1SD_LISTEN_FDS(3)                 sd_listen_fds                SD_LISTEN_FDS(3)
2
3
4

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);
15
16       int sd_listen_fds_with_names(int unset_environment, char*** names);
17

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       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
40       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
44       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.
50
51       If the unset_environment parameter is non-zero, sd_listen_fds() will
52       unset the $LISTEN_FDS, $LISTEN_PID and $LISTEN_FDNAMES environment
53       variables before returning (regardless of whether the function call
54       itself succeeded or not). Further calls to sd_listen_fds() will then
55       return zero, but the variables are no longer inherited by child
56       processes.
57
58       sd_listen_fds_with_names() is like sd_listen_fds(), but optionally also
59       returns an array of strings with identification names for the passed
60       file descriptors, if that is available and the names parameter is
61       non-NULL. This information is read from the $LISTEN_FDNAMES variable,
62       which may contain a colon-separated list of names. For socket-activated
63       services, these names may be configured with the FileDescriptorName=
64       setting in socket unit files, see systemd.socket(5) for details. For
65       file descriptors pushed into the file descriptor store (see above), the
66       name is set via the FDNAME= field transmitted via
67       sd_pid_notify_with_fds(). The primary usecase for these names are
68       services which accept a variety of file descriptors which are not
69       recognizable with functions like sd_is_socket() alone, and thus require
70       identification via a name. It is recommended to rely on named file
71       descriptors only if identification via sd_is_socket() and related calls
72       is not sufficient. Note that the names used are not unique in any way.
73       The returned array of strings has as many entries as file descriptors
74       have been received, plus a final NULL pointer terminating the array.
75       The caller needs to free the array itself and each of its elements with
76       libc's free() call after use. If the names parameter is NULL, the call
77       is entirely equivalent to sd_listen_fds().
78
79       Under specific conditions, the following automatic file descriptor
80       names are returned:
81
82       Table 1.  Special names
83       ┌─────────────┬────────────────────────────┐
84Name         Description                
85       ├─────────────┼────────────────────────────┤
86       │"unknown"    │ The process received no    │
87       │             │ name for the specific file │
88       │             │ descriptor from the        │
89       │             │ service manager.           │
90       ├─────────────┼────────────────────────────┤
91       │"stored"     │ The file descriptor        │
92       │             │ originates in the service  │
93       │             │ manager's per-service file │
94       │             │ descriptor store, and the  │
95       │             │ FDNAME= field was absent   │
96       │             │ when the file descriptor   │
97       │             │ was submitted to the       │
98       │             │ service manager.           │
99       ├─────────────┼────────────────────────────┤
100       │"connection" │ The service was activated  │
101       │             │ in per-connection style    │
102       │             │ using Accept=yes in the    │
103       │             │ socket unit file, and the  │
104       │             │ file descriptor is the     │
105       │             │ connection socket.         │
106       └─────────────┴────────────────────────────┘
107

RETURN VALUE

109       On failure, these calls returns a negative errno-style error code. If
110       $LISTEN_FDS/$LISTEN_PID was not set or was not correctly set for this
111       daemon and hence no file descriptors were received, 0 is returned.
112       Otherwise, the number of file descriptors passed is returned. The
113       application may find them starting with file descriptor
114       SD_LISTEN_FDS_START, i.e. file descriptor 3.
115

NOTES

117       These APIs are implemented as a shared library, which can be compiled
118       and linked to with the libsystemd pkg-config(1) file.
119
120       Internally, sd_listen_fds() checks whether the $LISTEN_PID environment
121       variable equals the daemon PID. If not, it returns immediately.
122       Otherwise, it parses the number passed in the $LISTEN_FDS environment
123       variable, then sets the FD_CLOEXEC flag for the parsed number of file
124       descriptors starting from SD_LISTEN_FDS_START. Finally, it returns the
125       parsed number.  sd_listen_fds_with_names() does the same but also
126       parses $LISTEN_FDNAMES if set.
127

ENVIRONMENT

129       $LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
130           Set by the service manager for supervised processes that use
131           socket-based activation. This environment variable specifies the
132           data sd_listen_fds() and sd_listen_fds_with_names() parses. See
133           above for details.
134

SEE ALSO

136       systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3),
137       sd_is_socket_inet(3), sd_is_socket_unix(3), sd_pid_notify_with_fds(3),
138       daemon(7), systemd.service(5), systemd.socket(5)
139
140
141
142systemd 241                                                   SD_LISTEN_FDS(3)
Impressum