Provided by: libsystemd-dev_255.4-1ubuntu8.5_amd64 
NAME
sd_listen_fds, sd_listen_fds_with_names, SD_LISTEN_FDS_START - Check for file descriptors passed by the
system manager
SYNOPSIS
#include <systemd/sd-daemon.h>
#define SD_LISTEN_FDS_START 3
int sd_listen_fds(int unset_environment);
int sd_listen_fds_with_names(int unset_environment, char*** names);
DESCRIPTION
sd_listen_fds() may be invoked by a daemon to check for file descriptors passed by the service manager as
part of the socket-based activation and file descriptor store logic. It returns the number of received
file descriptors. If no file descriptors have been received, zero is returned. The first file descriptor
may be found at file descriptor number 3 (i.e. SD_LISTEN_FDS_START), the remaining descriptors follow at
4, 5, 6, ..., if any.
The file descriptors passed this way may be closed at will by the processes receiving them: it's up to
the processes themselves to close them after use or whether to leave them open until the process exits
(in which case the kernel closes them automatically). Note that the file descriptors received by daemons
are duplicates of the file descriptors the service manager originally allocated and bound and of which it
continuously keeps a copy (except if Accept=yes is used). This means any socket option changes and other
changes made to the sockets will be visible to the service manager too. Most importantly this means it's
generally not a good idea to invoke shutdown(2) on such sockets, since it will shut down communication on
the file descriptor the service manager holds for the same socket too. Also note that if a daemon is
restarted (and its associated sockets are not) it will receive file descriptors to the very same sockets
as the earlier invocations, thus all socket options applied then will still apply.
If a daemon receives more than one file descriptor, they will be passed in the same order as configured
in the systemd socket unit file (see systemd.socket(5) for details) — if there's only one such file (see
below). Nonetheless, it is recommended to verify the correct socket types before using them. To simplify
this checking, the functions sd_is_fifo(3), sd_is_socket(3), sd_is_socket_inet(3), sd_is_socket_unix(3)
are provided. In order to maximize flexibility, it is recommended to make these checks as loose as
possible without allowing incorrect setups. i.e. often, the actual port number a socket is bound to
matters little for the service to work, hence it should not be verified. On the other hand, whether a
socket is a datagram or stream socket matters a lot for the most common program logics and should be
checked.
This function call will set the FD_CLOEXEC flag for all passed file descriptors to avoid further
inheritance to children of the calling process.
If multiple socket units activate the same service, the order of the file descriptors passed to its main
process is undefined. If additional file descriptors have been passed to the service manager using
sd_pid_notify_with_fds(3)'s "FDSTORE=1" messages, these file descriptors are passed last, in arbitrary
order, and with duplicates removed.
If the unset_environment parameter is non-zero, sd_listen_fds() will unset the $LISTEN_FDS, $LISTEN_PID
and $LISTEN_FDNAMES environment variables before returning (regardless of whether the function call
itself succeeded or not). Further calls to sd_listen_fds() will then return zero, but the variables are
no longer inherited by child processes.
sd_listen_fds_with_names() is like sd_listen_fds(), but optionally also returns an array of strings with
identification names for the passed file descriptors, if that is available and the names parameter is
non-NULL. This information is read from the $LISTEN_FDNAMES variable, which may contain a colon-separated
list of names. For socket-activated services, these names may be configured with the FileDescriptorName=
setting in socket unit files, see systemd.socket(5) for details. For file descriptors pushed into the
file descriptor store (see above), the name is set via the FDNAME= field transmitted via
sd_pid_notify_with_fds(). The primary use case for these names are services which accept a variety of
file descriptors which are not recognizable with functions like sd_is_socket() alone, and thus require
identification via a name. It is recommended to rely on named file descriptors only if identification via
sd_is_socket() and related calls is not sufficient. Note that the names used are not unique in any way.
The returned array of strings has as many entries as file descriptors have been received, plus a final
NULL pointer terminating the array. The caller needs to free the array itself and each of its elements
with libc's free() call after use. If the names parameter is NULL, the call is entirely equivalent to
sd_listen_fds().
Under specific conditions, the following automatic file descriptor names are returned:
Table 1. Special names
┌─────────────┬───────────────────────────────────────┐
│Name │ Description │
├─────────────┼───────────────────────────────────────┤
│"unknown" │ The process received no name for the │
│ │ specific file descriptor from the │
│ │ service manager. │
├─────────────┼───────────────────────────────────────┤
│"stored" │ The file descriptor originates in the │
│ │ service manager's per-service file │
│ │ descriptor store, and the FDNAME= │
│ │ field was absent when the file │
│ │ descriptor was submitted to the │
│ │ service manager. │
├─────────────┼───────────────────────────────────────┤
│"connection" │ The service was activated in │
│ │ per-connection style using Accept=yes │
│ │ in the socket unit file, and the file │
│ │ descriptor is the connection socket. │
└─────────────┴───────────────────────────────────────┘
For further information on the file descriptor store see the File Descriptor Store[1] overview.
RETURN VALUE
On failure, these calls returns a negative errno-style error code. If $LISTEN_FDS/$LISTEN_PID was not set
or was not correctly set for this daemon and hence no file descriptors were received, 0 is returned.
Otherwise, the number of file descriptors passed is returned. The application may find them starting with
file descriptor SD_LISTEN_FDS_START, i.e. file descriptor 3.
NOTES
Functions described here are available as a shared library, which can be compiled against and linked to
with the libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that
the code calling the functions described here must not call setenv(3) from a parallel thread. It is
recommended to only do calls to setenv() from an early phase of the program when no other threads have
been started.
Internally, sd_listen_fds() checks whether the $LISTEN_PID environment variable equals the daemon PID. If
not, it returns immediately. Otherwise, it parses the number passed in the $LISTEN_FDS environment
variable, then sets the FD_CLOEXEC flag for the parsed number of file descriptors starting from
SD_LISTEN_FDS_START. Finally, it returns the parsed number. sd_listen_fds_with_names() does the same but
also parses $LISTEN_FDNAMES if set.
These functions are not designed for services that specify StandardInput=socket as the $LISTEN_FDS
variable is not set in their environment.
ENVIRONMENT
$LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
Set by the service manager for supervised processes that use socket-based activation. This
environment variable specifies the data sd_listen_fds() and sd_listen_fds_with_names() parses. See
above for details.
HISTORY
sd_listen_fds_with_names() was added in version 227.
SEE ALSO
systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3), sd_is_socket_inet(3), sd_is_socket_unix(3), sd_pid_notify_with_fds(3), daemon(7), systemd.service(5), systemd.socket(5)
NOTES
1. File Descriptor Store
https://systemd.io/FILE_DESCRIPTOR_STORE