dirs_links/t_unlink.c

This is dirs_links/t_unlink.c (Listing 18-1, page 347), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* 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);
}

 

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