archive_read.3

.\" Copyright (c) 2003-2007 Tim Kientzle
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.Dd August 19, 2006
.Dt archive_read 3
.Os
.Sh NAME
.Nm archive_read_new ,
.Nm archive_read_support_compression_all ,
.Nm archive_read_support_compression_bzip2 ,
.Nm archive_read_support_compression_compress ,
.Nm archive_read_support_compression_gzip ,
.Nm archive_read_support_compression_none ,
.Nm archive_read_support_compression_program ,
.Nm archive_read_support_format_all ,
.Nm archive_read_support_format_cpio ,
.Nm archive_read_support_format_empty ,
.Nm archive_read_support_format_iso9660 ,
.Nm archive_read_support_format_tar ,
.Nm archive_read_support_format_zip ,
.Nm archive_read_open ,
.Nm archive_read_open2 ,
.Nm archive_read_open_fd ,
.Nm archive_read_open_FILE ,
.Nm archive_read_open_filename ,
.Nm archive_read_open_memory ,
.Nm archive_read_next_header ,
.Nm archive_read_data ,
.Nm archive_read_data_block ,
.Nm archive_read_data_skip ,
.\" #if ARCHIVE_API_VERSION < 3
.Nm archive_read_data_into_buffer ,
.\" #endif
.Nm archive_read_data_into_fd ,
.Nm archive_read_extract ,
.Nm archive_read_extract_set_progress_callback ,
.Nm archive_read_close ,
.Nm archive_read_finish
.Nd functions for reading streaming archives
.Sh SYNOPSIS
.In archive.h
.Ft struct archive *
.Fn archive_read_new "void"
.Ft int
.Fn archive_read_support_compression_all "struct archive *"
.Ft int
.Fn archive_read_support_compression_bzip2 "struct archive *"
.Ft int
.Fn archive_read_support_compression_compress "struct archive *"
.Ft int
.Fn archive_read_support_compression_gzip "struct archive *"
.Ft int
.Fn archive_read_support_compression_none "struct archive *"
.Ft int
.Fn archive_read_support_compression_program "struct archive *" "const char *cmd"
.Ft int
.Fn archive_read_support_format_all "struct archive *"
.Ft int
.Fn archive_read_support_format_cpio "struct archive *"
.Ft int
.Fn archive_read_support_format_empty "struct archive *"
.Ft int
.Fn archive_read_support_format_iso9660 "struct archive *"
.Ft int
.Fn archive_read_support_format_tar "struct archive *"
.Ft int
.Fn archive_read_support_format_zip "struct archive *"
.Ft int
.Fn archive_read_open "struct archive *" "void *client_data" "archive_open_callback *" "archive_read_callback *" "archive_close_callback *"
.Ft int
.Fn archive_read_open2 "struct archive *" "void *client_data" "archive_open_callback *" "archive_read_callback *" "archive_skip_callback *" "archive_close_callback *"
.Ft int
.Fn archive_read_open_FILE "struct archive *" "FILE *file"
.Ft int
.Fn archive_read_open_fd "struct archive *" "int fd" "size_t block_size"
.Ft int
.Fn archive_read_open_filename "struct archive *" "const char *filename" "size_t block_size"
.Ft int
.Fn archive_read_open_memory "struct archive *" "void *buff" "size_t size"
.Ft int
.Fn archive_read_next_header "struct archive *" "struct archive_entry **"
.Ft ssize_t
.Fn archive_read_data "struct archive *" "void *buff" "size_t len"
.Ft int
.Fn archive_read_data_block "struct archive *" "const void **buff" "size_t *len" "off_t *offset"
.Ft int
.Fn archive_read_data_skip "struct archive *"
.\" #if ARCHIVE_API_VERSION < 3
.Ft int
.Fn archive_read_data_into_buffer "struct archive *" "void *" "ssize_t len"
.\" #endif
.Ft int
.Fn archive_read_data_into_fd "struct archive *" "int fd"
.Ft int
.Fn archive_read_extract "struct archive *" "struct archive_entry *" "int flags"
.Ft void
.Fn archive_read_extract_set_progress_callback "struct archive *" "void (*func)(void *)" "void *user_data"
.Ft int
.Fn archive_read_close "struct archive *"
.Ft int
.Fn archive_read_finish "struct archive *"
.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.
The following summary describes the functions in approximately the
order they would be used:
.Bl -tag -compact -width indent
.It Fn archive_read_new
Allocates and initializes a
.Tn struct archive
object suitable for reading from an archive.
.It Fn archive_read_support_compression_all , Fn archive_read_support_compression_bzip2 , Fn archive_read_support_compression_compress , Fn archive_read_support_compression_gzip , Fn archive_read_support_compression_none
Enables auto-detection code and decompression support for the
specified compression.
