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.Dd February 2, 2012

.Dt ARCHIVE_READ 3

.Os

.Sh NAME

.Nm archive_read

.Nd functions for reading streaming archives

.Sh LIBRARY

Streaming Archive Library (libarchive, -larchive)

.Sh SYNOPSIS

.In archive.h

.Sh DESCRIPTION

These functions provide a complete API for reading streaming archives.

The general process is to first create the

.Tn struct archive

object, set options, initialize the reader, iterate over the archive

headers and associated data, then close the archive and release all

resources.

.\"

.Ss Create archive object

See

.Xr archive_read_new 3 .

.Pp

To read an archive, you must first obtain an initialized

.Tn struct archive

object from

.Fn archive_read_new .

.\"

.Ss Enable filters and formats

See

.Xr archive_read_filter 3

and

.Xr archive_read_format 3 .

.Pp

You can then modify this object for the desired operations with the

various

.Fn archive_read_set_XXX

and

.Fn archive_read_support_XXX

functions.

In particular, you will need to invoke appropriate

.Fn 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

.Fn archive_read_support_filter_all

and

.Fn archive_read_support_format_all

to enable auto-detect for all formats and compression types

currently supported by the library.

.\"

.Ss Set options

See

.Xr archive_read_set_options 3 .

.\"

.Ss Open archive

See

.Xr archive_read_open 3 .

.Pp

Once you have prepared the

.Tn struct archive

object, you call

.Fn 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,

.Ft "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.

.\"

.Ss Consume archive

See

.Xr archive_read_header 3 ,

.Xr archive_read_data 3

and

.Xr archive_read_extract 3 .

.Pp

Each archive entry consists of a header followed by a certain

amount of data.

You can obtain the next header with

.Fn archive_read_next_header ,

which returns a pointer to an

.Tn 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

.Fn archive_read_data

(which works much like the

.Xr read 2

system call)

to read this data from the archive, or

.Fn archive_read_data_block

which provides a slightly more efficient interface.

You may prefer to use the higher-level

.Fn archive_read_data_skip ,

which reads and discards the data for this entry,

.Fn archive_read_data_into_fd ,

which copies the data to the provided file descriptor, or

.Fn archive_read_extract ,

which recreates the specified entry on disk and copies data

from the archive.

In particular, note that

.Fn archive_read_extract

uses the

.Tn 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.

.\"

.Ss Release resources

See

.Xr archive_read_free 3 .

.Pp

Once you have finished reading data from the archive, you

should call

.Fn archive_read_close

to close the archive, then call

.Fn archive_read_free

to release all resources, including all memory allocated by the library.

.\"

.Sh EXAMPLE

The following illustrates basic usage of the library.

In this example,

the callback functions are simply wrappers around the standard

.Xr open 2 ,

.Xr read 2 ,

and

.Xr close 2

system calls.

.Bd -literal -offset indent

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\en",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);

}

.Ed

.\" .Sh ERRORS

.Sh SEE ALSO

.Xr tar 1 ,

.Xr libarchive 3 ,

.Xr archive_read_new 3 ,

.Xr archive_read_data 3 ,

.Xr archive_read_extract 3 ,

.Xr archive_read_filter 3 ,

.Xr archive_read_format 3 ,

.Xr archive_read_header 3 ,

.Xr archive_read_open 3 ,

.Xr archive_read_set_options 3 ,

.Xr archive_util 3 ,

.Xr tar 5

.Sh HISTORY

The

.Nm libarchive

library first appeared in

.Fx 5.3 .

.Sh AUTHORS

.An -nosplit

The

.Nm libarchive

library was written by

.An Tim Kientzle Aq kientzle@acm.org .

.Sh BUGS

Many traditional archiver programs treat

empty files as valid empty archives.

For example, many implementations of

.Xr 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

.Dq empty

format.