1SD_BUS_GET_FD(3) sd_bus_get_fd SD_BUS_GET_FD(3)
2
6 sd_bus_get_fd, sd_bus_get_events, sd_bus_get_timeout - Get the file
7 descriptor, I/O events and timeout to wait for from a message bus
8 object
9
11 #include <systemd/sd-bus.h>
12 int sd_bus_get_fd(sd_bus *bus);
14 int sd_bus_get_events(sd_bus *bus);
16 int sd_bus_get_timeout(sd_bus *bus, uint64_t *timeout_usec);
18
20 sd_bus_get_fd() returns the file descriptor used to communicate from a
21 message bus object. This descriptor can be used with poll(3) or a
22 similar function to wait for I/O events on the specified bus connection
23 object. If the bus object was configured with the sd_bus_set_fd()
24 function, then the input_fd file descriptor used in that call is
25 returned.
26 sd_bus_get_events() returns the I/O events to wait for, suitable for
28 passing to poll() or a similar call. Returns a combination of POLLIN,
29 POLLOUT, ... events, or negative on error.
30 sd_bus_get_timeout() returns the absolute time-out in μs, from which
32 the relative time-out to pass to poll() (or a similar call) can be
33 derived, when waiting for events on the specified bus connection. The
34 returned timeout may be zero, in which case a subsequent I/O polling
35 call should be invoked in non-blocking mode. The returned timeout may
36 be UINT64_MAX in which case the I/O polling call may block
37 indefinitely, without any applied timeout. Note that the returned
38 timeout should be considered only a maximum sleeping time. It is
39 permissible (and even expected) that shorter timeouts are used by the
40 calling program, in case other event sources are polled in the same
41 event loop. Note that the returned time-value is absolute, based of
42 CLOCK_MONOTONIC and specified in microseconds. When converting this
43 value in order to pass it as third argument to poll() (which expects
44 relative milliseconds), care should be taken to convert to a relative
45 time and use a division that rounds up to ensure the I/O polling
46 operation doesn't sleep for shorter than necessary, which might result
47 in unintended busy looping (alternatively, use ppoll(2) instead of
48 plain poll(), which understands timeouts with nano-second granularity).
49 These three functions are useful to hook up a bus connection object
51 with an external or manual event loop involving poll() or a similar I/O
52 polling call. Before each invocation of the I/O polling call, all three
53 functions should be invoked: the file descriptor returned by
54 sd_bus_get_fd() should be polled for the events indicated by
55 sd_bus_get_events(), and the I/O call should block for that up to the
56 timeout returned by sd_bus_get_timeout(). After each I/O polling call
57 the bus connection needs to process incoming or outgoing data, by
58 invoking sd_bus_process(3).
59 Note that these functions are only one of three supported ways to
61 implement I/O event handling for bus connections. Alternatively use
62 sd_bus_attach_event(3) to attach a bus connection to an sd-event(3)
63 event loop. Or use sd_bus_wait(3) as a simple synchronous, blocking I/O
64 waiting call.
65
67 On success, sd_bus_get_fd() returns the file descriptor used for
68 communication. On failure, it returns a negative errno-style error
69 code.
70 On success, sd_bus_get_events() returns the I/O event mask to use for
72 I/O event watching. On failure, it returns a negative errno-style error
73 code.
74 On success, sd_bus_get_timeout() returns a non-negative integer. On
76 failure, it returns a negative errno-style error code.
77 Errors
79 Returned errors may indicate the following problems:
80 -EINVAL
82 An invalid bus object was passed.
83 -ECHILD
85 The bus connection was allocated in a parent process and is being
86 reused in a child process after fork().
87 -ENOTCONN
89 The bus connection has been terminated.
90 -EPERM
92 Two distinct file descriptors were passed for input and output
93 using sd_bus_set_fd(), which sd_bus_get_fd() cannot return.
94 -ENOPKG
96 The bus cannot be resolved.
97
99 Functions described here are available as a shared library, which can
100 be compiled against and linked to with the libsystemd pkg-config(1)
101 file.
102 The code described here uses getenv(3), which is declared to be not
104 multi-thread-safe. This means that the code calling the functions
105 described here must not call setenv(3) from a parallel thread. It is
106 recommended to only do calls to setenv() from an early phase of the
107 program when no other threads have been started.
108
110 systemd(1), sd-bus(3), sd_bus_process(3), sd_bus_attach_event(3),
111 sd_bus_wait(3), sd_bus_set_fd(3), poll(3)
112systemd 254 SD_BUS_GET_FD(3)