interface/wx/file.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        file.h
   3 // Purpose:     interface of wxTempFile, wxFile
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9
  10 /**
  11     @class wxTempFile
  12
  13     wxTempFile provides a relatively safe way to replace the contents of the
  14     existing file. The name is explained by the fact that it may be also used as
  15     just a temporary file if you don't replace the old file contents.
  16
  17     Usually, when a program replaces the contents of some file it first opens it for
  18     writing, thus losing all of the old data and then starts recreating it.
  19     This approach is not very safe because during the regeneration of the file bad
  20     things may happen: the program may find that there is an internal error preventing
  21     it from completing file generation, the user may interrupt it (especially if file
  22     generation takes long time) and, finally, any other external interrupts (power
  23     supply failure or a disk error) will leave you without either the original file
  24     or the new one.
  25
  26     wxTempFile addresses this problem by creating a temporary file which is meant to
  27     replace the original file - but only after it is fully written. So, if the user
  28     interrupts the program during the file generation, the old file won't be lost.
  29     Also, if the program discovers itself that it doesn't want to replace the old
  30     file there is no problem - in fact, wxTempFile will @b not replace the old
  31     file by default, you should explicitly call wxTempFile::Commit() to do it.
  32     Calling wxTempFile::Discard() explicitly discards any modifications: it
  33     closes and deletes the temporary file and leaves the original file unchanged.
  34     If you call neither Commit() nor Discard(), the destructor will
  35     call Discard() automatically.
  36
  37     To summarize: if you want to replace another file, create an instance of
  38     wxTempFile passing the name of the file to be replaced to the constructor.
  39     (You may also use default constructor and pass the file name to wxTempFile::Open.)
  40     Then you can write to wxTempFile using wxFile-like functions and later call
  41     wxTempFile::Commit() to replace the old file (and close this one) or call
  42     wxTempFile::Discard() to cancel the modifications.
  43
  44     @library{wxbase}
  45     @category{file}
  46 */
  47 class wxTempFile
  48 {
  49 public:
  50     /**
  51         Associates wxTempFile with the file to be replaced and opens it.
  52
  53         @warning
  54         You should use IsOpened() to verify that the constructor succeeded.
  55     */
  56     wxTempFile(const wxString& strName);
  57
  58     /**
  59         Destructor calls Discard() if temporary file is still open.
  60     */
  61     ~wxTempFile();
  62
  63     /**
  64         Validate changes: deletes the old file of name m_strName and renames the new
  65         file to the old name. Returns @true if both actions succeeded.
  66
  67         If @false is returned it may unfortunately mean two quite different things:
  68         either that the old file couldn't be deleted or that the new file
  69         couldn't be renamed to the old name.
  70     */
  71     bool Commit();
  72
  73     /**
  74         Discard changes: the old file contents are not changed, the temporary
  75         file is deleted.
  76     */
  77     void Discard();
  78
  79     /**
  80         Flush the data written to the file to disk.
  81
  82         This simply calls wxFile::Flush() for the underlying file and may be
  83         necessary with file systems such as XFS and Ext4 under Linux. Calling
  84         this function may however have serious performance implications and
  85         also is not necessary with many other file systems so it is not done by
  86         default -- but you can call it before calling Commit() to absolutely
  87         ensure that the data was indeed written to the disk correctly.
  88      */
  89     bool Flush();
  90
  91     /**
  92         Returns @true if the file was successfully opened.
  93     */
  94     bool IsOpened() const;
  95
  96     /**
  97         Returns the length of the file.
  98
  99         This method may return ::wxInvalidOffset if the length couldn't be
 100         determined or 0 even for non-empty files if the file is not seekable.
 101
 102         In general, the only way to determine if the file for which this function
 103         returns 0 is really empty or not is to try reading from it.
 104     */
 105     wxFileOffset Length() const;
 106
 107     /**
 108         Open the temporary file, returns @true on success, @false if an error
 109         occurred.
 110         @a strName is the name of file to be replaced. The temporary file is always
 111         created in the directory where @a strName is. In particular, if @a strName
 112         doesn't include the path, it is created in the current directory and the
 113         program should have write access to it for the function to succeed.
 114     */
 115     bool Open(const wxString& strName);
 116
 117     /**
 118         Seeks to the specified position.
 119     */
 120     wxFileOffset Seek(wxFileOffset ofs,
 121                       wxSeekMode mode = wxFromStart);
 122
 123     /**
 124         Returns the current position or ::wxInvalidOffset if file is not opened or
 125         if another error occurred.
