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

       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 details.

       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 246                                                   SD_LISTEN_FDS(3)