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  20 .TH PCAP_SETFILTER 3PCAP "24 January 2025"
  21 .SH NAME
  22 pcap_setfilter \- set the filter
  23 .SH SYNOPSIS
  24 .nf
  25 .ft B
  26 #include <pcap/pcap.h>
  27 .ft
  28 .LP
  29 .ft B
  30 int pcap_setfilter(pcap_t *p, struct bpf_program *fp);
  31 .ft
  32 .fi
  33 .SH DESCRIPTION
  34 .BR pcap_setfilter ()
  35 is used to specify a filter program.
  36 .I fp
  37 is a pointer to a
  38 .I bpf_program
  39 struct, usually the result of a call to
  40 .BR \%pcap_compile (3PCAP).
  41 .PP
  42 If the calls to
  43 .BR \%pcap_compile ()
  44 and
  45 .BR \%pcap_setfilter ()
  46 use the same
  47 .B pcap_t
  48 handle, packet filtering should either work correctly or fail correctly
  49 without additional efforts.  Otherwise the filter program must have been
  50 compiled for the same link-layer header type as the one used by the
  51 .B pcap_t
  52 handle; also on Linux the filter must not use BPF extensions, see
  53 .BR pcap_compile ()
  54 for more information.
  55 .SH RETURN VALUE
  56 .BR pcap_setfilter ()
  57 returns
  58 .B 0
  59 on success,
  60 .B PCAP_ERROR_NOT_ACTIVATED
  61 if called on a capture handle that has been created but not activated, or
  62 .B PCAP_ERROR
  63 on other errors.  If
  64 .B PCAP_ERROR
  65 is returned,
  66 .BR pcap_geterr (3PCAP)
  67 or
  68 .BR pcap_perror (3PCAP)
  69 may be called with
  70 .I p
  71 as an argument to fetch or display the error text.
  72 .SH SEE ALSO
  73 .BR pcap (3PCAP)