path: root/lib/libarchive/archive_read.3

.\" Copyright (c) 2003-2004 Tim Kientzle
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.Dd October 1, 2003
.Dt archive_read 3
.Os
.Sh NAME
.Nm archive_read_new ,
.Nm archive_read_set_bytes_per_block ,
.Nm archive_read_support_compression_none ,
.Nm archive_read_support_compression_gzip ,
.Nm archive_read_support_compression_bzip2 ,
.Nm archive_read_support_compression_all ,
.Nm archive_read_support_format_tar ,
.Nm archive_read_support_format_gnutar ,
.Nm archive_read_support_format_cpio ,
.Nm archive_read_support_format_all ,
.Nm archive_read_open ,
.Nm archive_read_open_fd ,
.Nm archive_read_open_file ,
.Nm archive_read_next_header ,
.Nm archive_read_data ,
.Nm archive_read_data_skip ,
.Nm archive_read_data_into_buffer ,
.Nm archive_read_data_into_file ,
.Nm archive_read_extract ,
.Nm archive_read_finish
.Nd functions for reading tar archives
.Sh SYNOPSIS
.In archive.h
.Ft struct archive *
.Fn archive_read_new "void"
.Ft int
.Fn archive_read_set_bytes_per_block "struct archive *" "int"
.Ft int
.Fn archive_read_support_compression_none "struct archive *"
.Ft int
.Fn archive_read_support_compression_gzip "struct archive *"
.Ft int
.Fn archive_read_support_compression_bzip2 "struct archive *"
.Ft int
.Fn archive_read_support_compression_all "struct archive *"
.Ft int
.Fn archive_read_support_format_tar "struct archive *"
.Ft int
.Fn archive_read_support_format_gnutar "struct archive *"
.Ft int
.Fn archive_read_support_format_cpio "struct archive *"
.Ft int
.Fn archive_read_support_format_all "struct archive *"
.Ft int
.Fn archive_read_open "struct archive *" "void *client_data" "archive_read_archive_callback *" "archive_open_archive_callback *" "archive_close_archive_callback *"
.Ft int
.Fn archive_read_open_fd "struct archive *" "int fd"
.Ft int
.Fn archive_read_open_file "struct archive *" "const char *filename"
.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_skip "struct archive *"
.Ft int
.Fn archive_read_data_into_buffer "struct archive *" "void *"
.Ft int
.Fn archive_read_data_into_file "struct archive *" "int fd"
.Ft int
.Fn archive_read_extract "struct archive *" "int flags"
.Ft void
.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_set_bytes_per_block
Sets the block size used for reading the archive data.
This controls the size that will be used when invoking the read
callback function.
The default is 20 records or 10240 bytes for tar formats.
.It Fn archive_read_support_compression_XXX
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_format_XXX
Enables support---including auto-detection code---for the
specified archive format.
In particular,
.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.
Note that there is no default.
.It Fn archive_read_open
Freeze the settings, open the archive, and prepare for reading entries.
This is the most generic version of this call, which accepts
three callback functions.
The library invokes these client-provided functions to obtain
raw bytes from the archive.
Note: The API permits a decompression method to fork and invoke the
callbacks from another process.
Although none of the current decompression methods use this technique,
future decompression methods may utilize this technique.
If the decompressor forks, it will ensure that the open and close
callbacks are invoked within the same process as the read callback.
In particular, clients should not attempt to use shared variables to
communicate between the open/read/close callbacks and the mainline code.
.It Fn archive_read_open_fd
Like
.Fn archive_read_open ,
except that it accepts a file descriptor rather than
a trio of function pointers.
Note that the file descriptor will not be automatically closed at
end-of-archive.
.It Fn archive_read_open_file
Like
.Fn archive_read_open ,
except that it accepts a simple filename.
A NULL filename represents standard input.
.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.
.It Fn archive_read_data_skip
A convenience function that repeatedly calls
.Fn archive_read_data
to skip all of the data for this archive entry.
.It Fn archive_read_data_into_buffer
A convenience function that repeatedly calls
.Fn archive_read_data
to copy the entire entry into the client-supplied buffer.
Note that the client is responsible for sizing the buffer appropriately.
.It Fn archive_read_data_into_file
A convenience function that repeatedly calls
.Fn archive_read_data
to copy the entire entry to the provided file descriptor.
.It Fn archive_read_extract
A convenience function that recreates the specified object on
disk and reads the entry data into that object.
The
.Va flags
argument modifies how the object is recreated.
It consists of a bitwise OR of one or more of the following values:
.Bl -tag -compact -width "indent"
.It Cm ARCHIVE_EXTRACT_OWNER
The user and group IDs should be set on the restored file.
By default, the user and group IDs are not restored.
.It Cm ARCHIVE_EXTRACT_PERM
The permissions (mode bits) should be restored for all objects.
By default, permissions are only restored for regular files.
.It Cm ARCHIVE_EXTRACT_TIME
The timestamps (mtime, ctime, and atime) should be restored.
By default, they are ignored.
Note that restoring of atime is not currently supported.
.It Cm ARCHIVE_EXTRACT_NO_OVERWRITE
Existing files on disk will not be overwritten.
By default, existing files are unlinked before the new entry is written.
.It Cm ARCHIVE_EXTRACT_UNLINK
Existing files on disk will be unlinked and recreated from scratch.
By default, existing files are truncated and rewritten, but
the file is not recreated.
In particular, the default behavior does not break existing hard links.
.El
.It Fn archive_read_finish
Complete the archive, invoke the close callback, and release
all resources.
.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
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
.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
These callback functions are called whenever the library requires
raw bytes from the archive.
Note that it is the client's responsibility to correctly
block the input.
.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 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_READ_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);
}

int
myclose(struct archive *a, void *client_data)
{
  struct mydata *mydata = client_data;

  if (mydata->fd > 0)
    close(mydata->fd);
  return (0);
}
.Ed
.Sh RETURN VALUES
Most functions return zero on success, non-zero on error.
The possible return codes include:
.Cm ARCHIVE_READ_OK
(the operation succeeded)
.Cm ARCHIVE_READ_WARN
(the operation succeeded but a non-critical error was encountered)
.Cm ARCHIVE_READ_EOF
(the operation failed because end-of-archive was encountered),
.Cm ARCHIVE_READ_RETRY
(the operation failed but can be retried),
and
.Cm ARCHIVE_READ_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 errors eventually cause
.Cm ARCHIVE_FATAL
to be returned.)
.\" .Sh ERRORS
.Sh SEE ALSO
.Xr tar 1 ,
.Xr archive 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
The support for GNU tar formats is somewhat limited and should be improved.