I believe it’s the mode of the inode…
umode_t i_mode;
To access the field see stat(2) man page:
The following POSIX macros are defined to check the file type using the st_mode field:
S_ISREG(m) is it a regular file?
S_ISDIR(m) directory?
S_ISCHR(m) character device?
S_ISBLK(m) block device?
S_ISFIFO(m) FIFO (named pipe)?
S_ISLNK(m) symbolic link? (Not in POSIX.1-1996.)
S_ISSOCK(m) socket? (Not in POSIX.1-1996.)
Here is some example code from the Linux driver for minix FS:
434 void minix_set_inode(struct inode *inode, dev_t rdev)
435 {
436 if (S_ISREG(inode->i_mode)) {
437 inode->i_op = &minix_file_inode_operations;
438 inode->i_fop = &minix_file_operations;
439 inode->i_mapping->a_ops = &minix_aops;
440 } else if (S_ISDIR(inode->i_mode)) {
441 inode->i_op = &minix_dir_inode_operations;
442 inode->i_fop = &minix_dir_operations;
443 inode->i_mapping->a_ops = &minix_aops;
444 } else if (S_ISLNK(inode->i_mode)) {
445 inode->i_op = &minix_symlink_inode_operations;
446 inode->i_mapping->a_ops = &minix_aops;
447 } else
448 init_special_inode(inode, inode->i_mode, rdev);
449 }
450