This is loginacct/utmpx_login.c (Listing 40-3, page 828), 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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Download loginacct/utmpx_login.c

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/* utmpx_login.c

   Demonstrate the steps required to update the utmp and wtmp files on user
   login and logout.

   Note: updating utmp and wtmp (normally) requires root privileges.

   This program is Linux-specific.
#define _GNU_SOURCE
#include <time.h>
#include <utmpx.h>
#include <paths.h>              /* Definitions of _PATH_UTMP and _PATH_WTMP */
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    struct utmpx ut;
    char *devName;

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s username [sleep-time]\n", argv[0]);

    /* Initialize login record for utmp and wtmp files */

    memset(&ut, 0, sizeof(struct utmpx));
    ut.ut_type = USER_PROCESS;          /* This is a user login */
    strncpy(ut.ut_user, argv[1], sizeof(ut.ut_user));
    if (time((time_t *) &ut.ut_tv.tv_sec) == -1)
        errExit("time");                /* Stamp with current time */
    ut.ut_pid = getpid();

    /* Set ut_line and ut_id based on the terminal associated with
       'stdin'. This code assumes terminals named "/dev/[pt]t[sy]*".
       The "/dev/" dirname is 5 characters; the "[pt]t[sy]" filename
       prefix is 3 characters (making 8 characters in all). */

    devName = ttyname(STDIN_FILENO);
    if (devName == NULL)
    if (strlen(devName) <= 8)           /* Should never happen */
        fatal("Terminal name is too short: %s", devName);

    strncpy(ut.ut_line, devName + 5, sizeof(ut.ut_line));
    strncpy(ut.ut_id, devName + 8, sizeof(ut.ut_id));

    printf("Creating login entries in utmp and wtmp\n");
    printf("        using pid %ld, line %.*s, id %.*s\n",
            (long) ut.ut_pid, (int) sizeof(ut.ut_line), ut.ut_line,
            (int) sizeof(ut.ut_id), ut.ut_id);

    setutxent();                        /* Rewind to start of utmp file */
    if (pututxline(&ut) == NULL)        /* Write login record to utmp */
    updwtmpx(_PATH_WTMP, &ut);          /* Append login record to wtmp */

    /* Sleep a while, so we can examine utmp and wtmp files */

    sleep((argc > 2) ? getInt(argv[2], GN_NONNEG, "sleep-time") : 15);

    /* Now do a "logout"; use values from previously initialized 'ut',
       except for changes below */

    ut.ut_type = DEAD_PROCESS;          /* Required for logout record */
    time((time_t *) &ut.ut_tv.tv_sec);  /* Stamp with logout time */
    memset(&ut.ut_user, 0, sizeof(ut.ut_user));
                                        /* Logout record has null username */

    printf("Creating logout entries in utmp and wtmp\n");
    setutxent();                        /* Rewind to start of utmp file */
    if (pututxline(&ut) == NULL)        /* Overwrite previous utmp record */
    updwtmpx(_PATH_WTMP, &ut);          /* Append logout record to wtmp */



Download loginacct/utmpx_login.c

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