 126     */
 127     wxFileOffset Tell() const;
 128
 129     /**
 130         Write to the file, return @true on success, @false on failure.
 131         The second argument is only meaningful in Unicode build of wxWidgets when
 132         @a conv is used to convert @a str to multibyte representation.
 133     */
 134     bool Write(const wxString& str,
 135                const wxMBConv& conv = wxConvUTF8);
 136 };
 137
 138
 139
 140 /**
 141     @class wxFile
 142
 143     A wxFile performs raw file I/O. This is a very small class designed to
 144     minimize the overhead of using it - in fact, there is hardly any overhead at
 145     all, but using it brings you automatic error checking and hides differences
 146     between platforms and compilers. wxFile also automatically closes the file in
 147     its destructor so you won't forget to do so.
 148     wxFile is a wrapper around @c file descriptor. - see also wxFFile for a
 149     wrapper around @c FILE structure.
 150
 151     ::wxFileOffset is used by the wxFile functions which require offsets as
 152     parameter or return them. If the platform supports it, wxFileOffset is a
 153     typedef for a native 64 bit integer, otherwise a 32 bit integer is used for
 154     ::wxFileOffset.
 155
 156     @library{wxbase}
 157     @category{file}
 158 */
 159 class wxFile
 160 {
 161 public:
 162
 163     /**
 164         The OpenMode enumeration defines the different modes for opening a file with wxFile.
 165         It is also used with wxFile::Access function.
 166     */
 167     enum OpenMode {
 168
 169         /** Open file for reading or test if it can be opened for reading with Access() */
 170         read,
 171
 172         /** Open file for writing deleting the contents of the file if it already exists
 173             or test if it can be opened for writing with Access(). */
 174         write,
 175
 176         /** Open file for reading and writing; can not be used with Access() */
 177         read_write,
 178
 179         /** Open file for appending: the file is opened for writing, but the old contents
 180             of the file are not erased and the file pointer is initially placed at the end
 181             of the file; can not be used with Access().
 182
 183             This is the same as OpenMode::write if the file doesn't exist.
 184         */
 185         write_append,
 186
 187         /**
 188             Open the file securely for writing (Uses O_EXCL | O_CREAT).
 189             Will fail if the file already exists, else create and open it atomically.
 190             Useful for opening temporary files without being vulnerable to race exploits.
 191         */
 192         write_excl
 193     };
 194
 195     /**
 196        Standard file descriptors
 197     */
 198     enum { fd_invalid = -1, fd_stdin, fd_stdout, fd_stderr };
 199
 200     /**
 201        Default constructor.
 202     */
 203     wxFile();
 204
 205     /**
 206         Opens a file with a filename.
 207
 208         @param filename
 209             The filename.
 210         @param mode
 211             The mode in which to open the file.
 212
 213         @warning
 214         You should use IsOpened() to verify that the constructor succeeded.
 215     */
 216     wxFile(const wxString& filename,
 217            wxFile::OpenMode mode = wxFile::read);
 218
 219     /**
 220         Associates the file with the given file descriptor, which has already been
 221         opened. See Attach() for the list of predefined descriptors.
 222
 223         @param fd
 224             An existing file descriptor.
 225     */
 226     wxFile(int fd);
 227
 228     /**
 229         Destructor will close the file.
 230         @note This destructor is not virtual so you should not use wxFile polymorphically.
 231     */
 232     ~wxFile();
 233
 234     /**
 235         This function verifies if we may access the given file in specified mode.
 236         Only values of @c wxFile::read or @c wxFile::write really make sense here.
 237     */
 238     static bool Access(const wxString& name, wxFile::OpenMode mode);
 239
 240     /**
 241         Attaches an existing file descriptor to the wxFile object.
 242         Examples of predefined file descriptors are 0, 1 and 2 which correspond to
 243         stdin, stdout and stderr (and have symbolic names of @c wxFile::fd_stdin,
 244         @c wxFile::fd_stdout and @c wxFile::fd_stderr).
 245
 246         The descriptor should be already opened and it will be closed by wxFile
 247         object.
 248     */
 249     void Attach(int fd);
 250
 251     /**
 252         Closes the file.
 253     */
 254     bool Close();
 255
 256     /**
 257         Creates a file for writing.
 258
 259         If the file already exists, setting @b overwrite to @true will ensure
 260         it is overwritten.
 261
 262         @a access may be an OR combination of the ::wxPosixPermissions enumeration
 263         values.
