FIO08-C. Take care when calling remove() on an open file

Invoking remove() on an open file is implementation-defined. Removing an open file is sometimes recommended to hide the names of temporary files that may be prone to attack. (See FIO21-C. Do not create temporary files in shared directories.)

In cases requiring the removal of an open file, a more strongly defined function, such as the POSIX unlink() function, should be considered. To be strictly conforming and portable, remove() should not be called on an open file.

Noncompliant Code Example

This noncompliant code example shows a case where a file is removed while it is still open:

char *file_name;
FILE *file;

/* Initialize file_name */

file = fopen(file_name, "w+");
if (file == NULL) {
  /* Handle error condition */
}

/* ... */

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

/* Continue performing I/O operations on file */

fclose(file);

Some implementations will not remove the file specified by file_name because the stream is still open.

Implementation Details

Code compiled for Microsoft Windows prevents the remove() call from succeeding when the file is open, meaning that the file link will remain after execution completes.

Compliant Solution (POSIX)

This compliant solution uses the POSIX unlink() function to remove the file. The unlink() function is guaranteed to unlink the file from the file system hierarchy but keep the file on disk until all open instances of the file are closed [IEEE Std 1003.1:2013].

FILE *file;
char *file_name;

/* Initialize file_name */

file = fopen(file_name, "w+");
if (file == NULL) {
  /* Handle error condition */
}

if (unlink(file_name) != 0) {
  /* Handle error condition */
}

/* Continue performing I/O operations on file */

fclose(file);

Note that there is a race window between the fopen() call and the unlink() call, which could be exploited. This exploitation can be mitigated if the operations occur in a secure directory; see FIO45-C. Avoid TOCTOU race conditions while accessing files for more information.

Risk Assessment

Calling remove() on an open file has different implications for different implementations and may cause abnormal termination if the removed file is written to or read from, or it may result in unintended information disclosure from files not deleted as intended.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography