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POlibpcap not receiving in real time, seems to be buffering packets
 

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    <p>So I'm working with a device where I need to send and receive raw ethernet frames. It's a wireless radio and it uses ethernet to send status messages to its host. The protocol it uses is actually IPX, but I figured it would be easier to send raw ethernet frames using libpcap than to dig through decades old code implementing IPX (which got replaced by TCP/IP, so it's quite old).</p> <p>My program sends a request packet (this packet is exactly the same every time, it's stateless) and the device returns a response packet with the data I need. I'm using pcap_inject to send the frame and pcap_loop in another thread to do the receiving. I originally had it in one thread, but tried 2 threads to see if it fixed the issue I'm having. </p> <p>The issue is that libpcap doesn't seem to be receiving the packets in real time. It seems to buffer about 5 of them and then process them all at once. I want to be able to read them as fast as they come. Is there some way to disable this buffering on libpcap, or increase the refresh rate?</p> <p>Some example output (I just printed out the time that a packet was received). Notice how there is about a second of time between each group</p> <pre><code>Time: 1365792602.805750 Time: 1365792602.805791 Time: 1365792602.805806 Time: 1365792602.805816 Time: 1365792602.805825 Time: 1365792602.805834 Time: 1365792603.806886 Time: 1365792603.806925 Time: 1365792603.806936 Time: 1365792603.806944 Time: 1365792603.806952 Time: 1365792604.808007 Time: 1365792604.808044 Time: 1365792604.808055 Time: 1365792604.808063 Time: 1365792604.808071 Time: 1365792605.809158 Time: 1365792605.809194 Time: 1365792605.809204 Time: 1365792605.809214 Time: 1365792605.809223 </code></pre> <p>Here's the inject code:</p> <pre><code>char errbuf[PCAP_ERRBUF_SIZE]; char *dev="en0"; if(dev==NULL){ fprintf(stderr,"Pcap error: %s\n",errbuf); return 2; } printf("Device: %s\n",dev); pcap_t *handle; handle=pcap_open_live(dev, BUFSIZ, 1, 1000, errbuf); if(handle==NULL){ fprintf(stderr, "Device open error: %s\n",errbuf); return 2; } //Construct the packet that will get sent to the radio struct ether_header header; header.ether_type=htons(0x0170); int i; for(i=0;i&lt;6;i++){ header.ether_dhost[i]=radio_ether_address[i]; header.ether_shost[i]=my_ether_address[i]; } unsigned char frame[sizeof(struct ether_header)+sizeof(radio_request_packet)]; memcpy(frame, &amp;header, sizeof(struct ether_header)); memcpy(frame+sizeof(struct ether_header), radio_request_packet, sizeof(radio_request_packet)); if(pcap_inject(handle, frame, sizeof(frame))==-1){ pcap_perror(handle, errbuf); fprintf(stderr, "Couldn't send frame: %s\n",errbuf); return 2; } bpf_u_int32 mask; bpf_u_int32 net; if(pcap_lookupnet(dev,&amp;net,&amp;mask,errbuf)==-1){ pcap_perror(handle, errbuf); fprintf(stderr,"Net mask error: %s\n",errbuf); return 2; } char *filter="ether src 00:30:30:01:b1:35"; struct bpf_program fp; if(pcap_compile(handle, &amp;fp, filter, 0, net)==-1){ pcap_perror(handle, errbuf); fprintf(stderr,"Filter error: %s\n",errbuf); return 2; } if(pcap_setfilter(handle, &amp;fp)==-1){ pcap_perror(handle, errbuf); fprintf(stderr, "Install filter error: %s\n",errbuf); return 2; } printf("Starting capture\n"); pthread_t recvThread; pthread_create(&amp;recvThread, NULL, (void *(*)(void *))thread_helper, handle); while(1){ if(pcap_inject(handle, frame, sizeof(frame))==-1){ pcap_perror(handle, errbuf); fprintf(stderr, "Couldn't inject frame: %s\n",errbuf); return 2; } usleep(200000); } pcap_close(handle); return 0; </code></pre> <p>And the receiving code:</p> <pre><code>void got_packet(u_char *args,const struct pcap_pkthdr * header,const u_char * packet){ struct timeval tv; gettimeofday(&amp;tv, NULL); double seconds=(double)tv.tv_sec + ((double)tv.tv_usec)/1000000.0; printf("Time: %.6f\n",seconds); } void *thread_helper(pcap_t *handle){ pcap_loop(handle, -1, got_packet, NULL); return NULL; } </code></pre>
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