sd_bus_set_fd - Man Page
Set the file descriptors to use for bus communication
int sd_bus_set_fd(sd_bus *bus, int input_fd, int output_fd);
sd_bus_set_fd() sets the file descriptors used to communicate by a bus connection object. Both input_fd and output_fd must be valid file descriptors, referring to stream-based file objects (e.g. a stream socket, a pair of pipes or FIFOs, or even a TTY device). input_fd must be readable, and output_fd must be writable. The same file descriptor may be used (and typically is used) as both the input and the output file descriptor. This function must be called before the bus connection is started via sd_bus_start(3).
The bus connection object will take possession of the passed file descriptors and will close them automatically when it is freed. Use sd_bus_set_close_on_exit(3) to turn off this behaviour.
On success, sd_bus_set_fd() returns a non-negative integer. On failure, it returns a negative errno-style error code.
Returned errors may indicate the following problems:
An invalid bus object was passed.
The bus connection was allocated in a parent process and is being reused in a child process after fork().
An invalid file descriptor was passed to sd_bus_set_fd().
The bus cannot be resolved.
The bus connection has already been started.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.
systemd(1), sd-bus(3), sd_bus_get_fd(3), sd_bus_start(3)
sd_bus_get_fd(3), sd_bus_start(3), systemd.directives(7), systemd.index(7).