A Gentle Programming Primer

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.

The most obvious is probably that all I/O operations return a GnomeVFSResult indicating the success or failure of the operation. More on this later.
The types may be slightly different (but still parallel), for example rather than using an int for a file-descriptor, GnomeVFS uses GnomeVFSHandle, a handle to a particular URI.
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 gnome_vfs_open and gnome_vfs_open_uri.

By way of example, consider the basic read command:

	ssize_t read (int fd, void *buf, size_t count);

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 bytes_read:

	GnomeVFSResult gnome_vfs_read (GnomeVFSHandle *handle,
	                               gpointer buffer,
                                       GnomeVFSFileSize bytes,
                                       GnomeVFSFileSize *bytes_read);

So gnome_vfs_read takes a GnomeVFSHandle, which functions like a file descriptor, and attempts to read bytes bytes out of handle into buffer. The number of bytes succesfully read into buffer is returned in the pointer bytes_read. The return value of the function, a GnomeVFSResult indicates the success of the operation or any errors that might have occurred (for example, permission denied). GnomeVFSResult is just an enumeration.

Simple Sample Program

Now lets write a simple program to copy a fixed number of bytes from one file and append it to another file.


#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;
}

A Gentle Programming Primer

Simple Sample Program

Conversion of a Sample Code Block