pipes/fifo_seqnum_client.c

+/* fifo_seqnum_client.c
+
+   A simple client that uses a well-known FIFO to request a (trivial)
+   "sequence number service". This client creates its own FIFO (using a
+   convention agreed upon by client and server) which is used to receive a reply
+   from the server. The client then sends a request to the server consisting of
+   its PID and the length of the sequence it wishes to be allocated. The client
+   then reads the server's response and displays it on stdout.
+
+   See fifo_seqnum.h for the format of request and response messages.
+
+   The server is in fifo_seqnum_server.c.
+*/
 #include "fifo_seqnum.h"
 
 static char clientFifo[CLIENT_FIFO_NAME_LEN];
 
 static void             /* Invoked on exit to delete client FIFO */
 removeFifo(void)
 {
     unlink(clientFifo);
 }
 
 int
 main(int argc, char *argv[])
 {
     int serverFd, clientFd;
     struct request req;
     struct response resp;
 
     if (argc > 1 && strcmp(argv[1], "--help") == 0)
         usageErr("%s [seq-len]\n", argv[0]);
 
     /* Create our FIFO (before sending request, to avoid a race) */
 
     umask(0);                   /* So we get the permissions we want */
     snprintf(clientFifo, CLIENT_FIFO_NAME_LEN, CLIENT_FIFO_TEMPLATE,
             (long) getpid());
     if (mkfifo(clientFifo, S_IRUSR | S_IWUSR | S_IWGRP) == -1
                 && errno != EEXIST)
         errExit("mkfifo %s", clientFifo);
 
     if (atexit(removeFifo) != 0)
         errExit("atexit");
 
     /* Construct request message, open server FIFO, and send message */
 
     req.pid = getpid();
     req.seqLen = (argc > 1) ? getInt(argv[1], GN_GT_0, "seq-len") : 1;
 
     serverFd = open(SERVER_FIFO, O_WRONLY);
     if (serverFd == -1)
         errExit("open %s", SERVER_FIFO);
 
     if (write(serverFd, &req, sizeof(struct request)) !=
             sizeof(struct request))
         fatal("Can't write to server");
 
     /* Open our FIFO, read and display response */
 
     clientFd = open(clientFifo, O_RDONLY);
     if (clientFd == -1)
         errExit("open %s", clientFifo);
 
     if (read(clientFd, &resp, sizeof(struct response))
             != sizeof(struct response))
         fatal("Can't read response from server");
 
     printf("%d\n", resp.seqNum);
     exit(EXIT_SUCCESS);
 }