pcap_file.3pcap man page
pcap_file — get the standard I/O stream for a savefile being read
#include <pcap/pcap.h> FILE *pcap_file(pcap_t *p);
pcap_file() returns the standard I/O stream of the “savefile,” if a “savefile” was opened with pcap_open_offline(), or NULL, if a network device was opened with pcap_create() and pcap_activate(), or with pcap_open_live().
Note that the Packet Capture library is usually built with large file support, so the standard I/O stream of the “savefile” might refer to a file larger than 2 gigabytes; applications that use pcap_file() should, if possible, use calls that support large files on the return value of pcap_file() or the value returned by fileno() when passed the return value of pcap_file().