The rename() function has the following prototype.

int rename(char const *old, char const *new);

If the file referenced by new exists prior to calling rename(), the behavior is implementation-defined. On POSIX systems, the file is removed. On Windows systems, the rename() fails.

This creates issues when trying to write portable code, or when trying to implement alternative behavior.

New File Removed

Occasionally, a programmer might want to implements a behavior other than the implementation-defined behavior for rename() on that platform. If the desired behavior is to ensure that any file referenced by new is removed, Windows programmers must write additional code.

Non-Compliant Code Example (Windows)

If the intent of the programmer is to remove the file referenced by new if it exists prior to calling rename(), this code example is non-compliant on Windows platforms because it will fail, returns a nonzero value, and sets errno to EACCES.

char const *old = /* ... */;
char const *new = /* ... */;
if (rename(old, new) != 0) {
  /* Handle Error */
}

Compliant Solution (Windows)

On Windows systems, it is necessary to remove the file referenced by new before calling rename(), if you want the file to be overwritten and the rename() operation to succeed.

char const *old = /* ... */;
char const *new = /* ... */;

if (remove(new) != 0) {
  /* Handle error condition */
}

if (rename(old, new) != 0) {
  /* Handle error condition */
}

This code contains an unavoidable race condition between the call to remove() and the call to rename() and can consequently only be safely executed within a secure directory (see FIO17-A. Ensure that file operations are performed in a secure directory).

Compliant Solution (POSIX)

On POSIX systems, if the file referenced by new exists prior to calling rename(), the file is removed.

char const *old = /* ... */;
char const *new = /* ... */;
if (rename(old, new) != 0) {
  /* Handle error condition */
}

New File Not Removed

If the desired behavior is to ensure that any file referenced by new is not removed, POSIX programmers must write additional code.

Non-Compliant Code Example (POSIX)

If the intent of the programmer is to not remove the file referenced by new if it exists prior to calling rename(), this code example is non-compliant because on POSIX systems the file is removed.

char const *old = /* ... */;
char const *new = /* ... */;
if (rename(old, new) != 0) {
  /* Handle Error */
}

Compliant Solution (POSIX)

If the intent of the programmer is to not remove the file referenced by new if it exists prior to calling rename(), the POSIX access() function which can check for the existence of a file explicitly [[Open Group 04]]. This compliant solution only renames the file referenced by old if the file referenced by new does not exist.

char const *old = /* ... */;
char const *new = /* ... */;

if (access(new, F_OK) != 0) {
  if (rename(old, new) != 0) {
    /* Handle error condition */
  }
} 
else {
  /* Handle error condition */
}

This code contains an unavoidable race condition between the call to access() and the call to rename() and can consequently only be safely executed within a secure directory (see FIO17-A. Ensure that file operations are performed in a secure directory).

While the likelihood of access() returning a false negative is lower than that of fopen(), on file systems where the program does not have sufficient permissions in the directory to view the file, access() may return -1 even when the file exists. In such cases, rename() would also fail because the program lacks adequate permissions to perform the operation.

Compliant Solution (Windows)

On Windows, the rename() function fails if: [[MSDN]]

File or directory specified by newname already exists or could not be created (invalid path)

Consequently, it is unnecessary to explicitly check for the existence of the file referenced by new before calling rename().

char const *old = /* ... */;
char const *new = /* ... */;
if (rename(old, new) != 0) {
  /* Handle Error */
}

Portable Behavior

A programmer that wants to an application to behave the same on any C99 implementation must first determine what behavior to implement.

Compliant Solution (New File Removed)

This compliant solution ensures that any file referenced by new is portably removed.

char const *old = /* ... */;
char const *new = /* ... */;

if (remove(new) != 0) {
  /* Handle error condition */
}

if (rename(old, new) != 0) {
  /* Handle error condition */
}

This code contains an unavoidable race condition between the call to remove() and the call to rename() and can consequently only be safely executed within a secure directory (see FIO17-A. Ensure that file operations are performed in a secure directory).

Compliant Solution (New File Not Removed)

This compliant solution only renames the file referenced by old if the file referenced by new does not exist.

char const *old = /* ... */;
char const *new = /* ... */;

if (access(new, F_OK) != 0) {
  if (rename(old, new) != 0) {
    /* Handle error condition */
  }
} 
else {
  /* Handle error condition */
}

This code contains an unavoidable race condition between the call to remove() and the call to rename() and can consequently only be safely executed within a secure directory (see FIO17-A. Ensure that file operations are performed in a secure directory).

Risk Assessment

Calling rename() has implementation-defined behavior when the new file name refers to an existing file. Incorrect use of rename() can result in a file being unexpectedly overwritten or other unexpected behavior.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899:1999]] Section 7.9.4.2, "The rename function"
[[MSDN]] rename()
[[Open Group 04]] access()

FIO09-A. Be careful with binary data when transferring data across systems      09. Input Output (FIO)       FIO11-A. Take care when specifying the mode parameter of fopen()