process-send-string

process-send-string is a built-in function in `C source code'.

(process-send-string PROCESS STRING)

Send PROCESS the contents of STRING as input.
PROCESS may be a process, a buffer, the name of a process or buffer, or
nil, indicating the current buffer's process.
If STRING is more than 500 characters long,
it is sent in several bunches. This may happen even for shorter strings.
Output from processes can arrive in between bunches.