sd_listen_fds(3) — Linux manual page


SD_LISTEN_FDS(3)              sd_listen_fds             SD_LISTEN_FDS(3)

NAME         top

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

SYNOPSIS         top

       #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         top

       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

       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         top

       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         top

       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

       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         top

           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         top

       sd_listen_fds_with_names() was added in version 227.

SEE ALSO         top

       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),

NOTES         top

        1. File Descriptor Store

COLOPHON         top

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       manager) project.  Information about the project can be found at
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       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
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systemd 255                                             SD_LISTEN_FDS(3)

Pages that refer to this page: systemd(1)systemd-socket-activate(1)systemd-stdio-bridge(1)sd-daemon(3)sd_is_fifo(3)sd_notify(3)systemd.exec(5)systemd.service(5)systemd.socket(5)daemon(7)systemd.directives(7)systemd.index(7)