pcap_is_swapped.3pcap man page
pcap_is_swapped — find out whether a savefile has the native byte order
#include <pcap/pcap.h> int pcap_is_swapped(pcap_t *p);
pcap_is_swapped() returns true (1) if p refers to a “savefile” that uses a different byte order than the current system. For a live capture, it always returns false (0).
It must not be called on a pcap descriptor created by pcap_create() that has not yet been activated by pcap_activate().
pcap_datalink() returns true (1) or false (0) on success and PCAP_ERROR_NOT_ACTIVATED if called on a capture handle that has been created but not activated.