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.Dd February 2, 2012
.Dt ARCHIVE_WRITE 3
.Os
.Sh NAME
.Nm archive_write
.Nd functions for creating archives
.Sh LIBRARY
Streaming Archive Library (libarchive, -larchive)
.Sh SYNOPSIS
.In archive.h
.Sh DESCRIPTION
These functions provide a complete API for creating streaming
archive files.
The general process is to first create the
.Tn struct archive
object, set any desired options, initialize the archive, append entries, then
close the archive and release all resources.
.\"
.Ss Create archive object
See
.Xr archive_write_new 3 .
.Pp
To write an archive, you must first obtain an initialized
.Tn struct archive
object from
.Fn archive_write_new .
.\"
.Ss Enable filters and formats, configure block size and padding
See
.Xr archive_write_filter 3 ,
.Xr archive_write_format 3
and
.Xr archive_write_blocksize 3 .
.Pp
You can then modify this object for the desired operations with the
various
.Fn archive_write_set_XXX
functions.
In particular, you will need to invoke appropriate
.Fn archive_write_add_XXX
and
.Fn archive_write_set_XXX
functions to enable the corresponding compression and format
support.
.\"
.Ss Set options
See
.Xr archive_read_set_options 3 .
.\"
.Ss Open archive
See
.Xr archive_write_open 3 .
.Pp
Once you have prepared the
.Tn struct archive
object, you call
.Fn archive_write_open
to actually open the archive and prepare it for writing.
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 write the archive data.
.\"
.Ss Produce archive
See
.Xr archive_write_header 3
and
.Xr archive_write_data 3 .
.Pp
Individual archive entries are written in a three-step
process:
You first initialize a
.Tn struct archive_entry
structure with information about the new entry.
At a minimum, you should set the pathname of the
entry and provide a
.Va struct stat
with a valid
.Va st_mode
field, which specifies the type of object and
.Va st_size
field, which specifies the size of the data portion of the object.
.\"
.Ss Release resources
See
.Xr archive_write_free 3 .
.Pp
After all entries have been written, use the
.Fn archive_write_free
function to release all resources.
.\"
.Sh EXAMPLE
The following sketch illustrates basic usage of the library.
In this example,
the callback functions are simply wrappers around the standard
.Xr open 2 ,
.Xr write 2 ,
and
.Xr close 2
system calls.
.Bd -literal -offset indent
#ifdef __linux__
#define	_FILE_OFFSET_BITS 64
#endif
#include <sys/stat.h>
#include <archive.h>
#include <archive_entry.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

struct mydata {
  const char *name;
  int fd;
};

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

  mydata->fd = open(mydata->name, O_WRONLY | O_CREAT, 0644);
  if (mydata->fd >= 0)
    return (ARCHIVE_OK);
  else
    return (ARCHIVE_FATAL);
}

la_ssize_t
mywrite(struct archive *a, void *client_data, const void *buff, size_t n)
{
  struct mydata *mydata = client_data;

  return (write(mydata->fd, buff, n));
}

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

  if (mydata->fd > 0)
    close(mydata->fd);
  return (0);
}

void
write_archive(const char *outname, const char **filename)
{
  struct mydata *mydata = malloc(sizeof(struct mydata));
  struct archive *a;
  struct archive_entry *entry;
  struct stat st;
  char buff[8192];
  int len;
  int fd;

  a = archive_write_new();
  mydata->name = outname;
  /* Set archive format and filter according to output file extension.
   * If it fails, set default format. Platform depended function.
   * See supported formats in archive_write_set_format_filter_by_ext.c */
  if (archive_write_set_format_filter_by_ext(a, outname) != ARCHIVE_OK)  {
    archive_write_add_filter_gzip(a);
    archive_write_set_format_ustar(a);
  }  
  archive_write_open(a, mydata, myopen, mywrite, myclose);
  while (*filename) {
    stat(*filename, &st);
    entry = archive_entry_new();
    archive_entry_copy_stat(entry, &st);
    archive_entry_set_pathname(entry, *filename);
    archive_write_header(a, entry);
    if ((fd = open(*filename, O_RDONLY)) != -1) {
      len = read(fd, buff, sizeof(buff));
      while (len > 0) {
        archive_write_data(a, buff, len);
        len = read(fd, buff, sizeof(buff));
      }
      close(fd);
    }
    archive_entry_free(entry);
    filename++;
  }
  archive_write_free(a);
}

int main(int argc, const char **argv)
{
  const char *outname;
  argv++;
  outname = *argv++;
  write_archive(outname, argv);
  return 0;
}
.Ed
.Sh SEE ALSO
.Xr tar 1 ,
.Xr libarchive 3 ,
.Xr archive_write_set_options 3 ,
.Xr cpio 5 ,
.Xr mtree 5 ,
.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
There are many peculiar bugs in historic tar implementations that may cause
certain programs to reject archives written by this library.
For example, several historic implementations calculated header checksums
incorrectly and will thus reject valid archives; GNU tar does not fully support
pax interchange format; some old tar implementations required specific
field terminations.
.Pp
The default pax interchange format eliminates most of the historic
tar limitations and provides a generic key/value attribute facility
for vendor-defined extensions.
One oversight in POSIX is the failure to provide a standard attribute
for large device numbers.
This library uses
.Dq SCHILY.devminor
and
.Dq SCHILY.devmajor
for device numbers that exceed the range supported by the backwards-compatible
ustar header.
These keys are compatible with Joerg Schilling's
.Nm star
archiver.
Other implementations may not recognize these keys and will thus be unable
to correctly restore device nodes with large device numbers from archives
created by this library.