NAME¶
archive_read —
functions for reading
streaming archives
LIBRARY¶
Streaming Archive Library (libarchive, -larchive)
SYNOPSIS¶
#include <archive.h>
DESCRIPTION¶
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.
Create archive object¶
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.
Set options¶
See
archive_read_set_options(3).
Open archive¶
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.
Consume archive¶
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_to_file(), 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.
Release resources¶
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.
EXAMPLE¶
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);
}
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);
}
SEE ALSO¶
tar(1),
libarchive(3),
archive_read_new(3),
archive_read_data(3),
archive_read_extract(3),
archive_read_filter(3),
archive_read_format(3),
archive_read_header(3),
archive_read_open(3),
archive_read_set_options(3),
archive_util(3),
tar(5)
HISTORY¶
The
libarchive library first appeared in
FreeBSD 5.3.
AUTHORS¶
The
libarchive library was written by
Tim
Kientzle ⟨kientzle@acm.org⟩.
BUGS¶
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.