CMake/Utilities/cmlibarchive/libarchive/archive_read.3

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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.