source: trunk/third/gnome-vfs2/doc/about.sgml @ 20794

<chapter id="about">
  <title>Introduction to GnomeVFS</title>

  <sect1>
    <title>Uses and Purpose</title>
    <para>
      GnomeVFS is a filesystem abstraction library allowing applications
      plugable transparent access to a variety of "real" filesystems, from
      WebDAV to digital cameras, to the local filesystem. It also contains
      a number of other convenient file utilities such as a comphrehensive
      MIME database / Application registry, and a copy engine. Use of GnomeVFS
      ensures that an application or component will be usable by Nautilus
      or other GnomeVFS applications for handling the display of data from
      various URIs, as well 
    </para>
    <sect2>
      <title>User's Perspective</title>
      <para>
        From a user's perspective GnomeVFS enabled applications provide consistent
        access to their data, whether it be stored on remote servers or on their
        local harddisk, or even a peripheral device such as a Rio or a digital
        camera. Rather than having to work around the distinction between storage
        you can work off of and storage you can only "download" from or "upload" to,
        GnomeVFS allows users to store their documents and data wherever it is
        most convenient.
      </para>
    </sect2>
    <sect2>
      <title>Developer's Perspective</title>
      <para>
        Besides providing transparent access to data methods that you might
        otherwise have to implement, GnomeVFS provides a number of convenience
        libraries for processing URIs, detecting the MIME type of files, and
        even figuring out which applications or components to launch to view
        a file or what icon to use. Writing a GnomeVFS module may also be an
        appropriate solution to some data access problems as it allows the
        developer to implement a relatively small number of functions to gain
        general filesystem semantics (and of course, writing a GnomeVFS module
        benefits other applications too!).
      </para>
    </sect2>
  </sect1>

  <sect1 id="gnome-vfs-first-steps">
    <title>A Gentle Programming Primer</title>
    <para>
      Using GnomeVFS in an existing application, or writing a new application
      with it, is actually very simple since GnomeVFS tries to mimic POSIX
      file access syntax and semantics. That means that most "standard unix calls" 
      have a GnomeVFS equivalent that operates in a fairly similar manner. There are
      a few differences to keep in mind. 

      <itemizedlist>
        
        <listitem>
          <para>
            The most obvious is probably that all I/O operations return a <type>GnomeVFSResult</type>
            indicating the success or failure of the operation. More on this later.
          </para>
        </listitem>
        <listitem>
          <para>
            The types may be slightly different (but still parallel), for example rather than using an 
            <type>int</type> for a file-descriptor, GnomeVFS uses <type>GnomeVFSHandle</type>, a handle
            to a particular URI.
          </para>
        </listitem>
        <listitem>
          <para>
            Most operations come in Handle (think file descriptor) and URI form. The URI form may be
            more convenient if you do not want to track handles, etc, but just be aware that both are
            at your disposal and may be used interchangably. For example <function>gnome_vfs_open</function>
            and <function>gnome_vfs_open_uri</function>.
          </para>
        </listitem>
      </itemizedlist>
    </para>
    <para>
      By way of example, consider the basic read command:
      <programlisting>
        ssize_t read (int fd, void *buf, size_t count);
      </programlisting>
    </para>
    <para>
      The GnomeVFS equivalent is very similar, but you will notice slightly different data types. The
      consistent returning of a GnomeVFSResult also necessitated moving the return value of read into
      a pass-back-value pointer <parameter>bytes_read</parameter>:
      <programlisting>
        GnomeVFSResult gnome_vfs_read (GnomeVFSHandle *handle,
                                       gpointer buffer,
                                       GnomeVFSFileSize bytes,
                                       GnomeVFSFileSize *bytes_read);
      </programlisting>
    </para>
    <para>
      So <function>gnome_vfs_read</function> takes a <type>GnomeVFSHandle</type>, which functions
      like a file descriptor, and attempts to read <parameter>bytes</parameter> bytes out of
      <parameter>handle</parameter> into <parameter>buffer</parameter>. The number of bytes succesfully
      read into <parameter>buffer</parameter> is returned in the pointer <parameter>bytes_read</parameter>.
      The return value of the function, a <type>GnomeVFSResult</type> indicates the success of the
      operation or any errors that might have occurred (for example, permission denied).
      <type>GnomeVFSResult</type> is just an enumeration.
    </para>
    <sect2>
      <title>Simple Sample Program</title>
      <para>
        Now lets write a simple program to copy a fixed number of bytes from one file and append
        it to another file.
      </para>
      <para>
        <programlisting>
<![CDATA[
#include <libgnomevfs/gnome-vfs.h>
#include <gnome.h>

#define BYTES_TO_PROCESS 256

int print_error (GnomeVFSResult result, const char *uri_string);

int
main (int argc, char **argv)
{
  GnomeVFSHandle *read_handle, *write_handle;
  const char *input_uri_string = argv[1];
  const char *output_uri_string = argv[2];
  GnomeVFSFileSize bytes_read, bytes_written;
  guint buffer[BYTES_TO_PROCESS];
  GnomeVFSResult result;

  /* remember to initialize GnomeVFS! */
  if (!gnome_vfs_init ()) {
    printf ("Could not initialize GnomeVFS\n");
    return 1;
  }

  /* open the input file for read access */
  result = gnome_vfs_open (&read_handle, input_uri_string, GNOME_VFS_OPEN_READ);
  /* if the operation was not successful, print the error and abort */
  if (result != GNOME_VFS_OK) return print_error (result, input_uri_string);

  /* we use create instead of open, because open will not create the file if it does
     not already exist. The last argument is the permissions to use if the file is created,
     the second to last tells GnomeVFS that its ok if the file already exists, and just open it */
  result = gnome_vfs_create (&write_handle, output_uri_string, GNOME_VFS_OPEN_WRITE, FALSE, 0777);
  if (result != GNOME_VFS_OK) return print_error (result, output_uri_string);

  /* read data from the input uri */
  result = gnome_vfs_read (read_handle, buffer, BYTES_TO_PROCESS, &bytes_read);
  if (result != GNOME_VFS_OK) return print_error (result, input_uri_string);

  /* seek to the end of the output uri so we will append rather than overwrite */
  /* therefore, we seek 0 bytes relative to the end of the file */
  result = gnome_vfs_seek (write_handle, GNOME_VFS_SEEK_END, 0);

  /* now write the data we read out to the output uri */
  result = gnome_vfs_write (write_handle, buffer, bytes_read, &bytes_written);
  if (result != GNOME_VFS_OK) return print_error (result, output_uri_string);

  return 0;
}

int
print_error (GnomeVFSResult result, const char *uri_string)
{
  const char *error_string;
  /* get the string corresponding to this GnomeVFSResult value */
  error_string = gnome_vfs_result_to_string (result);
  printf ("Error %s occured opening location %s\n", error_string, uri_string);
  return 1;
}

]]>
        </programlisting>
</para>
    </sect2>
    <sect2>
        <title>Conversion of a Sample Code Block</title>
        <para>
        </para>
      </sect2>
  </sect1>
</chapter>