *fs,            const char *options); For more details see mnt_match_options(). Parameters fs filesystem options comma delimited list of options (and nooptions) Returns 1 if fs type is matching to options , else 0. The function returns 0 when types is NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-match-target ```text int mnt_fs_match_target (struct libmnt_fs *fs,            const char *target,            struct libmnt_cache *cache); Possible are three attempts: 1) compare target with fs->target 2) realpath(target ) with fs->target 3) realpath(target ) with realpath(fs->target ) if fs is not from /proc/self/mountinfo. However, if mnt_cache_set_targets(cache, mtab) was called, and the path target or fs->target is found in the mtab , the canonicalization is is not performed (see mnt_resolve_target()). The 2nd and 3rd attempts are not performed when cache is NULL. Parameters fs filesystem target mountpoint path cache tags/paths cache or NULL Returns 1 if fs target is equal to target , else 0. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-prepend-options ```text int mnt_fs_prepend_options (struct libmnt_fs *fs,             const char *optstr); Parses (splits) optstr and prepends the results to VFS, FS and userspace lists of options. If optstr is NULL, then fs is not modified and 0 is returned. Parameters fs fstab/mtab/mountinfo entry optstr mount options Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-print-debug ```text int mnt_fs_print_debug (struct libmnt_fs *fs,           FILE *file); Parameters fs fstab/mtab/mountinfo entry file file stream Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-comment ```text int mnt_fs_set_comment (struct libmnt_fs *fs,           const char *comm); Note that the comment has to be terminated by '\n' (new line), otherwise the whole filesystem entry will be written as a comment to the tabfile (e.g. fstab). Parameters fs fstab entry pointer comm comment string Returns 0 on success or <0 in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-freq ```text int mnt_fs_set_freq (struct libmnt_fs *fs,          int freq); Parameters fs fstab/mtab entry pointer freq dump frequency in days Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-fstype ```text int mnt_fs_set_fstype (struct libmnt_fs *fs,           const char *fstype); This function creates a private copy (strdup()) of fstype . Parameters fs fstab/mtab/mountinfo entry fstype filesystem type Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-options ```text int mnt_fs_set_options (struct libmnt_fs *fs,           const char *optstr); Splits optstr to VFS, FS and userspace mount options and updates relevant parts of fs . Parameters fs fstab/mtab/mountinfo entry pointer optstr options string Returns 0 on success, or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-passno ```text int mnt_fs_set_passno (struct libmnt_fs *fs,           int passno); Parameters fs fstab/mtab entry pointer passno pass number Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-source ```text int mnt_fs_set_source (struct libmnt_fs *fs,           const char *source); This function creates a private copy (strdup()) of source . Parameters fs fstab/mtab/mountinfo entry source new source Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-target ```text int mnt_fs_set_target (struct libmnt_fs *fs,           const char *tgt); This function creates a private copy (strdup()) of tgt . Parameters fs fstab/mtab/mountinfo entry tgt mountpoint Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-streq-target ```text int mnt_fs_streq_target (struct libmnt_fs *fs,            const char *path); Compares fs target path with path . The redundant slashes are ignored. This function compares strings and does not canonicalize the paths. See also more generic mnt_fs_match_target(). Parameters fs fs path mount point Returns 1 if fs target path equal to path , otherwise 0. