Simple readline algorithm for Networking

Are there any simple standardized way of reading a full line from a socket that mallocs the memory for you? If so, what are they? If not, is there a way to do it in approximately O(1) time (obviously not including the time for the memory allocation).

The better way is to read data into a buffer, and after there's no more data (at this moment), read all complete lines from the buffer. Implementation is left as an exercise for the reader.