/usr/include/net-snmp/system/openbsd4.h is in libsnmp-dev 5.7.3+dfsg-1.8ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | /* openbsd4 is a superset of openbsd3 */
#include "openbsd.h"
#define openbsd3 openbsd3
|