1SD_LISTEN_FDS(3) sd_listen_fds SD_LISTEN_FDS(3)
2
6 sd_listen_fds, SD_LISTEN_FDS_START - Check for file descriptors passed
7 by the system manager
8
10 #include <systemd/sd-daemon.h>
11 #define SD_LISTEN_FDS_START 3
13 int sd_listen_fds(int unset_environment);
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17 sd_listen_fds() shall be called by a daemon to check for file
18 descriptors passed by the init system as part of the socket-based
19 activation logic.
20 If the unset_environment parameter is non-zero, sd_listen_fds() will
22 unset the $LISTEN_FDS and $LISTEN_PID environment variables before
23 returning (regardless of whether the function call itself succeeded or
24 not). Further calls to sd_listen_fds() will then fail, but the
25 variables are no longer inherited by child processes.
26 If a daemon receives more than one file descriptor, they will be passed
28 in the same order as configured in the systemd socket unit file (see
29 systemd.socket(5) for details). Nonetheless, it is recommended to
30 verify the correct socket types before using them. To simplify this
31 checking, the functions sd_is_fifo(3), sd_is_socket(3),
32 sd_is_socket_inet(3), sd_is_socket_unix(3) are provided. In order to
33 maximize flexibility, it is recommended to make these checks as loose
34 as possible without allowing incorrect setups. i.e. often, the actual
35 port number a socket is bound to matters little for the service to
36 work, hence it should not be verified. On the other hand, whether a
37 socket is a datagram or stream socket matters a lot for the most common
38 program logics and should be checked.
39 This function call will set the FD_CLOEXEC flag for all passed file
41 descriptors to avoid further inheritance to children of the calling
42 process.
43 If multiple socket units activate the same service the order of the
45 file descriptors passed to its main process is undefined. If additional
46 file descriptors have been passed to the service manager using
47 sd_pid_notify_with_fds(3)'s "FDSTORE=1" messages, these file
48 descriptors are passed last, in arbitrary order, and with duplicates
49 removed.
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52 On failure, this call returns a negative errno-style error code. If
53 $LISTEN_FDS/$LISTEN_PID was not set or was not correctly set for this
54 daemon and hence no file descriptors were received, 0 is returned.
55 Otherwise, the number of file descriptors passed is returned. The
56 application may find them starting with file descriptor
57 SD_LISTEN_FDS_START, i.e. file descriptor 3.
58
60 These APIs are implemented as a shared library, which can be compiled
61 and linked to with the libsystemd pkg-config(1) file.
62 Internally, this function checks whether the $LISTEN_PID environment
64 variable equals the daemon PID. If not, it returns immediately.
65 Otherwise, it parses the number passed in the $LISTEN_FDS environment
66 variable, then sets the FD_CLOEXEC flag for the parsed number of file
67 descriptors starting from SD_LISTEN_FDS_START. Finally, it returns the
68 parsed number.
69
71 $LISTEN_PID, $LISTEN_FDS
72 Set by the init system for supervised processes that use
73 socket-based activation. This environment variable specifies the
74 data sd_listen_fds() parses. See above for details.
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77 systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3),
78 sd_is_socket_inet(3), sd_is_socket_unix(3), daemon(7),
79 systemd.service(5), systemd.socket(5)
80systemd 219 SD_LISTEN_FDS(3)