 264     */
 265     bool Create(const wxString& filename,
 266                 bool overwrite = false,
 267                 int access = wxS_DEFAULT);
 268
 269     /**
 270         Get back a file descriptor from wxFile object - the caller is responsible for
 271         closing the file if this descriptor is opened.
 272         IsOpened() will return @false after call to Detach().
 273     */
 274     void Detach();
 275
 276     /**
 277         Returns @true if the end of the file has been reached.
 278         Note that the behaviour of the file pointer-based class wxFFile is
 279         different as wxFFile::Eof() will return @true here only if an
 280         attempt has been made to read @b past the last byte of the file, while
 281         wxFile::Eof() will return @true even before such attempt is made if the
 282         file pointer is at the last position in the file.
 283
 284         Note also that this function doesn't work on unseekable file descriptors
 285         (examples include pipes, terminals and sockets under Unix) and an attempt to
 286         use it will result in an error message.
 287
 288         So, to read the entire file into memory, you should write a loop which uses
 289         Read() repeatedly and tests its return condition instead of using Eof()
 290         as this will not work for special files under Unix.
 291     */
 292     bool Eof() const;
 293
 294     /**
 295         Returns @true if the given name specifies an existing regular file
 296         (not a directory or a link).
 297     */
 298     static bool Exists(const wxString& filename);
 299
 300     /**
 301         Flushes the file descriptor.
 302
 303         Note that Flush() is not implemented on some Windows compilers due to a
 304         missing fsync function, which reduces the usefulness of this function
 305         (it can still be called but it will do nothing on unsupported compilers).
 306     */
 307     bool Flush();
 308
 309     /**
 310         Returns the type of the file.
 311     */
 312     wxFileKind GetKind() const;
 313
 314     /**
 315         Returns @true if the file has been opened.
 316     */
 317     bool IsOpened() const;
 318
 319     /**
 320         Returns the length of the file.
 321     */
 322     wxFileOffset Length() const;
 323
 324     /**
 325         Opens the file, returning @true if successful.
 326
 327         @param filename
 328             The filename.
 329         @param mode
 330             The mode in which to open the file.
 331         @param access
 332             An OR-combination of ::wxPosixPermissions enumeration values.
 333     */
 334     bool Open(const wxString& filename, wxFile::OpenMode mode = wxFile::read,
 335               int access = wxS_DEFAULT);
 336
 337     /**
 338         Reads from the file into a memory buffer.
 339
 340         @param buffer
 341            Buffer to write in
 342         @param count
 343            Bytes to read
 344
 345         @return The number of bytes read, or the symbol ::wxInvalidOffset.
 346     */
 347     ssize_t Read(void* buffer, size_t count);
 348
 349     /**
 350         Seeks to the specified position.
 351
 352         @param ofs
 353             Offset to seek to.
 354         @param mode
 355             One of wxFromStart, wxFromEnd, wxFromCurrent.
 356
 357         @return The actual offset position achieved, or ::wxInvalidOffset on
 358                 failure.
 359     */
 360     wxFileOffset Seek(wxFileOffset ofs,
 361                       wxSeekMode mode = wxFromStart);
 362
 363     /**
 364         Moves the file pointer to the specified number of bytes relative to the
 365         end of the file. For example, @c SeekEnd(-5) would position the pointer 5
 366         bytes before the end.
 367
 368         @param ofs
 369             Number of bytes before the end of the file.
 370
 371         @return The actual offset position achieved, or ::wxInvalidOffset on
 372                 failure.
 373     */
 374     wxFileOffset SeekEnd(wxFileOffset ofs = 0);
 375
 376     /**
 377         Returns the current position or ::wxInvalidOffset if file is not opened or
 378         if another error occurred.
 379     */
 380     wxFileOffset Tell() const;
 381
 382     /**
 383        Write data to the file (descriptor).
 384
 385        @param buffer
 386           Buffer from which to read data
 387        @param count
 388          Number of bytes to write
 389
 390        @return The number of bytes written.
 391     */
 392     size_t Write(const void *buffer, size_t count);
 393
 394     /**
 395         Writes the contents of the string to the file, returns @true on success.
 396         The second argument is only meaningful in Unicode build of wxWidgets when
 397         @a conv is used to convert @a s to a multibyte representation.
 398
 399         Note that this method only works with @c NUL-terminated strings, if you want
 400         to write data with embedded @c NULs to the file you should use the other
 401         Write() overload.
 402     */
 403     bool Write(const wxString& s, const wxMBConv& conv = wxConvUTF8);
 404
 405     /**
 406         Returns the file descriptor associated with the file.
 407     */
 408     int fd() const;
 409 };
 410