Beej's Guide to Network Programming Using Internet Sockets

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Beej’s Guide to Network Programming Using Internet Sockets 59Return ValueReturns the number of elements in the ufds array that have had event occur on them; this canbe zero if the timeout occurred. Also returns -1 on error (and errno will be set accordingly.)Exampleint s1, s2;int rv;char buf1[256], buf2[256];struct pollfd ufds[2];s1 = socket(PF_INET, SOCK_STREAM, 0);s2 = socket(PF_INET, SOCK_STREAM, 0);// pretend we’ve connected both to a server at this point//connect(s1, ...)...//connect(s2, ...)...// set up the array of file descriptors.//// in this example, we want to know when there’s normal or out-of-band// data ready to be recv()’d...ufds[0].fd = s1;ufds[0].events = POLLIN | POLLPRI; // check for normal or out-of-bandufds[1] = s2;ufds[1].events = POLLIN; // check for just normal data// wait for events on the sockets, 3.5 second timeoutrv = poll(ufds, 2, 3500);if (rv == -1) {perror("poll"); // error occurred in poll()} else if (rv == 0) {printf("Timeout occurred! No data after 3.5 seconds.\n");} else {// check for events on s1:if (ufds[0].revents & POLLIN) {recv(s1, buf1, sizeof(buf1), 0); // receive normal data}if (ufds[0].revents & POLLPRI) {recv(s1, buf1, sizeof(buf1), MSG_OOB); // out-of-band data}}// check for events on s2:if (ufds[1].revents & POLLIN) {recv(s1, buf2, sizeof(buf2), 0);}See Alsoselect()
Beej’s Guide to Network Programming Using Internet Sockets 608.15. recv(), recvfrom()Recieve data on a socketPrototypes#include #include ssize_t recv(int s, void *buf, size_t len, int flags);ssize_t recvfrom(int s, void *buf, size_t len, int flags,struct sockaddr *from, socklen_t *fromlen);DescriptionOnce you have a socket up and connected, you can read incoming data from the remoteside using the recv() (for TCP SOCK_STREAM sockets) and recvfrom() (for UDP SOCK_DGRAMsockets).Both functions take the socket descriptor s, a pointer to the buffer buf, the size (in bytes) ofthe buffer len, and a set of flags that control how the functions work.Additionally, the recvfrom() takes a struct sockaddr*, from that will tell you where thedata came from, and will fill in fromlen with the size of struct sockaddr. (You must alsoinitialize fromlen to be the size of from or struct sockaddr.)So what wonderous flags can you pass into this function? Here are some of them, but youshould check your local man pages for more information and what is actually supported on yoursystem. You bitwise-or these together, or just set flags to 0 if you want it to be a regular vanillarecv().MSG_OOBMSG_PEEKMSG_WAITALLRecieve Out of Band data. This is how to get data that has beensent to you with the MSG_OOB flag in send(). As the recieving side,you will have had signal SIGURG raised telling you there is urgentdata. In your handler for that signal, you could call recv() withthis MSG_OOB flag.If you want to call recv() “just for pretend”, you can call itwith this flag. This will tell you what’s waiting in the buffer forwhen you call recv() “for real” (i.e. without the MSG_PEEK flag.It’s like a sneak preview into the next recv() call.Tell recv() to not return until all the data you specified in thelen parameter. It will ignore your wishes in extreme circumstances,however, like if a signal interrupts the call or if some error occursor if the remote side closes the connection, etc. Don’t be mad withit.
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Beej's Guide to Network Programming Using Internet Sockets

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