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For OpenBSD and Linux operating systems, any file created will have mode S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH (0666), as modified by the process's umask value. (See fopen(3) in the OpenBSD Manual Pages [OpenBSD].)

Compliant Solution (fopen_s(), C11 Annex K)

The C11 Annex K function fopen_s() can be used to create a file with restricted permissions [ISO/IEC 9899:2011]:

If the file is being created, and the first character of the mode string is not 'u', to the extent that the underlying system supports it, the file shall have a file permission that prevents other users on the system from accessing the file. If the file is being created and the first character of the mode string is 'u', then by the time the file has been closed, it shall have the system default file access permissions.

The u character can be thought of as standing for "umask," meaning that these are the same permissions that the file would have been created with had it been created by fopen(). In this compliant solution, the u mode character is omitted so that the file is opened with restricted privileges (regardless of the umask):

char *file_name;
FILE *fp;

/* Initialize file_name */

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

On Windows, fopen_s() will create the file with security permissions based on the user executing the application. For more controlled permission schemes, consider using the CreateFile() function and specifying the SECURITY_ATTRIBUTES parameter.

Noncompliant Code Example (open(), POSIX)

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