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  20 .TH PCAP_NEXT_EX 3PCAP "5 March 2022"
  21 .SH NAME
  22 pcap_next_ex, pcap_next \- read the next packet from a pcap_t
  23 .SH SYNOPSIS
  24 .nf
  25 .ft B
  26 #include <pcap/pcap.h>
  27 .ft
  28 .LP
  29 .ft B
  30 int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
  31     const u_char **pkt_data);
  32 const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h);
  33 .ft
  34 .fi
  35 .SH DESCRIPTION
  36 .BR pcap_next_ex ()
  37 reads the next packet and returns a success/failure indication.
  38 If the packet was read without problems, the pointer pointed to by the
  39 .I pkt_header
  40 argument is set to point to the
  41 .B pcap_pkthdr
  42 struct for the packet, and the
  43 pointer pointed to by the
  44 .I pkt_data
  45 argument is set to point to the data in the packet.  The
  46 .B struct pcap_pkthdr
  47 and the packet data are not to be freed by the caller, and are not
  48 guaranteed to be valid after the next call to
  49 .BR pcap_next_ex (),
  50 .BR pcap_next (),
  51 .BR pcap_loop (3PCAP),
  52 or
  53 .BR pcap_dispatch (3PCAP);
  54 if the code needs them to remain valid, it must make a copy of them.
  55 .PP
  56 .BR pcap_next ()
  57 reads the next packet (by calling
  58 .BR pcap_dispatch ()
  59 with a
  60 .I cnt
  61 of 1) and returns a
  62 .B u_char
  63 pointer to the data in that packet.  The
  64 packet data is not to be freed by the caller, and is not
  65 guaranteed to be valid after the next call to
  66 .BR pcap_next_ex (),
  67 .BR pcap_next (),
  68 .BR pcap_loop (),
  69 or
  70 .BR pcap_dispatch ();
  71 if the code needs it to remain valid, it must make a copy of it.
  72 The
  73 .B pcap_pkthdr
  74 structure pointed to by
  75 .I h
  76 is filled in with the appropriate values for the packet.
  77 .PP
  78 The bytes of data from the packet begin with a link-layer header.  The
  79 format of the link-layer header is indicated by the return value of the
  80 .BR pcap_datalink (3PCAP)
  81 routine when handed the
  82 .B pcap_t
  83 value also passed to
  84 .BR pcap_loop ()
  85 or
  86 .BR pcap_dispatch ().
  87 .I https://www.tcpdump.org/linktypes.html
  88 lists the values
  89 .BR pcap_datalink ()
  90 can return and describes the packet formats that
  91 correspond to those values.  The value it returns will be valid for all
  92 packets received unless and until
  93 .BR pcap_set_datalink (3PCAP)
  94 is called; after a successful call to
  95 .BR pcap_set_datalink (),
  96 all subsequent packets will have a link-layer header of the type
  97 specified by the link-layer header type value passed to
  98 .BR pcap_set_datalink ().
  99 .PP
 100 Do
 101 .B NOT
 102 assume that the packets for a given capture or ``savefile`` will have
 103 any given link-layer header type, such as
 104 .B DLT_EN10MB
 105 for Ethernet.  For example, the "any" device on Linux will have a
 106 link-layer header type of
 107 .B DLT_LINUX_SLL
 108 or
 109 .B DLT_LINUX_SLL2
 110 even if all devices on the system at the time the "any" device is opened
 111 have some other data link type, such as
 112 .B DLT_EN10MB
 113 for Ethernet.
 114 .SH RETURN VALUE
 115 .BR pcap_next_ex ()
 116 returns
 117 .B 1
 118 if the packet was read without problems,
 119 .B 0
 120 if packets are
 121 being read from a live capture and the packet buffer timeout expired,
 122 .B PCAP_ERROR_BREAK
 123 if packets
 124 are being read from a ``savefile'' and there are no more packets to read
 125 from the savefile,
 126 .B PCAP_ERROR_NOT_ACTIVATED
 127 if called on a capture handle that has been created but not activated,
 128 or
 129 .B PCAP_ERROR
 130 if an error occurred while reading the packet.  If
 131 .B PCAP_ERROR
 132 is returned,
 133 .BR pcap_geterr (3PCAP)
 134 or
 135 .BR pcap_perror (3PCAP)
 136 may be called with
 137 .I p
 138 as an argument to fetch or display the error text.
 139 .PP
 140 .BR pcap_next ()
 141 returns a pointer to the packet data on success, and returns
 142 .B NULL
 143 if an error occurred, or if no packets were read from a live capture
 144 (if, for example, they were discarded because they didn't pass the
 145 packet filter, or if, on platforms that support a packet buffer timeout
 146 that starts before any packets arrive, the timeout expires before any
 147 packets arrive, or if the file descriptor for the capture device is in
 148 non-blocking mode and no packets were available to be read), or if no
 149 more packets are available in a ``savefile''. Unfortunately, there is no
 150 way to determine whether an error occurred or not.
 151 .SH SEE ALSO
 152 .BR pcap (3PCAP)