altio/t_select.c

This is altio/t_select.c (Listing 63-1, page 1334), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* t_select.c
+
+   Example of the use of the select() system call to monitor multiple
+   file descriptors.
+
+   Usage as shown in usageError().
+*/
 #include <sys/time.h>
+#if ! defined(__hpux)
+/* HP-UX 11 doesn't have this header file */
 #include <sys/select.h>
+#endif
 #include "tlpi_hdr.h"
 
 static void
 usageError(const char *progName)
 {
     fprintf(stderr, "Usage: %s {timeout|-} fd-num[rw]...\n", progName);
     fprintf(stderr, "    - means infinite timeout; \n");
     fprintf(stderr, "    r = monitor for read\n");
     fprintf(stderr, "    w = monitor for write\n\n");
     fprintf(stderr, "    e.g.: %s - 0rw 1w\n", progName);
     exit(EXIT_FAILURE);
 }
 
 int
 main(int argc, char *argv[])
 {
     fd_set readfds, writefds;
     int ready, nfds, fd, numRead, j;
     struct timeval timeout;
     struct timeval *pto;
     char buf[10];                       /* Large enough to hold "rw\0" */
 
     if (argc < 2 || strcmp(argv[1], "--help") == 0)
         usageError(argv[0]);
 
     /* Timeout for select() is specified in argv[1] */
 
     if (strcmp(argv[1], "-") == 0) {
         pto = NULL;                     /* Infinite timeout */
     } else {
         pto = &timeout;
         timeout.tv_sec = getLong(argv[1], 0, "timeout");
         timeout.tv_usec = 0;            /* No microseconds */
     }
 
     /* Process remaining arguments to build file descriptor sets */
 
     nfds = 0;
     FD_ZERO(&readfds);
     FD_ZERO(&writefds);
 
     for (j = 2; j < argc; j++) {
         numRead = sscanf(argv[j], "%d%2[rw]", &fd, buf);
         if (numRead != 2)
             usageError(argv[0]);
         if (fd >= FD_SETSIZE)
             cmdLineErr("file descriptor exceeds limit (%d)\n", FD_SETSIZE);
 
         if (fd >= nfds)
             nfds = fd + 1;              /* Record maximum fd + 1 */
         if (strchr(buf, 'r') != NULL)
             FD_SET(fd, &readfds);
         if (strchr(buf, 'w') != NULL)
             FD_SET(fd, &writefds);
     }
 
     /* We've built all of the arguments; now call select() */
 
     ready = select(nfds, &readfds, &writefds, NULL, pto);
                                         /* Ignore exceptional events */
     if (ready == -1)
         errExit("select");
 
     /* Display results of select() */
 
     printf("ready = %d\n", ready);
     for (fd = 0; fd < nfds; fd++)
         printf("%d: %s%s\n", fd, FD_ISSET(fd, &readfds) ? "r" : "",
                 FD_ISSET(fd, &writefds) ? "w" : "");
 
     if (pto != NULL)
         printf("timeout after select(): %ld.%03ld\n",
                (long) timeout.tv_sec, (long) timeout.tv_usec / 1000);
     exit(EXIT_SUCCESS);
 }

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