sd_bus_message_copy man page
sd_bus_message_copy — Copy the contents of one message to another
int sd_bus_message_copy(sd_bus_message *m, sd_bus_message *source, int all);
sd_bus_message_copy() copies the contents from message source to m. If all is false, a single complete type is copied (basic or container). If all is true, the contents are copied until the end of the currently open container or the end of source.
On success, this call returns true if anything was copied, and false if there was nothing to copy. On failure, it returns a negative errno-style error code.
Returned errors may indicate the following problems:
source or m are NULL.
Message m has been sealed or source has not been sealed.
Destination message is in invalid state.
Destination message cannot be appended to.
Memory allocation failed.
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_message_append(3)
sd-bus(3), systemd.directives(7), systemd.index(7).