ARCHIVE_ENTRY_LINKIFY(3) BSD Library Functions Manual ARCHIVE_ENTRY_LINKIFY(3)NAME
archive_entry_linkify — hardlink resolver functions
Streaming Archive Library (libarchive, -larchive)
struct archive_entry_linkresolver *
archive_entry_linkresolver_set_strategy(struct archive_entry_linkresolver *resolver,
archive_entry_linkresolver_free(struct archive_entry_linkresolver *resolver);
archive_entry_linkify(struct archive_entry_linkresolver *resolver,
struct archive_entry **entry, struct archive_entry **sparse);
Programs that want to create archives have to deal with hardlinks.
Hardlinks are handled in different ways by the archive formats. The
basic strategies are:
1. Ignore hardlinks and store the body for each reference (old cpio,
2. Store the body the first time an inode is seen (ustar, pax).
3. Store the body the last time an inode is seen (new cpio).
The archive_entry_linkresolver functions help by providing a unified
interface and handling the complexity behind the scene.
The archive_entry_linkresolver functions assume that archive_entry
instances have valid nlinks, inode and device values. The inode and
device value is used to match entries. The nlinks value is used to
determined if all references have been found and if the internal refer‐
ences can be recycled.
The archive_entry_linkresolver_new() function allocates a new link
resolver. The instance can be freed using
archive_entry_linkresolver_free(). All deferred entries are flushed and
the internal storage is freed.
The archive_entry_linkresolver_set_strategy() function selects the opti‐
mal hardlink strategy for the given format. The format code can be
obtained from archive_format(3). The function can be called more than
once, but it is recommended to flush all deferred entries first.
The archive_entry_linkify() function is the core of
archive_entry_linkresolver. The entry() argument points to the
archive_entry that should be written. Depending on the strategy one of
the following actions is taken:
1. For the simple archive formats *entry is left unmodified and *sparse
is set to NULL.
2. For tar like archive formats, *sparse is set to NULL. If *entry is
NULL, no action is taken. If the hardlink count of *entry is larger
than 1 and the file type is a regular file or symbolic link, the
internal list is searched for a matching inode. If such an inode is
found, the link count is decremented and the file size of *entry is
set to 0 to notify that no body should be written. If no such inode
is found, a copy of the entry is added to the internal cache with a
link count reduced by one.
3. For new cpio like archive formats a value for *entry of NULL is used
to flush deferred entries. In that case *entry is set to an arbi‐
trary deferred entry and the entry itself is removed from the inter‐
nal list. If the internal list is empty, *entry is set to NULL. In
either case, *sparse is set to NULL and the function returns. If
the hardlink count of *entry is one or the file type is a directory
or device, *sparse is set to NULL and no further action is taken.
Otherwise, the internal list is searched for a matching inode. If
such an inode is not found, the entry is added to the internal list,
both *entry and *sparse are set to NULL and the function returns.
If such an inode is found, the link count is decremented. If it
remains larger than one, the existing entry on the internal list is
swapped with *entry after retaining the link count. The existing
entry is returned in *entry. If the link count reached one, the new
entry is also removed from the internal list and returned in
*sparse. Otherwise *sparse is set to NULL.
The general usage is therefore:
1. For each new archive entry, call archive_entry_linkify().
2. Keep in mind that the entries returned may have a size of 0 now.
3. If *entry is not NULL, archive it.
4. If *sparse is not NULL, archive it.
5. After all entries have been written to disk, call
archive_entry_linkify() with *entry set to NULL and archive the
returned entry as long as it is not NULL.
RETURN VALUESarchive_entry_linkresolver_new() returns NULL on malloc(3) failures.
SEE ALSOarchive_entry(3)BSD February 2, 2012 BSD