ARCHIVE_ENTRY_LINKIFY(3) Introduction to Library Functions

NAME

archive_entry_linkresolver, archive_entry_linkresolver_new,
archive_entry_linkresolver_set_strategy,
archive_entry_linkresolver_free, archive_entry_linkify - hardlink
resolver functions

LIBRARY

Streaming Archive Library (libarchive, -larchive)

SYNOPSIS

#include <archive_entry.h>

struct archive_entry_linkresolver *
archive_entry_linkresolver_new(void);

void
archive_entry_linkresolver_set_strategy(struct archive_entry_linkresolver *resolver,
int format);

void
archive_entry_linkresolver_free(struct archive_entry_linkresolver *resolver);

void
archive_entry_linkify(struct archive_entry_linkresolver *resolver,
struct archive_entry **entry, struct archive_entry **sparse);

DESCRIPTION

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,
zip).

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
references 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
optimal 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
arbitrary deferred entry and the entry itself is removed from the
internal 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 VALUES

archive_entry_linkresolver_new() returns NULL on malloc(3) failures.

SEE ALSO

archive_entry(3)

illumos February 2, 2012 illumos