When Emacs writes process output directly into a multibyte buffer,
it decodes the output according to the process output coding system.
If the coding system is raw-text or no-conversion, Emacs
converts the unibyte output to multibyte using
string-to-multibyte, and inserts the resulting multibyte text.
You can use set-process-coding-system to specify which coding
system to use (see Process Information). Otherwise, the coding
system comes from coding-system-for-read, if that is
non-nil; or else from the defaulting mechanism (see Default Coding Systems). If the text output by a process contains null
bytes, Emacs by default uses no-conversion for it; see
inhibit-null-byte-detection, for how to
control this behavior.
Warning: Coding systems such as undecided, which
determine the coding system from the data, do not work entirely
reliably with asynchronous subprocess output. This is because Emacs
has to process asynchronous subprocess output in batches, as it
arrives. Emacs must try to detect the proper coding system from one
batch at a time, and this does not always work. Therefore, if at all
possible, specify a coding system that determines both the character
code conversion and the end of line conversion—that is, one like
latin-1-unix, rather than undecided or latin-1.
When Emacs calls a process filter function, it provides the process
output as a multibyte string or as a unibyte string according to the
process’s filter coding system. Emacs
decodes the output according to the process output coding system,
which usually produces a multibyte string, except for coding systems
such as binary and raw-text.