Creación de un archivo de tamaño arbitrario usando API de Windows C ++

Question

Me gustaría crear un archivo de tamaño arbitrario usando la API de Windows C / C ++. Estoy utilizando Windows XP Service Pack 2 con un espacio de memoria virtual de direcciones de 32 bits. Estoy familiarizado con CreateFile.

Sin embargo CreateFile no tiene un tamaño arument, La razón por la que quiero pasar en un argumento tamaño es permitir a mí para crear archivos de asignación de memoria que permiten al usuario acceso a las estructuras de datos de tamaño predeterminado. ¿Podría por favor avise de la correcta función API de Windows C / C ++ que me permiten crear un archivo de arbritrary tamaño predeterminado? Gracias

Answer 1

To do this on UNIX, seek to (RequiredFileSize - 1) and then write a byte. The value of the byte can be anything, but zero is the obvious choice.

Answer 2

You CreateFile as usual, SetFilePointerEx to the desired size and then call SetEndOfFile.

Answer 3

You don't need a file, you can use the pagefile as the backing for your memory mapped file, from the MSDN CreateFileMapping function page:

If hFile is INVALID_HANDLE_VALUE, the calling process must also specify a size for the file mapping object in the dwMaximumSizeHigh and dwMaximumSizeLow parameters. In this scenario, CreateFileMapping creates a file mapping object of a specified size that is backed by the system paging file instead of by a file in the file system.

You can still share the mapping object by use of DuplicateHandle.

Answer 4

according to your comments, you actually need cross-platform solution, so check Boost Interprocess library. it provides cross-platform shared memory facilities and more

Answer 5

to do this on Linux, you can do the following:

/**
 *  Clear the umask permissions so we 
 *  have full control of the file creation (see man umask on Linux)
 */
mode_t origMask = umask(0);

int fd = open("/tmp/file_name",
      O_RDWR, 00666);

umask(origMask);
if (fd < 0)
{
  perror("open fd failed");
  return;
}


if (ftruncate(fd, size) == 0)
{
   int result = lseek(data->shmmStatsDataFd, size - 1, SEEK_SET);
   if (result == -1)
   {
     perror("lseek fd failed");
     close(fd);
     return ;
   }

   /* Something needs to be written at the end of the file to
    * have the file actually have the new size.
    * Just writing an empty string at the current file position will do.
    *newDataSize
    * Note:
    *  - The current position in the file is at the end of the stretched
    *    file due to the call to lseek().
    *  - An empty string is actually a single '\0' character, so a zero-byte
    *    will be written at the last byte of the file.
    */
   result = data->write(fd, "", 1);
   if (result != 1)
   {
     perror("write fd failed");
     close(fd);

     return;
   }
}