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-to-mntent ```text int mnt_fs_to_mntent (struct libmnt_fs *fs,          struct mntent **mnt); Copies the information from fs to struct mntent mnt . If mnt is already set, then the struct mntent items are reallocated and updated. See also mnt_free_mntent(). Parameters fs filesystem mnt mount description (as described in mntent.h) Returns 0 on success and a negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-new-fs ```text struct libmnt_fs * mnt_new_fs (void); The initial refcount is 1, and needs to be decremented to release the resources of the filesystem. Returns newly allocated struct libmnt_fs. ``` ### Functions specific/exclusive to `/proc/#/mountinfo` - `mnt_fs_get_devno ` - `mnt_fs_get_fs_options ` - `mnt_fs_get_fstype ` - `mnt_fs_get_id ` - `mnt_fs_get_optional_fields ` - `mnt_fs_get_parent_id ` - `mnt_fs_get_propagation ` - `mnt_fs_get_root ` - `mnt_fs_get_target ` - `mnt_fs_get_tid ` - `mnt_fs_get_vfs_options ` - `mnt_fs_get_vfs_options_all ` - `mnt_fs_match_fstype ` - `mnt_fs_match_target ` - `mnt_fs_print_debug ` - `mnt_fs_strdup_options ` - `mnt_fs_streq_target ` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-devno ```text dev_t mnt_fs_get_devno (struct libmnt_fs *fs); Parameters fs /proc/self/mountinfo entry Returns value of st_dev for files on filesystem or 0 in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-fs-options ```text const char * mnt_fs_get_fs_options (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns pointer to superblock (fs-depend) mount option string or NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-fstype ```text const char * mnt_fs_get_fstype (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns pointer to filesystem type. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-id ```text int mnt_fs_get_id (struct libmnt_fs *fs); Parameters fs /proc/self/mountinfo entry Returns mount ID (unique identifier of the mount) or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-optional-fields ```text const char * mnt_fs_get_optional_fields (struct libmnt_fs *fs); Parameters fs mountinfo entry pointer Returns pointer to string with mountinfo optional fields or NULL in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-parent-id ```text int mnt_fs_get_parent_id (struct libmnt_fs *fs); Parameters fs /proc/self/mountinfo entry Returns parent mount ID or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-propagation ```text int mnt_fs_get_propagation (struct libmnt_fs *fs,             unsigned long *flags); Note that this function sets flags to zero if no propagation flags are found in the mountinfo file. The kernel default is MS_PRIVATE, this flag is not stored in the mountinfo file. Parameters fs mountinfo entry flags returns propagation MS_* flags as present in the mountinfo file Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-root ```text const char * mnt_fs_get_root (struct libmnt_fs *fs); Parameters fs /proc/self/mountinfo entry Returns root of the mount within the filesystem or NULL ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-target ```text const char * mnt_fs_get_target (struct libmnt_fs *fs); Parameters fs fstab/mtab/mountinfo entry pointer Returns pointer to mountpoint path or NULL ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-tid ```text pid_t mnt_fs_get_tid (struct libmnt_fs *fs); Parameters fs /proc/tid/mountinfo entry Returns TID (task ID) for filesystems read from the mountinfo file ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-vfs-options ```text const char * mnt_fs_get_vfs_options (struct libmnt_fs *fs); Parameters fs fstab/mtab entry pointer Returns pointer to fs-independent (VFS) mount option string or NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-vfs-options-all ```text char * mnt_fs_get_vfs_options_all (struct libmnt_fs *fs); Parameters fs fstab/mtab entry pointer Returns pointer to newlly allocated string (can be freed by free(3)) or NULL in case of error. The string contains all (including defaults) mount options. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-match-fstype ```text int mnt_fs_match_fstype (struct libmnt_fs *fs,            const char *types); For more details see mnt_match_fstype(). Parameters fs filesystem types filesystem name or comma delimited list of filesystems Returns 1 if fs type is matching to types , else 0. The function returns 0 when types is NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-match-target ```text int mnt_fs_match_target (struct libmnt_fs *fs,            const char *target,            struct libmnt_cache *cache); Possible are three attempts: 1) compare target with fs->target 2) realpath(target ) with fs->target 3) realpath(target ) with realpath(fs->target ) if fs is not from /proc/self/mountinfo. However, if mnt_cache_set_targets(cache, mtab) was called, and the path target or fs->target is found in the mtab , the canonicalization is is not performed (see mnt_resolve_target()). The 2nd and 3rd attempts are not performed when cache is NULL. Parameters fs filesystem target mountpoint path cache tags/paths cache or NULL Returns 1 if fs target is equal to target , else 0. