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.