Note that
.Dq none
is always enabled by default.
For convenience,
.Fn archive_read_support_compression_all
enables all available decompression code.
.It Fn archive_read_support_compression_program
Data is fed through the specified external program before being dearchived.
Note that this disables automatic detection of the compression format,
so it makes no sense to specify this in conjunction with any other
decompression option.
.It Fn archive_read_support_format_all , Fn archive_read_support_format_cpio , Fn archive_read_support_format_empty , Fn archive_read_support_format_iso9660 , Fn archive_read_support_format_tar, Fn archive_read_support_format_zip
Enables support---including auto-detection code---for the
specified archive format.
For example,
.Fn archive_read_support_format_tar
enables support for a variety of standard tar formats, old-style tar,
ustar, pax interchange format, and many common variants.
For convenience,
.Fn archive_read_support_format_all
enables support for all available formats.
Only empty archives are supported by default.
.It Fn archive_read_open
The same as
.Fn archive_read_open2 ,
except that the skip callback is assumed to be
.Dv NULL .
.It Fn archive_read_open2
Freeze the settings, open the archive, and prepare for reading entries.
This is the most generic version of this call, which accepts
four callback functions.
Most clients will want to use
.Fn archive_read_open_filename ,
.Fn archive_read_open_FILE ,
.Fn archive_read_open_fd ,
or
.Fn archive_read_open_memory
instead.
The library invokes the client-provided functions to obtain
raw bytes from the archive.
.It Fn archive_read_open_FILE
Like
.Fn archive_read_open ,
except that it accepts a
.Ft "FILE *"
pointer.
This function should not be used with tape drives or other devices
that require strict I/O blocking.
.It Fn archive_read_open_fd
Like
.Fn archive_read_open ,
except that it accepts a file descriptor and block size rather than
a set of function pointers.
Note that the file descriptor will not be automatically closed at
end-of-archive.
This function is safe for use with tape drives or other blocked devices.
.It Fn archive_read_open_file
This is a deprecated synonym for
.Fn archive_read_open_filename .
.It Fn archive_read_open_filename
Like
.Fn archive_read_open ,
except that it accepts a simple filename and a block size.
A NULL filename represents standard input.
This function is safe for use with tape drives or other blocked devices.
.It Fn archive_read_open_memory
Like
.Fn archive_read_open ,
except that it accepts a pointer and size of a block of
memory containing the archive data.
.It Fn archive_read_next_header
Read the header for the next entry and return a pointer to
a
.Tn struct archive_entry .
.It Fn archive_read_data
Read data associated with the header just read.
Internally, this is a convenience function that calls
.Fn archive_read_data_block
and fills any gaps with nulls so that callers see a single
continuous stream of data.
.It Fn archive_read_data_block
Return the next available block of data for this entry.
Unlike
.Fn archive_read_data ,
the
.Fn archive_read_data_block
function avoids copying data and allows you to correctly handle
sparse files, as supported by some archive formats.
The library guarantees that offsets will increase and that blocks
will not overlap.
Note that the blocks returned from this function can be much larger
than the block size read from disk, due to compression
and internal buffer optimizations.
.It Fn archive_read_data_skip
A convenience function that repeatedly calls
.Fn archive_read_data_block
to skip all of the data for this archive entry.
.\" #if ARCHIVE_API_VERSION < 3
.It Fn archive_read_data_into_buffer
This function is deprecated and will be removed.
Use
.Fn archive_read_data
instead.
.\" #endif
.It Fn archive_read_data_into_fd
A convenience function that repeatedly calls
.Fn archive_read_data_block
to copy the entire entry to the provided file descriptor.
.It Fn archive_read_extract , Fn archive_read_extract_set_skip_file
A convenience function that wraps the corresponding
.Xr archive_write_disk 3
interfaces.
The first call to
.Fn archive_read_extract
creates a restore object using
.Xr archive_write_disk_new 3
and
.Xr archive_write_disk_set_standard_lookup 3 ,
then transparently invokes
.Xr archive_write_disk_set_options 3 ,
.Xr archive_write_header 3 ,
.Xr archive_write_data 3 ,
and
.Xr archive_write_finish_entry 3
to create the entry on disk and copy data into it.
The
.Va flags
argument is passed unmodified to
.Xr archive_write_disk_set_options 3 .
.It Fn archive_read_extract_set_progress_callback
Sets a pointer to a user-defined callback that can be used
for updating progress displays during extraction.
The progress function will be invoked during the extraction of large
regular files.
The progress function will be invoked with the pointer provided to this call.
Generally, the data pointed to should include a reference to the archive
object and the archive_entry object so that various statistics
can be retrieved for the progress display.
