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Many operating systems support file links including symbolic (soft) links, hard links, short cuts, and UNC (universal naming convention) shares. Symbolic links can be created in POSIX using the ln -s command, and hard links using the ln command. Hard links are indistinguisable indistinguishable from normal files on POSIX sysetmssystems.

There are three types of file links supported in the NTFS file system: hard links, junctions, and symbolic links. Symbolic links are available in NTFS starting with Windows Vista.

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When creating new files, it may be possible to use functions that only create a new file where a file does not already exist. This prevents the application from overwriting an existing file during file creation; see rule void FIO00-J. Do not overwrite an existing file while attempting to create a new file for more information. Existing Java classes such as FileOutputStream() and FileWriter() do not

allow a programmer to specify that opening a file should fail if the file already exists
indicate whether an existing file has been opened or a new file has been created

These limitations may lead to a program overwriting or accessing an unintended file.

Device Files

File names on many operating systems may be used to access device files. Reserved MS-DOS device names include AUX, CON, PRN, COM1, and LPT1. Character special files and block special files on POSIX systems are used to apply access rights and to direct operations on the files to the appropriate device drivers.

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On many systems, file can be be simlutaneouslysimultaneously accessed by concurrent processes.  Exclusive access grants unrestricted file access to the locking process while denying access to all other processes, eliminating the potential for a race condition on the locked region. The {{java.nio.channels.FileLock}} class facilitates file locking. According to the Java API \[[API 2006|AA. Bibliography#API 06]\] documentation

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Code Block

bgColor	#ffcccc

String filename = /* provided by user */
Path file = new File(filename).toPath();
try {
  BasicFileAttributes attr = Files.readAttributes(
    file, BasicFileAttributes.class, LinkOption.NOFOLLOW_LINKS
  );

  // Check
  if (!attr.isRegularFile()) {
    System.out.println("Not a regular file");
    return;
  }
  // other necessary checks

  // Use
  try (InputStream in = Files.newInputStream(file)) {

      // read file
    };
} catch (IOException x) {
  // handle error
}

This code is still vulnerabile vulnerable to a TOCTOU race condition, however. For example, an attacker can replace the regular file with a file link or device file after the code has completed its checks but before it opens the file.

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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="1d38b9fd73e2cb18-9ea4a56b-468b491f-bf929751-12e82fdfe9671bd459acd6ca"><ac:plain-text-body><![CDATA[	[[Darwin 2004	AA. Bibliography#Darwin 04]]	11.5 Creating a Transient File	]]></ac:plain-text-body></ac:structured-macro>
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Device Files