/usr/include/net-snmp/system/openbsd5.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 | /* openbsd5 is a superset of all since openbsd3 */
#include "openbsd4.h"
#define openbsd4 openbsd4
|