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-print-debug ```text int mnt_fs_print_debug (struct libmnt_fs *fs,           FILE *file); Parameters fs fstab/mtab/mountinfo entry file file stream Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-strdup-options ```text char * mnt_fs_strdup_options (struct libmnt_fs *fs); Merges all mount options (VFS, FS and userspace) to one options string and returns the result. This function does not modify fs . Parameters fs fstab/mtab/mountinfo entry pointer Returns pointer to string (can be freed by free(3)) or NULL in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-streq-target ```text int mnt_fs_streq_target (struct libmnt_fs *fs,            const char *path); Compares fs target path with path . The redundant slashes are ignored. This function compares strings and does not canonicalize the paths. See also more generic mnt_fs_match_target(). Parameters fs fs path mount point Returns 1 if fs target path equal to path , otherwise 0. ``` ### Functions specific/exclusive to `/run/mount/utab` - `mnt_fs_append_attributes ` - `mnt_fs_get_attribute ` - `mnt_fs_get_attributes ` - `mnt_fs_get_bindsrc ` - `mnt_fs_get_id ` - `mnt_fs_get_root ` - `mnt_fs_get_user_options ` - `mnt_fs_match_options ` - `mnt_fs_match_target ` - `mnt_fs_prepend_attributes ` - `mnt_fs_set_attributes ` - `mnt_fs_set_bindsrc ` - `mnt_fs_set_root ` - `mnt_fs_set_source ` - `mnt_fs_streq_target ` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-append-attributes ```text int mnt_fs_append_attributes (struct libmnt_fs *fs,              const char *optstr); Appends mount attributes. (See mnt_fs_set_attributes()). Parameters fs fstab/mtab/mountinfo entry optstr options string Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-attribute ```text int mnt_fs_get_attribute (struct libmnt_fs *fs,            const char *name,            char **value,            size_t *valsz); Parameters fs fstab/mtab/mountinfo entry pointer name option name value returns pointer to the beginning of the value (e.g. name=VALUE) or NULL valsz returns size of options value or 0 Returns 0 on success, 1 when name not found or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-attributes ```text const char * mnt_fs_get_attributes (struct libmnt_fs *fs); Parameters fs fstab/mtab entry pointer Returns pointer to attributes string or NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-bindsrc ```text const char * mnt_fs_get_bindsrc (struct libmnt_fs *fs); Parameters fs /run/mount/utab entry Returns full path that was used for mount(2) on MS_BIND ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-id ```text int mnt_fs_get_id (struct libmnt_fs *fs); Parameters fs /proc/self/mountinfo entry Returns mount ID (unique identifier of the mount) or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-root ```text const char * mnt_fs_get_root (struct libmnt_fs *fs); Parameters fs /proc/self/mountinfo entry Returns root of the mount within the filesystem or NULL ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-user-options ```text const char * mnt_fs_get_user_options (struct libmnt_fs *fs); Parameters fs fstab/mtab entry pointer Returns pointer to userspace mount option string or NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-match-options ```text int mnt_fs_match_options (struct libmnt_fs *fs,            const char *options); For more details see mnt_match_options(). Parameters fs filesystem options comma delimited list of options (and nooptions) Returns 1 if fs type is matching to options , else 0. The function returns 0 when types is NULL. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-match-target ```text int mnt_fs_match_target (struct libmnt_fs *fs,            const char *target,            struct libmnt_cache *cache); Possible are three attempts: 1) compare target with fs->target 2) realpath(target ) with fs->target 3) realpath(target ) with realpath(fs->target ) if fs is not from /proc/self/mountinfo. However, if mnt_cache_set_targets(cache, mtab) was called, and the path target or fs->target is found in the mtab , the canonicalization is is not performed (see mnt_resolve_target()). The 2nd and 3rd attempts are not performed when cache is NULL. Parameters fs filesystem target mountpoint path cache tags/paths cache or NULL Returns 1 if fs target is equal to target , else 0. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-prepend-attributes ```text int mnt_fs_prepend_attributes (struct libmnt_fs *fs,               const char *optstr); Prepends mount attributes. (See mnt_fs_set_attributes()). Parameters fs fstab/mtab/mountinfo entry optstr options string Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-attributes ```text int mnt_fs_set_attributes (struct libmnt_fs *fs,             const char *optstr); Sets mount attributes. The attributes are mount(2) and mount(8) independent options, these options are not sent to the kernel and are not interpreted by libmount. The attributes are stored in /run/mount/utab only. The attributes are managed by libmount in userspace only. It's possible that information stored in userspace will not be available for libmount after CLONE_FS unshare. Be careful, and don't use attributes if possible. Parameters fs fstab/mtab/mountinfo entry optstr options string Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-bindsrc ```text int mnt_fs_set_bindsrc (struct libmnt_fs *fs,           const char *src); Parameters fs filesystem src path Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-root ```text int mnt_fs_set_root (struct libmnt_fs *fs,          const char *path); Parameters fs mountinfo entry path root path Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-source ```text int mnt_fs_set_source (struct libmnt_fs *fs,           const char *source); This function creates a private copy (strdup()) of source . Parameters fs fstab/mtab/mountinfo entry source new source Returns 0 on success or negative number in case of error. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-streq-target ```text int mnt_fs_streq_target (struct libmnt_fs *fs,            const char *path); Compares fs target path with path . The redundant slashes are ignored. This function compares strings and does not canonicalize the paths. See also more generic mnt_fs_match_target(). Parameters fs fs path mount point Returns 1 if fs target path equal to path , otherwise 0. ``` ### Functions specific/exclusive to `/proc/swaps` - `mnt_fs_get_priority ` - `mnt_fs_get_size ` - `mnt_fs_get_swaptype ` - `mnt_fs_get_usedsize ` - `mnt_fs_set_priority ` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-priority ```text int mnt_fs_get_priority (struct libmnt_fs *fs); Parameters fs /proc/swaps entry Returns priority ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-size ```text off_t mnt_fs_get_size (struct libmnt_fs *fs); Parameters fs /proc/swaps entry Returns size ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-swaptype ```text const char * mnt_fs_get_swaptype (struct libmnt_fs *fs); Parameters fs /proc/swaps entry Returns swap type or NULL ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-get-usedsize ```text off_t mnt_fs_get_usedsize (struct libmnt_fs *fs); Parameters fs /proc/swaps entry Returns used size ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-priority ```text int mnt_fs_set_priority (struct libmnt_fs *fs,            int prio); Parameters fs /proc/swaps entry prio priority Returns 0 or -1 in case of error Since: 2.28 ``` M4 - What role does the `userdata` field in `libmnt_fs` play? For which file entry is it necessary? `fstab`, `mountinfo`, `utab`, `swaps`? https://github.com/util-linux/util-linux/blob/8aa25617467a1249669cff7240ca31973bf9a127/libmount/src/mountP.h#L237 ```C void *userdata; /* library independent data */ ``` M5 - Could anyone provide more information about the format used by the entries in `/run/mount/utab`? Are the definitions reproduced below correct? By reading the code for the `mnt_parse_utab_line` function, I identified the following keys for possible key-value pairs in `/run/mount/utab`. A cursory search on the web yielded these definitions: - `SRC`: the mounted device, - `TARGET`: the deviceâ??s mount point, - `ROOT`: ?? - `BINDSRC`: the source of a bind mount, - `OPTS`: mount options, - `ATTRS`: options independent from those used by the mount syscall and mount  command. They are neither sent to the kernel, nor interpreted by libmount. They  are stored in /run/mount/utab, and managed by libmount in userspace only. https://github.com/util-linux/util-linux/blob/8aa25617467a1249669cff7240ca31973bf9a127/libmount/src/tab_parse.c#L326-L368 ```C if (!fs->id && !strncmp(p, "ID=", 3)) { int rc = 0; end = next_s32(p + 3, &fs->id, &rc); if (!end || rc) return rc; } else if (!fs->source && !strncmp(p, "SRC=", 4)) { char *v = unmangle(p + 4, &end); if (!v) goto enomem; if (__mnt_fs_set_source_ptr(fs, v)) free(v); } else if (!fs->target && !strncmp(p, "TARGET=", 7)) { fs->target = unmangle(p + 7, &end); if (!fs->target) goto enomem; } else if (!fs->root && !strncmp(p, "ROOT=", 5)) { fs->root = unmangle(p + 5, &end); if (!fs->root) goto enomem; } else if (!fs->bindsrc && !strncmp(p, "BINDSRC=", 8)) { fs->bindsrc = unmangle(p + 8, &end); if (!fs->bindsrc) goto enomem; } else if (!fs->user_optstr && !strncmp(p, "OPTS=", 5)) { fs->user_optstr = unmangle(p + 5, &end); if (!fs->user_optstr) goto enomem; } else if (!fs->attrs && !strncmp(p, "ATTRS=", 6)) { fs->attrs = unmangle(p + 6, &end); if (!fs->attrs) goto enomem; } else { /* unknown variable */ while (*p && *p != ' ') p++; } ``` M6 - Can the `SRC` key mentioned in question `M5` have a: - tag (UUID, PARTUUID, LABEL, etc.) - network ID (Samba: `smb://ip-address-or-hostname/shared-dir`, SSHFS:  `user@ip-address-or-hostname:/shared-dir`, etc.)  as value? (e.g. SRC="UUID=ac4f36bf-191b-4fb0-b808-6d7fc9fc88be") M7 - What does `mnt_fs_set_root` do? Is it meant for `/proc/self/mountinfo`, `/run/mount/utab`, or both? Although `libmnt_fs` can represent a line in `/proc/self/mountinfo`, files in `/proc` are usually read-only, managed directly by the Linux kernel. However, the docstring of `mnt_fs_set_root` says that it is supposed to modify an entry in `/proc/self/mountinfo`. https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-root ```text int mnt_fs_set_root (struct libmnt_fs *fs,          const char *path); Parameters   fs mountinfo entry   path root path Returns  0 on success or negative number in case of error. ``` M8 - What is `mnt_context_apply_fstab` supposed to do? The documentation only says "This function is optional". https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Library-high-level-context.html#mnt-context-apply-fstab ```text int mnt_context_apply_fstab (struct libmnt_context *cxt); This function is optional. Parameters  cxt mount context Returns  0 on success, negative number in case of error. ``` M9 - Why does `mnt_optstr_prepend_option("", "ro", "recursive")` return `"ro=recursive,"`, but `mnt_optstr_append_option("", "ro", "recursive")` return `"ro=recursive"`? What is the extra comma after the keyword `recursive` output by `mnt_optstr_prepend_option` for? https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Options-string.html#mnt-optstr-prepend-option ```text int mnt_optstr_prepend_option (char **optstr,               const char *name,               const char *value); Parameters optstr option string or NULL, returns a reallocated string name value name value value Returns 0 on success or <0 in case of error. After an error the optstr should be unmodified. ``` https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Options-string.html#mnt-optstr-append-option ```text int mnt_optstr_append_option (char **optstr,              const char *name,              const char *value); Parameters optstr option string or NULL, returns a reallocated string name value name value value Returns 0 on success or <0 in case of error. After an error the optstr should be unmodified. ``` M10 - In option maps, what does using the mask `MNT_INVERT` do to mountflags (e.g. `noatime`, `suid`, others)? https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Option-maps.html#MNT-INVERT:CAPS ```text #define MNT_INVERT (1 << 1) /* invert the mountflag */ ``` M11 - Which definition is correct, or am I conflating two notions? The function `mnt_fs_set_freq` sets the backup frequency (in days) of a drive in `/etc/fstab`. According to `libmount`, it takes an integer value. But, from [An introduction to the Linux /etc/fstab file](https://www.redhat.com/en/blog/etc-fstab) the value in the fifth column on each line` in `/etc/fstab` can only take two values, `0 = no backup` or `1 = dump utility backup of a partition`. > Backup Operation: (the first digit) this is a binary system where 1 = dump > utility backup of a partition. 0 = no backup. This is an outdated backup > method and should NOT be used. https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.39/libmount-docs/libmount-Filesystem.html#mnt-fs-set-freq ```text int mnt_fs_set_freq (struct libmnt_fs *fs,          int freq); Parameters fs fstab/mtab entry pointer freq dump frequency in days Returns 0 on success or negative number in case of error. ``` Thank you for reading this far. Please accept my apologies for making you go through this wall of text m(__)m
