jybox/ZeroMS — comment on commit 40f35b56304135dcbbc91fa9cff5dabb3f21068f
File:  line 0
By whtsky:

Each instance of a backslash character (/) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines.Only the last backslash on any physical source lineshall be eligible for being part of such a splice. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character before any such splicing takes place.

C99 Rationale中进一步指出：

A backslash immediately before a newline has long been used to continue string literals, as well as preprocessing command lines. In the interest of easing machine generation of C, and of transporting code to machines with restrictive physical line lengths, the C89 Committee generalized this mechanism to permit any token to be continued by interposing a backslash/newline sequence.

这么规定的初衷有两个：
为了每一行都要以换行结束。
因为行尾的/表示连接下一行，如果一个文件最后一行(main函数的右边大括号)行尾有/，那么，紧跟它也被包含进来的下一个源文件的第一行就会被连接！而如果一个文件以一个空行结束就会避免这种情况的发生。