为什么我的老师写的 c 文件都是以 # 开头的？

Question

Why do all the C files written by my lecturer start with a #?

I'm going through some C course notes, and every C program source file begins with a single # on the first line of the program.

Then there are blank spaces, and following that other stuff followed by the main function.

What is the reason for the #?

(It's out of term now and I can't really ask the chap.)

Here's an example (there is an extra line at the end of the closing })

#

#include <stdio.h>
int main() {
   printf("Hello, World!");
   return 0;
}

转载于:https://stackoverflow.com/questions/45629176/why-do-all-the-c-files-written-by-my-lecturer-start-with-a

Answer 1

Wow, this requirement goes way back to the 1970s.

In the very early days of pre-standardised C, if you wanted to invoke the preprocessor, then you had to write a # as the first thing in the first line of a source file. Writing only a # at the top of the file affords flexibility in the placement of the other preprocessor directives.

From an original C draft by the great Dennis Ritchie himself:

12. Compiler control lines

[...] In order to cause [the] preprocessor to be invoked, it is necessary that the very first line of the program begin with #. Since null lines are ignored by the preprocessor, this line need contain no other information.

That document makes for great reading (and allowed me to jump on this question like a mad cat).

I suspect it's the lecturer simply being sentimental - it hasn't been required certainly since ANSI C.