ARCHIVE_READ(3) | Library Functions Manual | ARCHIVE_READ(3) |
archive_read
—
functions for reading streaming archives
Streaming Archive Library (libarchive, -larchive)
#include
<archive.h>
These functions provide a complete API for reading streaming archives. The general process is to first create the struct archive object, set options, initialize the reader, iterate over the archive headers and associated data, then close the archive and release all resources.
See archive_read_new(3).
To read an archive, you must first obtain an
initialized struct archive object from
archive_read_new
().
See archive_read_filter(3) and archive_read_format(3).
You can then modify this object for the
desired operations with the various
archive_read_set_XXX
()
and
archive_read_support_XXX
()
functions. In particular, you will need to invoke appropriate
archive_read_support_XXX
() functions to enable the
corresponding compression and format support. Note that these latter
functions perform two distinct operations: they cause the corresponding
support code to be linked into your program, and they enable the
corresponding auto-detect code. Unless you have specific constraints, you
will generally want to invoke
archive_read_support_filter_all
()
and
archive_read_support_format_all
()
to enable auto-detect for all formats and compression types currently
supported by the library.
See archive_read_open(3).
Once you have prepared the struct archive
object, you call
archive_read_open
()
to actually open the archive and prepare it for reading. There are several
variants of this function; the most basic expects you to provide pointers to
several functions that can provide blocks of bytes from the archive. There
are convenience forms that allow you to specify a filename, file descriptor,
FILE * object, or a block of memory from which to read
the archive data. Note that the core library makes no assumptions about the
size of the blocks read; callback functions are free to read whatever block
size is most appropriate for the medium.
See archive_read_header(3), archive_read_data(3) and archive_read_extract(3).
Each archive entry consists of a
header followed by a certain amount of data. You can obtain the next header
with
archive_read_next_header
(),
which returns a pointer to an struct archive_entry structure with
information about the current archive element. If the entry is a regular
file, then the header will be followed by the file data. You can use
archive_read_data
()
(which works much like the read(2) system
call) to read this data from the archive, or
archive_read_data_block
()
which provides a slightly more efficient interface. You may prefer to use
the higher-level
archive_read_data_skip
(),
which reads and discards the data for this entry,
archive_read_data_into_fd
(),
which copies the data to the provided file descriptor, or
archive_read_extract
(),
which recreates the specified entry on disk and copies data from the
archive. In particular, note that
archive_read_extract
() uses the struct archive_entry
structure that you provide it, which may differ from the entry just read
from the archive. In particular, many applications will want to override the
pathname, file permissions, or ownership.
See archive_read_free(3).
Once you have finished reading data from
the archive, you should call
archive_read_close
()
to close the archive, then call
archive_read_free
()
to release all resources, including all memory allocated by the library.
The following illustrates basic usage of the library. In this example, the callback functions are simply wrappers around the standard open(2), read(2), and close(2) system calls.
void list_archive(const char *name) { struct mydata *mydata; struct archive *a; struct archive_entry *entry; mydata = malloc(sizeof(struct mydata)); a = archive_read_new(); mydata->name = name; archive_read_support_filter_all(a); archive_read_support_format_all(a); archive_read_open(a, mydata, myopen, myread, myclose); while (archive_read_next_header(a, &entry) == ARCHIVE_OK) { printf("%s\n",archive_entry_pathname(entry)); archive_read_data_skip(a); } archive_read_free(a); free(mydata); } la_ssize_t myread(struct archive *a, void *client_data, const void **buff) { struct mydata *mydata = client_data; *buff = mydata->buff; return (read(mydata->fd, mydata->buff, 10240)); } int myopen(struct archive *a, void *client_data) { struct mydata *mydata = client_data; mydata->fd = open(mydata->name, O_RDONLY); return (mydata->fd >= 0 ? ARCHIVE_OK : ARCHIVE_FATAL); } int myclose(struct archive *a, void *client_data) { struct mydata *mydata = client_data; if (mydata->fd > 0) close(mydata->fd); return (ARCHIVE_OK); }
tar(1), archive_read_data(3), archive_read_extract(3), archive_read_filter(3), archive_read_format(3), archive_read_header(3), archive_read_new(3), archive_read_open(3), archive_read_set_options(3), archive_util(3), libarchive(3), tar(5)
The libarchive
library first appeared in
FreeBSD 5.3.
The libarchive
library was written by
Tim Kientzle ⟨kientzle@acm.org⟩.
Many traditional archiver programs treat empty files as valid empty archives. For example, many implementations of tar(1) allow you to append entries to an empty file. Of course, it is impossible to determine the format of an empty file by inspecting the contents, so this library treats empty files as having a special “empty” format.
February 2, 2012 | macOS 14.6 |