.It Fn archive_read_close
Complete the archive and invoke the close callback.
.It Fn archive_read_finish
Invokes
.Fn archive_read_close
if it was not invoked manually, then release all resources.
Note: In libarchive 1.x, this function was declared to return
.Ft void ,
which made it impossible to detect certain errors when
.Fn archive_read_close
was invoked implicitly from this function.
The declaration is corrected beginning with libarchive 2.0.
.El
.Pp
Note that the library determines most of the relevant information about
the archive by inspection.
In particular, it automatically detects
.Xr gzip 1
or
.Xr bzip2 1
compression and transparently performs the appropriate decompression.
It also automatically detects the archive format.
.Pp
A complete description of the
.Tn struct archive
and
.Tn struct archive_entry
objects can be found in the overview manual page for
.Xr libarchive 3 .
.Sh CLIENT CALLBACKS
The callback functions must match the following prototypes:
.Bl -item -offset indent
.It
.Ft typedef ssize_t
.Fn archive_read_callback "struct archive *" "void *client_data" "const void **buffer"
.It
.\" #if ARCHIVE_API_VERSION < 2
.Ft typedef int
.Fn archive_skip_callback "struct archive *" "void *client_data" "size_t request"
.\" #else
.\" .Ft typedef off_t
.\" .Fn archive_skip_callback "struct archive *" "void *client_data" "off_t request"
.\" #endif
.It
.Ft typedef int
.Fn archive_open_callback "struct archive *" "void *client_data"
.It
.Ft typedef int
.Fn archive_close_callback "struct archive *" "void *client_data"
.El
.Pp
The open callback is invoked by
.Fn archive_open .
It should return
.Cm ARCHIVE_OK
if the underlying file or data source is successfully
opened.
If the open fails, it should call
.Fn archive_set_error
to register an error code and message and return
.Cm ARCHIVE_FATAL .
.Pp
The read callback is invoked whenever the library
requires raw bytes from the archive.
The read callback should read data into a buffer,
set the
.Li const void **buffer
argument to point to the available data, and
return a count of the number of bytes available.
The library will invoke the read callback again
only after it has consumed this data.
The library imposes no constraints on the size
of the data blocks returned.
On end-of-file, the read callback should
return zero.
On error, the read callback should invoke
.Fn archive_set_error
to register an error code and message and
return -1.
.Pp
The skip callback is invoked when the
library wants to ignore a block of data.
The return value is the number of bytes actually
skipped, which may differ from the request.
If the callback cannot skip data, it should return
zero.
If the skip callback is not provided (the
function pointer is
.Dv NULL ),
the library will invoke the read function
instead and simply discard the result.
A skip callback can provide significant
performance gains when reading uncompressed
archives from slow disk drives or other media
that can skip quickly.
.Pp
The close callback is invoked by archive_close when
the archive processing is complete.
The callback should return
.Cm ARCHIVE_OK
on success.
On failure, the callback should invoke
.Fn archive_set_error
to register an error code and message and
return
.Cm ARCHIVE_FATAL.
.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_compression_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_finish(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);
}
.Ed
.Sh RETURN VALUES
Most functions return zero on success, non-zero on error.
The possible return codes include:
.Cm ARCHIVE_OK
(the operation succeeded),
.Cm ARCHIVE_WARN
(the operation succeeded but a non-critical error was encountered),
.Cm ARCHIVE_EOF
(end-of-archive was encountered),
.Cm ARCHIVE_RETRY
(the operation failed but can be retried),
and
.Cm ARCHIVE_FATAL
(there was a fatal error; the archive should be closed immediately).
Detailed error codes and textual descriptions are available from the
.Fn archive_errno
and
.Fn archive_error_string
functions.
.Pp
.Fn archive_read_new
returns a pointer to a freshly allocated
.Tn struct archive
object.
It returns
.Dv NULL
on error.
.Pp
.Fn archive_read_data
returns a count of bytes actually read or zero at the end of the entry.
On error, a value of
.Cm ARCHIVE_FATAL ,
.Cm ARCHIVE_WARN ,
or
.Cm ARCHIVE_RETRY
is returned and an error code and textual description can be retrieved from the
.Fn archive_errno
and
.Fn archive_error_string
functions.
.Pp
The library expects the client callbacks to behave similarly.
If there is an error, you can use
.Fn archive_set_error
to set an appropriate error code and description,
then return one of the non-zero values above.
(Note that the value eventually returned to the client may
not be the same; many errors that are not critical at the level
of basic I/O can prevent the archive from being properly read,
thus most I/O errors eventually cause
.Cm ARCHIVE_FATAL
to be returned.)
.\" .Sh ERRORS
.Sh SEE ALSO
.Xr tar 1 ,
.Xr archive 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.