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 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.        │

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

       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.

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.

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),

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug
       report for this manual page, see
       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨⟩ on 2020-09-18.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-09-18.)  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

systemd 246                                                 SD_LISTEN_FDS(3)

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