Ubuntu Manpage: sd_listen_fds, sd_listen_fds_with_names, SD_LISTEN_FDS_START

Provided by: libsystemd-dev_245.4-4ubuntu3.24_amd64

NAME

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

SYNOPSIS

       #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

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).
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 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

       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

       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