Why do all the C files written by my lecturer start with a #?
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;
}
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.
链接地址: http://www.djcxy.com/p/59856.html