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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | ENVIRONMENT | HISTORY | SEE ALSO | NOTES | COLOPHON |
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SD_LISTEN_FDS(3) sd_listen_fds SD_LISTEN_FDS(3)
sd_listen_fds, sd_listen_fds_with_names, SD_LISTEN_FDS_START -
Check for file descriptors passed by the system manager
#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);
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 is 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
is 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.
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.
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.
$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.
sd_listen_fds_with_names() was added in version 227.
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)
1. File Descriptor Store
https://systemd.io/FILE_DESCRIPTOR_STORE
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 SD_LISTEN_FDS(3)
Pages that refer to this page: systemd(1), systemd-socket-activate(1), systemd-stdio-bridge(1), systemd-vmspawn(1), sd-daemon(3), sd_is_fifo(3), sd_notify(3), org.freedesktop.systemd1(5), systemd.exec(5), systemd.service(5), systemd.socket(5), daemon(7), systemd.directives(7), systemd.index(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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