C language: meaning of operator "#" ?

Question

This code can be compiled by gcc -g -o main main.c, no warnings.

// main.c
# 41 "stdio.h"
# 22
#
int main(void){
    (void)printf("foo");
    return 0;
}

My question is:

1. What is the meaning of operator "#" before "include". We use #include <stdio.h> to include the header file, why not include <stdio.h> ?

2. Why # 22 or # 41 can be compiled, I have never seen it before.

Anybody can help me ?

Answer 1

This looks like the output of the preprocessor. The line

# 41 "stdio.h"

can be interpreted like:

• 41 has the meaning:

It specifies the line number which should be reported for the following line of input. Subsequent lines are counted from linenum.

• "stdio.h" is just the filename from which the lines are taken.

The following line and all subsequent lines are reported to come from the file it specifies, until something else happens to change that. filename is interpreted according to the normal rules for a string constant: backslash escapes are interpreted.

And all this information is taken from https://gcc.gnu.org/onlinedocs/cpp/Line-Control.html#Line-Control and https://gcc.gnu.org/onlinedocs/cpp/Preprocessor-Output.html#Preprocessor-Output

For the single #:

The null directive consists of a ‘#’ followed by a newline, with only whitespace (including comments) in between. A null directive is understood as a preprocessing directive but has no effect on the preprocessor output. The primary significance of the existence of the null directive is that an input line consisting of just a ‘#’ will produce no output, rather than a line of output containing just a ‘#’. Supposedly some old C programs contain such lines.

taken from https://gcc.gnu.org/onlinedocs/cpp/Other-Directives.html#Other-Directives

Answer 2

A # at the start of a line introduces a preprocessing directive. Preprocessing is conceptually an earlier stage of translation than compilation, which is why it uses an easily-recognisable syntax.

If the # is followed by anything other than include, define, undef, line, error or pragma, then it is a non-directive (6.10p1), and is ignored.

This means that # at the start of a line is a simple way for the compiler to pass information between stages of translation; here it is being used to pass source file and line number information.