Recursion using main() function

Question

I am writing a program to calculate factorial using recursion of main() function.

/* Print factorial */
#include <stdio.h>
#include <stdlib.h>

static char **p;

int main(int argc, char **argv)
{
        int n, rv;
        if (argc < 2) {
                printf("Usage: a.out <value>\n");
                exit(-1);
        }
        n = atoi(argv[1]);
        if (!n) {
                rv = 0;
        } else {
                if (n == 1) {
                        rv = 1;
                } else {
                        n = n - 1;
                        **p = n;
                        main(2, p);
                }
        }
        printf("%d\n", rv);
        return 0;
}

The program compiles using gcc but on execution, I am getting a Segmentation Fault at **p = n. Can somebody help me to modify the above program to get the correct result. Also, what is the logic to capture correct rv value between successive recursive calls in main()?

Why would you explicitly use something that you know is bad? — Mark Garcia, Dec 19 '12 at 08:51
@icepack: FUBAR - Fully Upvoted Because of Arbitrary Recursion — manav m-n, Dec 19 '12 at 08:59
`p` is unassigned, can't deference it. Do it correctly: try to write 'standard' code with good practices — Tio Pepe, Dec 19 '12 at 09:12
You should compile with all warnings and debugging info, that is with `gcc -Wall -g` on Linux, improve your code till no warnings are given, and learn to use the `gdb` debugger (and also `valgrind`) — Basile Starynkevitch, Dec 19 '12 at 09:12

nhahtdh · Accepted Answer · 2012-12-19T09:49:36.743

15

Since you don't seem to care about standard and stuffs, here is an implementation of the recursive main function for printing factorial, that compiles on gcc (I only test on Windows). Since it doesn't follow standard, there is no guarantee that it will compiles on other compiler/platform.

Writing such code for fun is OK, but never let the bad behavior follows into serious coding project or workplace.

/* Print factorial */
#include <stdio.h>
#include <stdlib.h>

char buf[16];

int main(int argc, char **argv)
{
        int n, rv;

        if (argc < 2) {
                printf("Usage: a.out <value>\n");
                exit(-1);
        }

        n = atoi(argv[1]);
        if (!n) {
                rv = 1;
        } else {
                if (n == 1) {
                    rv = 1;
                } else {
                    char *pt = buf;
                    char **pt2 = &pt - 1;

                    sprintf(buf, "%d", n - 1);
                    rv = main(2, pt2) * n;
                }
        }
        printf("%d\n", rv);

        return rv;
}

edited Dec 19 '12 at 09:49

answered Dec 19 '12 at 09:18

nhahtdh

55,989
15
126
162

1

YeeeHoooo... yes this solved my problem ! – manav m-n Dec 19 '12 at 09:21
How can you claim "here is a working implementation"? - at best it happens to work on some set of tested platforms, as far as you could tell. – Tony Delroy Dec 19 '12 at 09:47
1

@TonyD: Agree, edited. It is bad code anyway. – nhahtdh Dec 19 '12 at 09:50
1

Well -1 to +1 for the more appropriately cautioned example - hackers, live long and crash...! – Tony Delroy Dec 19 '12 at 11:00

score -3 · Answer 2 · answered Dec 19 '12 at 09:36

Only operating system can call main when it runs the program. Other than operating system no one can call any function named main. So if want to calculate factorial using recursion then you have to write another function to calculate that recursively and call that function from main.

You can ask why is this? Answer is this is the syntax.

Recursion using main() function

2 Answers2

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