snippets/C/select.c

   1 /*
   2  * select() example, you can test by doing:
   3  *
   4  * $ mkfifo my-fifo
   5  * $ cc -o select select.c
   6  * $ ./select my-fifo
   7  *
   8  * And, in other terminal:
   9  *
  10  * $ echo 'Hello, world!' > my-fifo
  11  *
  12  * Luiz Fernando N. Capitulino
  13  * <lcapitulino@gmail.com>
  14  */
  15 #include <stdio.h>
  16 #include <unistd.h>
  17 #include <stdlib.h>
  18 #include <fcntl.h>
  19 #include <sys/types.h>
  20 #include <sys/stat.h>
  21 #include <sys/select.h>
  22
  23 int main(int argc, char *argv[])
  24 {
  25         char c;
  26         int err, fd;
  27         fd_set rset;
  28         ssize_t bytes;
  29
  30         if (argc != 2) {
  31                 printf("usage: select < fifo-path >");
  32                 exit(1);
  33         }
  34
  35         fd = open(argv[1], O_RDONLY);
  36         if (fd < 0) {
  37                 perror("open()");
  38                 exit(1);
  39         }
  40
  41         /*
  42          * select() receives a set fo file-descriptors to
  43          * monitor. The 'fd_set' data type should be used
  44          * to define the set variable.
  45          *
  46          * Before calling select() you have to:
  47          *
  48          * 1. Initialize each set with FD_ZERO()
  49          * 2. Add file-descriptors to the set
  50          */
  51         FD_ZERO(&rset);
  52         FD_SET(fd, &rset);
  53
  54         /*
  55          * The first parameter is always confusing to
  56          * most people...
  57          *
  58          * The rule is simple: it's the highest-numbered
  59          * file descriptor in any of the sets, plus 1.
  60          *
  61          * In this example we only have one file-descriptor,
  62          * so all we have to do is to add 1 to it. But if you
  63          * have, say, 30 file-descriptors, you'll have to
  64          * find the highest one and add 1 to it.
  65          */
  66         err = select(fd + 1, &rset, NULL, NULL, NULL);
  67         if (err < 0) {
  68                 perror("select()");
  69                 exit(1);
  70         }
  71
  72         if (FD_ISSET(fd, &rset)) {
  73                 for (;;) {
  74                         bytes = read(fd, &c, 1);
  75                         if (bytes == 0)
  76                                 break;
  77                         if (bytes < 0) {
  78                                 perror("read()");
  79                                 exit(1);
  80                         }
  81                         if (bytes != 1) {
  82                                 fprintf(stderr,
  83                                         "Something strange has happend\n");
  84                                 exit(1);
  85                         }
  86                         printf("%c", c);
  87                 }
  88         } else {
  89                 fprintf(stderr, "select() returned but fd is not set\n");
  90                 exit(1);
  91         }
  92
  93         return 0;
  94 }