please don't change the format of the doc.

ok,i just know this ,sorry

you should use the string.buffer instead of implement this feature.

how to use string.buffer instead of implement this feature on lua land ,can you give some example? the copy occurs on the c land,but i need to split the big string on the lua land ,and also i need to make the header of the udp package on lua land. if i use the origin udpsock:send() method , sub string ( big_msg ) is inevitable ,.usually i can send big string
use udp like below，there is no need to make a new string pass to c land.

for idx = 1, totalPackage do
        local header_binary = struct.pack(">i2c1c32i4c1", packageSize + 42, '1', msg_id, totalPackage, zip_type or '0')
        local serial_binary
        if totalPackage == 1 then
            serial_binary = struct.pack(">i4", 0)
        else
            msgEndIndex = msgStartIndex + packageSize - 1
            serial_binary = struct.pack(">i4", idx - 1)
        end
        ok, err = udpsock:sendbuf(header_binary, serial_binary, big_msg, msgStartIndex, msgEndIndex)
        ---sub string use origin method
        ---ok, err = udpsock:send({ header_binary, serial_binary, string.sub(msg, msgStartIndex, msgEndIndex) })
        msgStartIndex = msgEndIndex + 1
    end

https://luajit.org/ext_buffer.html

Thanks a lot，but we can't assume that from the beginning the user can use string.buffer to build the string content on the lua land ,sometimes is really hard for it , and more scenario is that the user can only accept a large string which was already built before.

I think we still need to copy a string on lua land, event if we use the string buffer.that is because openresty only provides a udp send method which can accepts only a string or table object.

Limited by udp message size , it’s necessary to split an existing big string, so you need to avoid a new large number of substring , The new udp send method that i provided is to adapt to this scenario.

Thank you very much!

gotcha