How to tell if a file is on tmpfs given its path on Linux?

Question

This might be a dumb question, but suppose I'm given a file path and I'd like to know if it points to a file on tmpfs (that is, it's an in-memory file). How can I do that using only Linux system calls? (That is, I can't go to the shell.)

Answer 1

Use the statfs syscall and see if the returned f_type field is TMPFS_MAGIC.

Here's a small utility demonstrating this:

#include <sys/vfs.h>                                                         
#include <linux/magic.h>                                                     
#include <stdio.h>                                                           

int main(int argc, char** argv) {                                            
  struct statfs info;                                                        
  statfs(argv[1], &info);                                                    

  if (info.f_type == TMPFS_MAGIC) {                                          
    printf("It's tmpfs\n");                                                  
    return 0;                                                                
  } else {                                                                   
    printf("It's not tmpfs\n");                                              
    return 1;                                                                
  }                                                                          
}

Example:

$ ./isittmpfs /etc/passwd
It's not tmpfs

$ ./isittmpfs /dev/shm/pulse-shm-1358569836 
It's tmpfs

(NB: This is just an example of how to determine if a file is on tmpfs through syscalls. This answer does not suggest dropping to a shell even though the example code is invoked from a shell)