how to use lstat() to determine if hard link or not

问题

My OS is linux. I program in C. I know I can use the lstat() to recognize the soft link, i.e., use S_ISLNK(st.st_mode). But how can I recognize the link is hard link? if the link is hard link, it will be thought of as regular file. However, I also want to distinguish the regular file from the hard link. Are there any ways to handle this case?

回答1:

But how can I recognize the link is hard link?

You can't.

A "hard link" isn't actually anything special. It's just a directory entry that happens to point to the same data on disk as a directory entry somewhere else. The only way to reliably identify hard links is to map all the paths on your filesystem to inodes, and then see which ones point to the same value.

回答2:

struct stat has st_nlink member for number of hard links. It is > 1, file is being stated in one of the hard links to actual file content.

struct stat {
    dev_t     st_dev;     /* ID of device containing file */
    ino_t     st_ino;     /* inode number */
    mode_t    st_mode;    /* protection */
    nlink_t   st_nlink;   /* number of hard links */
    uid_t     st_uid;     /* user ID of owner */
    gid_t     st_gid;     /* group ID of owner */
    dev_t     st_rdev;    /* device ID (if special file) */
    off_t     st_size;    /* total size, in bytes */
    blksize_t st_blksize; /* blocksize for file system I/O */
    blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
    time_t    st_atime;   /* time of last access */
    time_t    st_mtime;   /* time of last modification */
    time_t    st_ctime;   /* time of last status change */
};

Here is sample program:

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
int main()
{
    struct stat buf = {0};
    lstat("origfile", &buf);
    printf("number of hard links for origfile: %d\n", buf.st_nlink);
}

output:

$ touch origfile
$ ./a.out
number of hard links for origfile: 1
$ ln origfile hardlink1
$ ./a.out
number of hard links for origfile: 2
$ ln origfile hardlink2
$ ./a.out
number of hard links for origfile: 3

来源：https://stackoverflow.com/questions/40564474/how-to-use-lstat-to-determine-if-hard-link-or-not

标签

Linux

symlink

stat

hardlink