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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(),

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C11 Annex K)

The ISO/IEC TR 24731-1 C11 Annex K function fopen_s() can be used to create a file with restricted permissions [ISO/IEC TR 24731-1:20079899:2011]:

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.

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char *file_name;
FILE *fp;

/* Initialize file_name */

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

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