Using stat to determine programmatically whether a file is a mount point.

mountpoint.c

#include <assert.h>
#include <sys/stat.h>
#include <stdint.h>
#include <libgen.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char * argv[]) {

  struct stat file_stat;
  struct stat parent_stat;
  char * file_name;
  char * parent_name;

  assert(argc == 2);

  file_name = argv[1];

  /* get the parent directory  of the file */
  parent_name = dirname(file_name);

  /* get the file's stat info */
  if( -1 == stat(file_name, &file_stat) ) {
    perror("Stat: ");
    goto fail;
  }

  /* determine whether the supplied file is a directory
    if it isn't, then it can't be a mountpoint. */
  if( !(file_stat.st_mode & S_IFDIR) ) {
    printf("%s is not a directory.\n", file_name);
    goto fail;
  }

  /* get the parent's stat info */
  if( -1 == stat(parent_name, &parent_stat) ) {
    perror("Stat: ");
    goto fail;
  }

  /* print out device ids and inodes for each file */
  printf("Parent directory: %s\n", parent_name);
  printf("Files's dev ID: %u\n", file_stat.st_dev);
  printf("Parent's dev ID: %u\n", parent_stat.st_dev);
  printf("Files's inode: %llu\n", (uint64_t)file_stat.st_ino);
  printf("Parent's inode: %llu\n", (uint64_t)parent_stat.st_ino);

  /* if file and parent have different device ids,
    then the file is a mount point
    or, if they refer to the same file,
    then it's probably the root directory '/'
    and therefore a mountpoint */
  if( file_stat.st_dev != parent_stat.st_dev || 
    ( file_stat.st_dev == parent_stat.st_dev &&
      file_stat.st_ino == parent_stat.st_ino 
    ) 
  ) {
    printf("%s IS a mountpoint.\n", file_name);
  } else {
    printf("%s is NOT a mountpoint.\n", file_name);
  }

  free(parent_name);
  return 0;

  fail:

  free(parent_name);
  return 1;
  
}