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.Dd February 2, 2012
.Dt ARCHIVE_READ_OPEN 3
.Os
.Sh NAME
.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
.Nd functions for reading streaming archives
.Sh LIBRARY
Streaming Archive Library (libarchive, -larchive)
.Sh SYNOPSIS
.In archive.h
.Ft int
.Fo archive_read_open
.Fa "struct archive *"
.Fa "void *client_data"
.Fa "archive_open_callback *"
.Fa "archive_read_callback *"
.Fa "archive_close_callback *"
.Fc
.Ft int
.Fo archive_read_open2
.Fa "struct archive *"
.Fa "void *client_data"
.Fa "archive_open_callback *"
.Fa "archive_read_callback *"
.Fa "archive_skip_callback *"
.Fa "archive_close_callback *"
.Fc
.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
.Fo archive_read_open_filename
.Fa "struct archive *"
.Fa "const char *filename"
.Fa "size_t block_size"
.Fc
.Ft int
.Fn archive_read_open_memory "struct archive *" "const void *buff" "size_t size"
.Sh DESCRIPTION
.Bl -tag -compact -width indent
.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.
.El
.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 la_ssize_t
.Fo archive_read_callback
.Fa "struct archive *"
.Fa "void *client_data"
.Fa "const void **buffer"
.Fc
.It
.Ft typedef la_int64_t
.Fo archive_skip_callback
.Fa "struct archive *"
.Fa "void *client_data"
.Fa "off_t request"
.Fc
.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
.\"
.Sh RETURN VALUES
These functions return
.Cm ARCHIVE_OK
on success, or
.Cm ARCHIVE_FATAL .
.\"
.Sh ERRORS
Detailed error codes and textual descriptions are available from the
.Fn archive_errno
and
.Fn archive_error_string
functions.
.\"
.Sh SEE ALSO
.Xr tar 1 ,
.Xr archive_read 3 ,
.Xr archive_read_data 3 ,
.Xr archive_read_filter 3 ,
.Xr archive_read_format 3 ,
.Xr archive_read_set_options 3 ,
.Xr archive_util 3 ,
.Xr libarchive 3 ,
.Xr tar 5
