1SD_LISTEN_FDS(3)                 sd_listen_fds                SD_LISTEN_FDS(3)
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NAME

6       sd_listen_fds, sd_listen_fds_with_names, SD_LISTEN_FDS_START - Check
7       for file descriptors passed by the system manager
8

SYNOPSIS

10       #include <systemd/sd-daemon.h>
11
12       #define SD_LISTEN_FDS_START 3
13
14       int sd_listen_fds(int unset_environment);
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16       int sd_listen_fds_with_names(int unset_environment, char*** names);
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DESCRIPTION

19       sd_listen_fds() may be invoked by a daemon to check for file
20       descriptors passed by the service manager as part of the socket-based
21       activation logic. It returns the number of received file descriptors.
22       If no file descriptors have been received, zero is returned. The first
23       file descriptor may be found at file descriptor number 3 (i.e.
24       SD_LISTEN_FDS_START), the remaining descriptors follow at 4, 5, 6, ...,
25       if any.
26
27       The file descriptors passed this way may be closed at will by the
28       processes receiving them: it's up to the processes themselves to close
29       them after use or whether to leave them open until the process exits
30       (in which case the kernel closes them automatically). Note that the
31       file descriptors received by daemons are duplicates of the file
32       descriptors the service manager originally allocated and bound and of
33       which it continously keeps a copy (except if Accept=yes is used). This
34       means any socket option changes and other changes made to the sockets
35       will visible to the service manager too. Most importanly this means
36       it's generally not a good idea to invoke shutdown(2) on such sockets,
37       since it will shut down communication on the file descriptor the
38       service manager holds for the same socket, too. Also note that if a
39       daemon is restarted (and its associated sockets are not) it will
40       receive file descriptors to the very same sockets as the earlier
41       invocations, thus all socket options applied then will still apply.
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43       If a daemon receives more than one file descriptor, they will be passed
44       in the same order as configured in the systemd socket unit file (see
45       systemd.socket(5) for details) — if there's only one such file (see
46       below). Nonetheless, it is recommended to verify the correct socket
47       types before using them. To simplify this checking, the functions
48       sd_is_fifo(3), sd_is_socket(3), sd_is_socket_inet(3),
49       sd_is_socket_unix(3) are provided. In order to maximize flexibility, it
50       is recommended to make these checks as loose as possible without
51       allowing incorrect setups. i.e. often, the actual port number a socket
52       is bound to matters little for the service to work, hence it should not
53       be verified. On the other hand, whether a socket is a datagram or
54       stream socket matters a lot for the most common program logics and
55       should be checked.
56
57       This function call will set the FD_CLOEXEC flag for all passed file
58       descriptors to avoid further inheritance to children of the calling
59       process.
60
61       If multiple socket units activate the same service, the order of the
62       file descriptors passed to its main process is undefined. If additional
63       file descriptors have been passed to the service manager using
64       sd_pid_notify_with_fds(3)'s "FDSTORE=1" messages, these file
65       descriptors are passed last, in arbitrary order, and with duplicates
66       removed.
67
68       If the unset_environment parameter is non-zero, sd_listen_fds() will
69       unset the $LISTEN_FDS, $LISTEN_PID and $LISTEN_FDNAMES environment
70       variables before returning (regardless of whether the function call
71       itself succeeded or not). Further calls to sd_listen_fds() will then
72       return zero, but the variables are no longer inherited by child
73       processes.
74
75       sd_listen_fds_with_names() is like sd_listen_fds(), but optionally also
76       returns an array of strings with identification names for the passed
77       file descriptors, if that is available and the names parameter is
78       non-NULL. This information is read from the $LISTEN_FDNAMES variable,
79       which may contain a colon-separated list of names. For socket-activated
80       services, these names may be configured with the FileDescriptorName=
81       setting in socket unit files, see systemd.socket(5) for details. For
82       file descriptors pushed into the file descriptor store (see above), the
83       name is set via the FDNAME= field transmitted via
84       sd_pid_notify_with_fds(). The primary usecase for these names are
85       services which accept a variety of file descriptors which are not
86       recognizable with functions like sd_is_socket() alone, and thus require
87       identification via a name. It is recommended to rely on named file
88       descriptors only if identification via sd_is_socket() and related calls
89       is not sufficient. Note that the names used are not unique in any way.
90       The returned array of strings has as many entries as file descriptors
91       have been received, plus a final NULL pointer terminating the array.
92       The caller needs to free the array itself and each of its elements with
93       libc's free() call after use. If the names parameter is NULL, the call
94       is entirely equivalent to sd_listen_fds().
95
96       Under specific conditions, the following automatic file descriptor
97       names are returned:
98
99       Table 1.  Special names
100       ┌─────────────┬────────────────────────────┐
101Name         Description                
102       ├─────────────┼────────────────────────────┤
103       │"unknown"    │ The process received no    │
104       │             │ name for the specific file │
105       │             │ descriptor from the        │
106       │             │ service manager.           │
107       ├─────────────┼────────────────────────────┤
108       │"stored"     │ The file descriptor        │
109       │             │ originates in the service  │
110       │             │ manager's per-service file │
111       │             │ descriptor store, and the  │
112       │             │ FDNAME= field was absent   │
113       │             │ when the file descriptor   │
114       │             │ was submitted to the       │
115       │             │ service manager.           │
116       ├─────────────┼────────────────────────────┤
117       │"connection" │ The service was activated  │
118       │             │ in per-connection style    │
119       │             │ using Accept=yes in the    │
120       │             │ socket unit file, and the  │
121       │             │ file descriptor is the     │
122       │             │ connection socket.         │
123       └─────────────┴────────────────────────────┘
124

RETURN VALUE

126       On failure, these calls returns a negative errno-style error code. If
127       $LISTEN_FDS/$LISTEN_PID was not set or was not correctly set for this
128       daemon and hence no file descriptors were received, 0 is returned.
129       Otherwise, the number of file descriptors passed is returned. The
130       application may find them starting with file descriptor
131       SD_LISTEN_FDS_START, i.e. file descriptor 3.
132

NOTES

134       These APIs are implemented as a shared library, which can be compiled
135       and linked to with the libsystemd pkg-config(1) file.
136
137       Internally, sd_listen_fds() checks whether the $LISTEN_PID environment
138       variable equals the daemon PID. If not, it returns immediately.
139       Otherwise, it parses the number passed in the $LISTEN_FDS environment
140       variable, then sets the FD_CLOEXEC flag for the parsed number of file
141       descriptors starting from SD_LISTEN_FDS_START. Finally, it returns the
142       parsed number.  sd_listen_fds_with_names() does the same but also
143       parses $LISTEN_FDNAMES if set.
144

ENVIRONMENT

146       $LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
147           Set by the service manager for supervised processes that use
148           socket-based activation. This environment variable specifies the
149           data sd_listen_fds() and sd_listen_fds_with_names() parses. See
150           above for details.
151

SEE ALSO

153       systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3),
154       sd_is_socket_inet(3), sd_is_socket_unix(3), sd_pid_notify_with_fds(3),
155       daemon(7), systemd.service(5), systemd.socket(5)
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159systemd 249                                                   SD_LISTEN_FDS(3)
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