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.