dirs_links/t_unlink.c

+/* t_unlink.c
+
+    Demonstrate that, when a file is unlinked, it is not actually removed from
+    the file system until after any open descriptors referring to it are closed.
+
+    Usage: t_unlink file
+*/
 #include <sys/stat.h>
 #include <fcntl.h>
 #include "tlpi_hdr.h"
 
 #define CMD_SIZE 200
 #define BUF_SIZE 1024
 
 int
 main(int argc, char *argv[])
 {
     int fd, j, numBlocks;
     char shellCmd[CMD_SIZE];            /* Command to be passed to system() */
     char buf[BUF_SIZE];                 /* Random bytes to write to file */
 
     if (argc < 2 || strcmp(argv[1], "--help") == 0)
         usageErr("%s temp-file [num-1kB-blocks] \n", argv[0]);
 
     numBlocks = (argc > 2) ? getInt(argv[2], GN_GT_0, "num-1kB-blocks")
                            : 100000;
 
+    /* O_EXCL so that we ensure we create a new file */
+
     fd = open(argv[1], O_WRONLY | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
     if (fd == -1)
         errExit("open");
 
     if (unlink(argv[1]) == -1)          /* Remove filename */
         errExit("unlink");
 
     for (j = 0; j < numBlocks; j++)     /* Write lots of junk to file */
         if (write(fd, buf, BUF_SIZE) != BUF_SIZE)
             fatal("partial/failed write");
 
     snprintf(shellCmd, CMD_SIZE, "df -k `dirname %s`", argv[1]);
     system(shellCmd);                   /* View space used in file system */
 
     if (close(fd) == -1)                /* File is now destroyed */
         errExit("close");
     printf("********** Closed file descriptor\n");
 
+    /* See the erratum for page 348 at http://man7.org/tlpi/errata/.
+       Depending on factors such as random scheduler decisions and the
+       size of the file created, the 'df' command executed by the second
+       system() call below does may not show a change in the amount
+       of disk space consumed, because the blocks of the closed file
+       have not yet been freed by the kernel. If this is the case,
+       then inserting a sleep(1) call here should be sufficient to
+       ensure that the the file blocks have been freed by the time
+       of the second 'df' command.
+    */
+
     system(shellCmd);                   /* Review space used in file system */
     exit(EXIT_SUCCESS);
 }