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
       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.

       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

       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

       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 usecase 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.          │

       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.

       These APIs are implemented as a shared library, which can be compiled and
       linked to with the libsystemd pkg-config(1) file.

       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.

           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

       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)

systemd 247                                                     SD_LISTEN_FDS(3)