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20 .TH PCAP_NEXT_EX 3PCAP "13 October 2013"
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 .ti +8
32 const u_char **pkt_data);
33 const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h);
34 .ft
35 .fi
36 .SH DESCRIPTION
37 .B pcap_next_ex()
38 reads the next packet and returns a success/failure indication.
39 If the packet was read without problems, the pointer pointed to by the
40 .I pkt_header
41 argument is set to point to the
42 .I pcap_pkthdr
43 struct for the packet, and the
44 pointer pointed to by the
45 .I pkt_data
46 argument is set to point to the data in the packet. The
47 .I struct pcap_pkthdr
48 and the packet data are not to be freed by the caller, and are not
49 guaranteed to be valid after the next call to
50 .BR pcap_next_ex() ,
51 .BR pcap_next() ,
52 .BR pcap_loop() ,
53 or
54 .BR pcap_dispatch() ;
55 if the code needs them to remain valid, it must make a copy of them.
56 .PP
57 .B pcap_next()
58 reads the next packet (by calling
59 .B pcap_dispatch()
60 with a
61 .I cnt
62 of 1) and returns a
63 .I u_char
64 pointer to the data in that packet. The
65 packet data is not to be freed by the caller, and is not
66 guaranteed to be valid after the next call to
67 .BR pcap_next_ex() ,
68 .BR pcap_next() ,
69 .BR pcap_loop() ,
70 or
71 .BR pcap_dispatch() ;
72 if the code needs it to remain valid, it must make a copy of it.
73 The
74 .I pcap_pkthdr
75 structure pointed to by
76 .I h
77 is filled in with the appropriate values for the packet.
78 .PP
79 The bytes of data from the packet begin with a link-layer header. The
80 format of the link-layer header is indicated by the return value of the
81 .B pcap_datalink()
82 routine when handed the
83 .B pcap_t
84 value also passed to
85 .B pcap_loop()
86 or
87 .BR pcap_dispatch() .
88 .I http://www.tcpdump.org/linktypes.html
89 lists the values
90 .B pcap_datalink()
91 can return and describes the packet formats that
92 correspond to those values. The value it returns will be valid for all
93 packets received unless and until
94 .B pcap_set_datalink()
95 is called; after a successful call to
96 .BR pcap_set_datalink() ,
97 all subsequent packets will have a link-layer header of the type
98 specified by the link-layer header type value passed to
99 .BR pcap_set_datalink() .
100 .PP
101 Do
102 .B NOT
103 assume that the packets for a given capture or ``savefile`` will have
104 any given link-layer header type, such as
105 .B DLT_EN10MB
106 for Ethernet. For example, the "any" device on Linux will have a
107 link-layer header type of
108 .B DLT_LINUX_SLL
109 even if all devices on the system at the time the "any" device is opened
110 have some other data link type, such as
111 .B DLT_EN10MB
112 for Ethernet.
113 .SH RETURN VALUE
114 .B pcap_next_ex()
115 returns 1 if the packet was read without problems, 0
116 if packets are being read from a live capture and the timeout expired,
117 \-1 if an error occurred while reading the packet, and \-2 if
118 packets are being read from a ``savefile'' and there are no more
119 packets to read from the savefile.
120 If \-1 is returned,
121 .B pcap_geterr()
122 or
123 .B pcap_perror()
124 may be called with
125 .I p
126 as an argument to fetch or display the error text.
127 .PP
128 .B pcap_next()
129 returns a pointer to the packet data on success, and returns
130 .B NULL
131 if an error occurred, or if no packets were read from a live
132 capture (if, for example, they were discarded because they didn't pass
133 the packet filter, or if, on platforms that support a read timeout that
134 starts before any packets arrive, the timeout expires before any packets
135 arrive, or if the file descriptor for the capture device is in
136 non-blocking mode and no packets were available to be read), or if no
137 more packets are available in a ``savefile.'' Unfortunately, there is
138 no way to determine whether an error occurred or not.
139 .SH SEE ALSO
140 pcap(3PCAP), pcap_geterr(3PCAP), pcap_dispatch(3PCAP),
141 pcap_datalink(3PCAP)