1SD_LOGIN_MONITOR_NEW(3)      sd_login_monitor_new      SD_LOGIN_MONITOR_NEW(3)


6       sd_login_monitor_new, sd_login_monitor_unref, sd_login_monitor_unrefp,
7       sd_login_monitor_flush, sd_login_monitor_get_fd,
8       sd_login_monitor_get_events, sd_login_monitor_get_timeout,
9       sd_login_monitor - Monitor login sessions, seats, users and virtual
10       machines/containers


13       #include <systemd/sd-login.h>
15       int sd_login_monitor_new(const char *category, sd_login_monitor **ret);
17       sd_login_monitor *sd_login_monitor_unref(sd_login_monitor *m);
19       void sd_login_monitor_unrefp(sd_login_monitor **m);
21       int sd_login_monitor_flush(sd_login_monitor *m);
23       int sd_login_monitor_get_fd(sd_login_monitor *m);
25       int sd_login_monitor_get_events(sd_login_monitor *m);
27       int sd_login_monitor_get_timeout(sd_login_monitor *m,
28                                        uint64_t *timeout_usec);


31       sd_login_monitor_new() may be used to monitor login sessions, users,
32       seats, and virtual machines/containers. Via a monitor object a file
33       descriptor can be integrated into an application defined event loop
34       which is woken up each time a user logs in, logs out or a seat is added
35       or removed, or a session, user, seat or virtual machine/container
36       changes state otherwise. The first parameter takes a string which can
37       be "seat" (to get only notifications about seats being added, removed
38       or changed), "session" (to get only notifications about sessions being
39       created or removed or changed), "uid" (to get only notifications when a
40       user changes state in respect to logins) or "machine" (to get only
41       notifications when a virtual machine or container is started or
42       stopped). If notifications shall be generated in all these conditions,
43       NULL may be passed. Note that in the future additional categories may
44       be defined. The second parameter returns a monitor object and needs to
45       be freed with the sd_login_monitor_unref() call after use.
47       sd_login_monitor_unref() may be used to destroy a monitor object. Note
48       that this will invalidate any file descriptor returned by
49       sd_login_monitor_get_fd().
51       sd_login_monitor_unrefp() is similar to sd_login_monitor_unref() but
52       takes a pointer to a pointer to an sd_login_monitor object. This call
53       is useful in conjunction with GCC's and LLVM's Clean-up Variable
54       Attribute[1]. Note that this function is defined as inline function.
55       Use a declaration like the following, in order to allocate a login
56       monitor object that is freed automatically as the code block is left:
58           {
59             __attribute__((cleanup(sd_login_monitor_unrefp))) sd_login_monitor *m = NULL;
60             int r;
61             ...
62             r = sd_login_monitor_new(NULL, &m);
63             if (r < 0)
64               fprintf(stderr, "Failed to allocate login monitor object: %s\n", strerror(-r));
65             ...
66           }
68       sd_login_monitor_flush() may be used to reset the wakeup state of the
69       monitor object. Whenever an event causes the monitor to wake up the
70       event loop via the file descriptor this function needs to be called to
71       reset the wake-up state. If this call is not invoked, the file
72       descriptor will immediately wake up the event loop again.
74       sd_login_monitor_unref() and sd_login_monitor_unrefp() execute no
75       operation if the passed in monitor object is NULL.
77       sd_login_monitor_get_fd() may be used to retrieve the file descriptor
78       of the monitor object that may be integrated in an application defined
79       event loop, based around poll(2) or a similar interface. The
80       application should include the returned file descriptor as wake-up
81       source for the events mask returned by sd_login_monitor_get_events().
82       It should pass a timeout value as returned by
83       sd_login_monitor_get_timeout(). Whenever a wake-up is triggered the
84       file descriptor needs to be reset via sd_login_monitor_flush(). An
85       application needs to reread the login state with a function like
86       sd_get_seats(3) or similar to determine what changed.
88       sd_login_monitor_get_events() will return the poll() mask to wait for.
89       This function will return a combination of POLLIN, POLLOUT and similar
90       to fill into the ".events" field of struct pollfd.
92       sd_login_monitor_get_timeout() will return a timeout value for usage in
93       poll(). This returns a value in microseconds since the epoch of
94       CLOCK_MONOTONIC for timing out poll() in timeout_usec. See
95       clock_gettime(2) for details about CLOCK_MONOTONIC. If there is no
96       timeout to wait for this will fill in (uint64_t) -1 instead. Note that
97       poll() takes a relative timeout in milliseconds rather than an absolute
98       timeout in microseconds. To convert the absolute '┬Ás' timeout into
99       relative 'ms', use code like the following:
101           uint64_t t;
102           int msec;
103           sd_login_monitor_get_timeout(m, &t);
104           if (t == (uint64_t) -1)
105             msec = -1;
106           else {
107             struct timespec ts;
108             uint64_t n;
109             clock_gettime(CLOCK_MONOTONIC, &ts);
110             n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
111             msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
112           }
114       The code above does not do any error checking for brevity's sake. The
115       calculated msec integer can be passed directly as poll()'s timeout
116       parameter.


119       On success, sd_login_monitor_new(), sd_login_monitor_flush() and
120       sd_login_monitor_get_timeout() return 0 or a positive integer. On
121       success, sd_login_monitor_get_fd() returns a Unix file descriptor. On
122       success, sd_login_monitor_get_events() returns a combination of POLLIN,
123       POLLOUT and suchlike. On failure, these calls return a negative
124       errno-style error code.
126       sd_login_monitor_unref() always returns NULL.
128   Errors
129       Returned errors may indicate the following problems:
131       -EINVAL
132           An input parameter was invalid (out of range, or NULL, where that
133           is not accepted). The specified category to watch is not known.
135       -ENOMEM
136           Memory allocation failed.


139       These APIs are implemented as a shared library, which can be compiled
140       and linked to with the libsystemd pkg-config(1) file.


143       systemd(1), sd-login(3), sd_get_seats(3), poll(2), clock_gettime(2)


146        1. Clean-up Variable Attribute
147           https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
151systemd 250                                            SD_LOGIN_MONITOR_NEW(3)