ARCHIVE_WRITE(3) manual page 
== NAME == 
'''archive_write''' 
- functions for creating archives 
== LIBRARY == 
Streaming Archive Library (libarchive, -larchive) 
== SYNOPSIS == 
'''<nowiki>#include <archive.h></nowiki>''' 
== DESCRIPTION == 
These functions provide a complete API for creating streaming 
archive files. 
The general process is to first create the 
'''struct archive''' 
object, set any desired options, initialize the archive, append entries, then 
close the archive and release all resources. 
=== Create archive object=== 
See 
[[ManPageArchiveWriteNew3]]. 

To write an archive, you must first obtain an initialized 
'''struct archive''' 
object from 
'''archive_write_new'''(). 
=== Enable filters and formats, configure block size and padding=== 
See 
[[ManPageArchiveWriteFilter3]], 
[[ManPageArchiveWriteFormat3]] 
and 
[[ManPageArchiveWriteBlocksize3]]. 

You can then modify this object for the desired operations with the 
various 
'''archive_write_set_XXX'''() 
functions. 
In particular, you will need to invoke appropriate 
'''archive_write_add_XXX'''() 
and 
'''archive_write_set_XXX'''() 
functions to enable the corresponding compression and format 
support. 
=== Set options=== 
See 
[[ManPageArchiveReadSetOptions3]]. 
=== Open archive=== 
See 
[[ManPageArchiveWriteOpen3]]. 

Once you have prepared the 
'''struct archive''' 
object, you call 
'''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, 
''FILE *'' 
object, or a block of memory from which to write the archive data. 
=== Produce archive=== 
See 
[[ManPageArchiveWriteHeader3]] 
and 
[[ManPageArchiveWriteData3]]. 

Individual archive entries are written in a three-step 
process: 
You first initialize a 
'''struct archive_entry''' 
structure with information about the new entry. 
At a minimum, you should set the pathname of the 
entry and provide a 
''struct'' stat 
with a valid 
''st_mode'' 
field, which specifies the type of object and 
''st_size'' 
field, which specifies the size of the data portion of the object. 
=== Release resources=== 
See 
[[ManPageArchiveWriteFree3]]. 

After all entries have been written, use the 
'''archive_write_free'''() 
function to release all resources. 
== EXAMPLE == 
The following sketch illustrates basic usage of the library. 
In this example, 
the callback functions are simply wrappers around the standard 
[[open(2)|http://www.freebsd.org/cgi/man.cgi?query=open&sektion=2]], 
[[write(2)|http://www.freebsd.org/cgi/man.cgi?query=write&sektion=2]], 
and 
[[close(2)|http://www.freebsd.org/cgi/man.cgi?query=close&sektion=2]] 
system calls. 
```text
#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;
}
```
== SEE ALSO == 
[[ManPageBsdtar1]], 
[[ManPageLibarchive3]], 
[[ManPageArchiveWriteSetOptions3]], 
[[ManPageCpio5]], 
[[ManPageMtree5]], 
[[ManPageTar5]] 
== HISTORY == 
The 
'''libarchive''' 
library first appeared in 
FreeBSD 5.3. 
== AUTHORS == 
The 
'''libarchive''' 
library was written by 
Tim Kientzle  &lt;kientzle@acm.org.&gt; 
== 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. 

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 
"SCHILY.devminor" 
and 
"SCHILY.devmajor" 
for device numbers that exceed the range supported by the backwards-compatible 
ustar header. 
These keys are compatible with Joerg Schilling's 
'''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. 
