12-07-2009
Here is an oversimplified analogy:
Think of a hard link as a pointer in the filesystem. Just like in C, it has to be dereferenced. But instead of what a pointer does: refer to a separate place in memory, with a hardlink we are referred to a place on the physical disk. The link acts as a proxy for the "real" filename.
The C pointer uses very little memory, but can reference a giant object in memory.
A hard link uses very little disks space but can reference a huge file.
How hard links are implemented is not by a universally defined object, but is up to the filesystem and the drivers to resolve. This is like C: a pointer can be a "big-endian" or a "little-endian" object in memory, and the compiler works out how to reference the object the pointer is "aimed" at. The filesystem + driver does the same. We do not worry about how to figure it out at all.
Links have consequences. One of them is that you do not ever want dangling links - links "aimed" at non-existent. It is possible to create a series: link -> link -> physical file. This is where it is easy to get in trouble. You can create circular links in some circumstances.
Copy duplicates a whole file. Link references a whole file.
Last edited by jim mcnamara; 12-07-2009 at 12:10 PM..
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LEARN ABOUT REDHAT
symlink
SYMLINK(2) Linux Programmer's Manual SYMLINK(2)
NAME
symlink - make a new name for a file
SYNOPSIS
#include <unistd.h>
int symlink(const char *oldpath, const char *newpath);
DESCRIPTION
symlink creates a symbolic link named newpath which contains the string oldpath.
Symbolic links are interpreted at run-time as if the contents of the link had been substituted into the path being followed to find a file
or directory.
Symbolic links may contain .. path components, which (if used at the start of the link) refer to the parent directories of that in which
the link resides.
A symbolic link (also known as a soft link) may point to an existing file or to a nonexistent one; the latter case is known as a dangling
link.
The permissions of a symbolic link are irrelevant; the ownership is ignored when following the link, but is checked when removal or renam-
ing of the link is requested and the link is in a directory with the sticky bit set.
If newpath exists it will not be overwritten.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EPERM The filesystem containing newpath does not support the creation of symbolic links.
EFAULT oldpath or newpath points outside your accessible address space.
EACCES Write access to the directory containing newpath is not allowed for the process's effective uid, or one of the directories in new-
path did not allow search (execute) permission.
ENAMETOOLONG
oldpath or newpath was too long.
ENOENT A directory component in newpath does not exist or is a dangling symbolic link, or oldpath is the empty string.
ENOTDIR
A component used as a directory in newpath is not, in fact, a directory.
ENOMEM Insufficient kernel memory was available.
EROFS newpath is on a read-only filesystem.
EEXIST newpath already exists.
ELOOP Too many symbolic links were encountered in resolving newpath.
ENOSPC The device containing the file has no room for the new directory entry.
EIO An I/O error occurred.
NOTES
No checking of oldpath is done.
Deleting the name referred to by a symlink will actually delete the file (unless it also has other hard links). If this behaviour is not
desired, use link.
CONFORMING TO
SVr4, SVID, POSIX, BSD 4.3. SVr4 documents additional error codes SVr4, SVID, BSD 4.3, X/OPEN. SVr4 documents additional error codes
EDQUOT and ENOSYS. See open(2) re multiple files with the same name, and NFS.
SEE ALSO
readlink(2), link(2), unlink(2), rename(2), open(2), lstat(2), ln(1)
Linux 2.0.30 1997-08-21 SYMLINK(2)