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char *file_name;

/* Initialize file_name */
FILE *fp;
errno_t res = fopen_s(&fp, file_name, "wx");
if (res != 0) {
  /* Handle error */
}

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The open() function, as defined in the Open Group Standard for Information Technology—Portable Operating System Interface (POSIX®), Base Specifications, Issue 6 [Open Group 20047 [IEEE Std 1003.1:2013], is available on many platforms and provides finer control than fopen(). In particular, open() accepts the O_CREAT and O_EXCL flags. When used together, these flags instruct the open() function to fail if the file specified by file_name already exists.

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For code that operates on FILE pointers and not file descriptors, the POSIX fdopen() function can be used to associate an open stream with the file descriptor returned by open(), as shown in this compliant solution [Open Group 2004IEEE Std 1003.1:2013]:

char *file_name;
int new_file_mode;
FILE *fp;
int fd;

/* Initialize file_name and new_file_mode */

fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, new_file_mode);
if (fd == -1) {
  /* Handle error */
}

fp = fdopen(fd, "w");
if (fp == NULL) {
  /* Handle error */
}

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The ability to determine whether an existing file has been opened or a new file has been created provides greater assurance that a file other than the intended file is not acted upon.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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