ISmount

ismount is a C library function used on Unix-like systems to determine whether a given path is a mount point—i.e., a directory where another filesystem is attached. On supported systems, it returns 1 if the path is a mount point, 0 if it is not, and -1 on error with errno set.

Most implementations determine this by comparing the device id (st_dev) of the path with that of its parent directory. A common approach is to stat the path and then stat path/..; if the device IDs differ, the path is a mount point. Note that behavior may vary with symbolic links and special mounts; some systems do not follow symlinks, or may fail to detect certain types of mounts such as bind mounts or automounted filesystems.

ismount is typically declared in the header sys/mount.h and is available on Linux and many BSDs, but it is not part of the POSIX standard. Availability and exact semantics can differ across platforms; in portable code you may supplement it with other checks or use higher-level utilities like the findmnt command or the mountpoint helper.

Typical usage patterns include checking a path before performing unmount operations or when code needs to be aware of filesystem boundaries. When ismount returns -1, programs should inspect errno to determine the cause, such as ENOENT for a nonexistent path or EACCES for permissions.