program that prints itself, how does it work?

问题

I came across a program that prints itself on this site, i.e. it prints the program code.

The program code is:

#include <stdio.h>
char *program = "#include <stdio.h>%cchar *program = %c%s%c;%cint main()%c{%cprintf(program, 10, 34, program, 34, 10, 10, 10, 10, 10, 10);%c    return 0;%c}%c";
//what is this line doing, what is the use of %c and %s and what properties of %c and %s are being used here?
int main()
{
        printf(program, 10, 34, program, 34, 10, 10, 10, 10, 10, 10);
        //what is this print function doing, and how?
        return 0;
}

And the explanation given is:

The two key tricks here are using a string with an embedded %s specifier to allow the string to contain itself when printed, and to use the %c format specifier to allow printing out special characters like newlines, which could not otherwise be embedded in the output string.

I didn't understand how the program is working. I have mentioned the lines i need the explanation about, how they work and what are they doing. Please explain.

回答1:

char *program = "#include <stdio.h>%cchar *program = %c%s%c;%cint main()%c{%cprintf(program, 10, 34, program, 34, 10, 10, 10, 10, 10, 10);%c    return 0;%c}%c";

There is a char pointer name "program" which is used to store the string and %c and %s are format specifiers for char and string arguments respectively.

printf(program, 10, 34, program, 34, 10, 10, 10, 10, 10, 10);

printf function is printing output to console, 10 here is ASCII code for NEWLINE and 34 for " printf parameters are doing

program , passing string to be printed
10 , passing 10 ASCII code for first %c (will be converted to character newline)
program , passing same string again to %s in program to print same string again
34 , passing 34 ASCII code for second %c (will be converted to character double qoutes)
10 , passing 10 ASCII code for 3rd %c (will be converted to character newline)
10 , passing 10 ASCII code for 4th %c (will be converted to character newline)
10 , passing 10 ASCII code for 5th %c (will be converted to character newline)
10 , passing 10 ASCII code for 6th %c (will be converted to character newline)
10 , passing 10 ASCII code for 7th %c (will be converted to character newline)
10 , passing 10 ASCII code for 8th %c (will be converted to character newline)

回答2:

Printf prints the string given as the first argument (in this case the string in *program) substituting the other arguments where you have a %s or %c

%s means the arguement is a string, %c is a character.

As the note says, it uses %s to print a copy of the program string inside the program string - hence making a copy, and uses the %c to print the characters 10 (new line) and 34 "

回答3:

For a better understanding, the variable program could have been written like this:

"#include <stdio.h>\nchar *program = \"%s\";\nint main()\n..."

The idea is, that you run the program, compile it's output, run that program and so on. But this can only been done with %c values 10 for linefeed and 34 for double quote.

回答4:

This can be done using File handling. Save the program with any name and put that name in the open directory in fopen command. Like my program's name is hello.cpp.

This is the following program

#include <stdio.h>
#include <iostream>
int main()
{
    FILE *fp;   
    fp=fopen("hello.cpp","r");
    char ch;
    while((ch=fgetc(fp))!=EOF)
    {
       printf("%c",ch);
     }
}

来源：https://stackoverflow.com/questions/7690862/program-that-prints-itself-how-does-it-work

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