pcap_next_ex(3PCAP) man page
#include <pcap/pcap.h> int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
const u_char **pkt_data); const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h);
pcap_next() reads the next packet (by calling pcap_dispatch() with a cnt of 1) and returns a u_char pointer to the data in that packet. The packet data is not to be freed by the caller, and is not guaranteed to be valid after the next call to pcap_next_ex(), pcap_next(), pcap_loop(), or pcap_dispatch(); if the code needs it to remain valid, it must make a copy of it. The pcap_pkthdr structure pointed to by h is filled in with the appropriate values for the packet.
The bytes of data from the packet begin with a link-layer header. The format of the link-layer header is indicated by the return value of the pcap_datalink(3PCAP) routine when handed the pcap_t value also passed to pcap_loop() or pcap_dispatch(). https://www.tcpdump.org/linktypes.html lists the values pcap_datalink() can return and describes the packet formats that correspond to those values. The value it returns will be valid for all packets received unless and until pcap_set_datalink(3PCAP) is called; after a successful call to pcap_set_datalink(), all subsequent packets will have a link-layer header of the type specified by the link-layer header type value passed to pcap_set_datalink().
Do NOT assume that the packets for a given capture or ``savefile`` will have any given link-layer header type, such as DLT_EN10MB for Ethernet. For example, the "any" device on Linux will have a link-layer header type of DLT_LINUX_SLL or DLT_LINUX_SLL2 even if all devices on the system at the time the "any" device is opened have some other data link type, such as DLT_EN10MB for Ethernet.
pcap_next() returns a pointer to the packet data on success, and returns NULL if an error occurred, or if no packets were read from a live capture (if, for example, they were discarded because they didn't pass the packet filter, or if, on platforms that support a packet buffer timeout that starts before any packets arrive, the timeout expires before any packets arrive, or if the file descriptor for the capture device is in non-blocking mode and no packets were available to be read), or if no more packets are available in a ``savefile.'' Unfortunately, there is no way to determine whether an error occurred or not.