sd_bus_message_append_string_memfd, sd_bus_message_append_string_iovec, sd_bus_message_append_string_space - Attach a string to a message
int sd_bus_message_append_string_memfd(sd_bus_message *m, int memfd);
int sd_bus_message_append_string_iovec(sd_bus_message *m, const struct iovec *iov, unsigned n);
int sd_bus_message_append_string_space(sd_bus_message *m, size_t size, char **s);
The functions sd_bus_message_append_string_memfd() and sd_bus_message_append_string_iovec() can be used to append a single string (item of type "s") to message m.
In case of sd_bus_message_append_string_memfd(), the contents of memfd are the string. They must satisfy the same constraints as described for the "s" type in sd_bus_message_append_basic(3).
In case of sd_bus_message_append_string_iovec(), the payload of iov is the string. It must satisfy the same constraints as described for the "s" type in sd_bus_message_append_basic(3).
The iov argument must point to n struct iovec structures. Each structure may have the iov_base field set, in which case the memory pointed to will be copied into the message, or unset, in which case a block of spaces (ASCII 32) of length iov_len will be inserted. The memory pointed at by iov may be changed after this call.
The sd_bus_message_append_string_space() function appends space for a string to message m. It behaves similar to sd_bus_message_append_basic() with type "s", but instead of copying a string into the message, it returns a pointer to the destination area to the caller in pointer p. Space for the string of length size plus the terminating NUL is allocated.
On success, those calls return 0 or a positive integer. On failure, they return a negative errno-style error code.
Returned errors may indicate the following problems:
Specified parameter is invalid.
Message has been sealed.
Message is in invalid state.
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_basic(3), The D-Bus specification 
The D-Bus specification