timers/timed_read.c

This is timers/timed_read.c (Listing 23-2, page 486), 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
+/* timed_read.c
+
+   Demonstrate the use of a timer to place a timeout on a blocking system call
+   (read(2) in this case).
+*/
 #include <signal.h>
 #include "tlpi_hdr.h"
 
 #define BUF_SIZE 200
 
 static void     /* SIGALRM handler: interrupts blocked system call */
 handler(int sig)
 {
     printf("Caught signal\n");          /* UNSAFE (see Section 21.1.2) */
 }
 
 int
 main(int argc, char *argv[])
 {
     struct sigaction sa;
     char buf[BUF_SIZE];
     ssize_t numRead;
     int savedErrno;
 
     if (argc > 1 && strcmp(argv[1], "--help") == 0)
         usageErr("%s [num-secs [restart-flag]]\n", argv[0]);
 
     /* Set up handler for SIGALRM. Allow system calls to be interrupted,
        unless second command-line argument was supplied. */
 
     sa.sa_flags = (argc > 2) ? SA_RESTART : 0;
     sigemptyset(&sa.sa_mask);
     sa.sa_handler = handler;
     if (sigaction(SIGALRM, &sa, NULL) == -1)
         errExit("sigaction");
 
     alarm((argc > 1) ? getInt(argv[1], GN_NONNEG, "num-secs") : 10);
 
     numRead = read(STDIN_FILENO, buf, BUF_SIZE);
 
     savedErrno = errno;                 /* In case alarm() changes it */
     alarm(0);                           /* Ensure timer is turned off */
     errno = savedErrno;
 
     /* Determine result of read() */
 
     if (numRead == -1) {
         if (errno == EINTR)
             printf("Read timed out\n");
         else
             errMsg("read");
     } else {
         printf("Successful read (%ld bytes): %.*s",
                 (long) numRead, (int) numRead, buf);
     }
 
     exit(EXIT_SUCCESS);
 }

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