1SD_LISTEN_FDS(3) sd_listen_fds SD_LISTEN_FDS(3)
2
6 sd_listen_fds, sd_listen_fds_with_names, SD_LISTEN_FDS_START - Check
7 for file descriptors passed 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);
15 int sd_listen_fds_with_names(int unset_environment, char*** names);
17
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 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) — if there's only one such file (see
30 below). Nonetheless, it is recommended to verify the correct socket
31 types before using them. To simplify this checking, the functions
32 sd_is_fifo(3), sd_is_socket(3), sd_is_socket_inet(3),
33 sd_is_socket_unix(3) are provided. In order to maximize flexibility, it
34 is recommended to make these checks as loose as possible without
35 allowing incorrect setups. i.e. often, the actual port number a socket
36 is bound to matters little for the service to work, hence it should not
37 be verified. On the other hand, whether a socket is a datagram or
38 stream socket matters a lot for the most common program logics and
39 should be checked.
40 This function call will set the FD_CLOEXEC flag for all passed file
42 descriptors to avoid further inheritance to children of the calling
43 process.
44 If multiple socket units activate the same service, the order of the
46 file descriptors passed to its main process is undefined. If additional
47 file descriptors have been passed to the service manager using
48 sd_pid_notify_with_fds(3)'s "FDSTORE=1" messages, these file
49 descriptors are passed last, in arbitrary order, and with duplicates
50 removed.
51 If the unset_environment parameter is non-zero, sd_listen_fds() will
53 unset the $LISTEN_FDS, $LISTEN_PID and $LISTEN_FDNAMES environment
54 variables before returning (regardless of whether the function call
55 itself succeeded or not). Further calls to sd_listen_fds() will then
56 return zero, but the variables are no longer inherited by child
57 processes.
58 sd_listen_fds_with_names() is like sd_listen_fds(), but optionally also
60 returns an array of strings with identification names for the passed
61 file descriptors, if that is available and the names parameter is
62 non-NULL. This information is read from the $LISTEN_FDNAMES variable,
63 which may contain a colon-separated list of names. For socket-activated
64 services, these names may be configured with the FileDescriptorName=
65 setting in socket unit files, see systemd.socket(5) for details. For
66 file descriptors pushed into the file descriptor store (see above), the
67 name is set via the FDNAME= field transmitted via
68 sd_pid_notify_with_fds(). The primary usecase for these names are
69 services which accept a variety of file descriptors which are not
70 recognizable with functions like sd_is_socket() alone, and thus require
71 identification via a name. It is recommended to rely on named file
72 descriptors only if identification via sd_is_socket() and related calls
73 is not sufficient. Note that the names used are not unique in any way.
74 The returned array of strings has as many entries as file descriptors
75 have been received, plus a final NULL pointer terminating the array.
76 The caller needs to free the array itself and each of its elements with
77 libc's free() call after use. If the names parameter is NULL, the call
78 is entirely equivalent to sd_listen_fds().
79 Under specific conditions, the following automatic file descriptor
81 names are returned:
82 Table 1. Special names
84 ┌─────────────┬────────────────────────────┐
85 │Name │ Description │
86 ├─────────────┼────────────────────────────┤
87 │"unknown" │ The process received no │
88 │ │ name for the specific file │
89 │ │ descriptor from the │
90 │ │ service manager. │
91 ├─────────────┼────────────────────────────┤
92 │"stored" │ The file descriptor │
93 │ │ originates in the service │
94 │ │ manager's per-service file │
95 │ │ descriptor store, and the │
96 │ │ FDNAME= field was absent │
97 │ │ when the file descriptor │
98 │ │ was submitted to the │
99 │ │ service manager. │
100 ├─────────────┼────────────────────────────┤
101 │"connection" │ The service was activated │
102 │ │ in per-connection style │
103 │ │ using Accept=yes in the │
104 │ │ socket unit file, and the │
105 │ │ file descriptor is the │
106 │ │ connection socket. │
107 └─────────────┴────────────────────────────┘
108
110 On failure, these calls returns a negative errno-style error code. If
111 $LISTEN_FDS/$LISTEN_PID was not set or was not correctly set for this
112 daemon and hence no file descriptors were received, 0 is returned.
113 Otherwise, the number of file descriptors passed is returned. The
114 application may find them starting with file descriptor
115 SD_LISTEN_FDS_START, i.e. file descriptor 3.
116
118 These APIs are implemented as a shared library, which can be compiled
119 and linked to with the libsystemd pkg-config(1) file.
120 Internally, sd_listen_fds() checks whether the $LISTEN_PID environment
122 variable equals the daemon PID. If not, it returns immediately.
123 Otherwise, it parses the number passed in the $LISTEN_FDS environment
124 variable, then sets the FD_CLOEXEC flag for the parsed number of file
125 descriptors starting from SD_LISTEN_FDS_START. Finally, it returns the
126 parsed number. sd_listen_fds_with_names() does the same but also
127 parses $LISTEN_FDNAMES if set.
128
130 $LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
131 Set by the service manager for supervised processes that use
132 socket-based activation. This environment variable specifies the
133 data sd_listen_fds() and sd_listen_fds_with_names() parses. See
134 above for details.
135
137 systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3),
138 sd_is_socket_inet(3), sd_is_socket_unix(3), sd_pid_notify_with_fds(3),
139 daemon(7), systemd.service(5), systemd.socket(5)
140systemd 246 SD_LISTEN_FDS(3)