c - how to check packet availability in libpcap -

i use libpcap capture packets, , need put packets fifo queue packet available. fifo queue shared 2 threads, 1 thread call pcap_next() , put packet fifo queue. thread fetch packet fifo queue. have relate mutex. below:

u_char* pkt; for(;;){     pkt = pcap_next();     lock(&mutex);     some_process(pkt);     insert(pkt, list);     unlock(&mutext);  } 

the pcap_next() related packet buffer, if there no packet in buffer, pcap_next() blocked. if there is/are packet(s), each call of pcap_next() returns 1 packet.

it can fetch oen packet each lock-unlock operation pair, if packet arrival not frequent, fine. if packet arrival frequent, in buffer there many pending packets, bit resource-consuming don lock-unlock operation pair 1 packet.

what hope is: after processing , inserting packet, can check whether there packets available in packet buffer. if there are, continue processing , insertion. otherwise, unlock mutex , go loop.

is there workaround this?

try such as

/*  * xxx - can fail on platforms , devices,  * , there may issues select() on this.  see  * pcap_get_selectable_fd() man page details.  */ pcap_fd = pcap_get_selectable_fd(p); pcap_setnonblock(p);  /* xxx - check failure */  (;;) {     fd_set fdset;     struct timeval timeout;      /*      * wait batch of packets available.      */     fd_zero(&fdset);     fd_set(pcap_fd, &fdset);     timeout.tv_sec = 1;     timeout.tv_usec = 0;     if (select(1, &fdset, null, null, &timeout) == -1) {         report error;     } else {         lock(&mutex);         pcap_dispatch(p, -1, callback, pointer-to-stuff);         unlock(&mutex);     } } 

that way, lock mutex, process entire batch of packets, , unlock mutex. many os capture mechanisms deliver multiple packets in batch, there 1 lock/unlock pair per batch in case.

callback some_process(pkt); , insert(pkt, list); stuff.

it possible that, once you're done batch, next batch available, doesn't achieve absolute minimum of lock/unlock pairs; however, absolute minimum might lock out other thread significant period of time, can never make progress, lock , unlock around each batch might best.