Inserts len bytes of val into string at pos.
Because len is provided, val may contain embedded
nuls and need not be nul-terminated. If pos is -1,
bytes are inserted at the end of the string.
Since this function does not stop at nul bytes, it is
the caller's responsibility to ensure that val has at
least len addressable bytes.
Inserts len bytes of val into string at pos. Because len is provided, val may contain embedded nuls and need not be nul-terminated. If pos is -1, bytes are inserted at the end of the string. Since this function does not stop at nul bytes, it is the caller's responsibility to ensure that val has at least len addressable bytes.