Creating a file of arbitrary size using Windows C++ API

Question

I would like to create a file of arbitrary size using the Windows C/C++ API. I am using Windows XP service pack 2 with a 32 bit virtual address memory space. I am familiar with CreateFile.

However CreateFile does not have a size arument, The reason I want to pass in a size argument is to allow me to create memory mapping files which allow the user to access data structures of predetermined size. Could you please advise of the proper Windows C/C++ API function which allow me to create a file of arbritrary predetermined size? Thank you

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;
   }
}