pcap_snapshot.3pcap man page
pcap_snapshot — get the snapshot length
#include <pcap/pcap.h> int pcap_snapshot(pcap_t *p);
pcap_snapshot() returns the snapshot length specified when pcap_set_snapshot() or pcap_open_live() was called, for a live capture, or the snapshot length from the capture file, for a “savefile”.
It must not be called on a pcap descriptor created by pcap_create() that has not yet been activated by pcap_activate().
pcap_snapshot() returns the snapshot length on success and PCAP_ERROR_NOT_ACTIVATED if called on a capture handle that has been created but not activated.