Perhaps the simplest solution uses one of my favorite little-known functions,
buffer[strcspn(buffer, "\n")] = 0;
If you want it to also handle
'\r' (say, if the stream is binary):
buffer[strcspn(buffer, "\r\n")] = 0; // works for LF, CR, CRLF, LFCR, ...
The function counts the number of characters until it hits a
'\r' or a
'\n' (in other words, it finds the first
'\n'). If it doesn’t hit anything, it stops at the
'\0' (returning the length of the string).
Note that this works fine even if there is no newline, because
strcspn stops at a
'\0'. In that case, the entire line is simply replacing