C - Using "&variable" in printf()

Question

I am running this program with two command line arguments: 10 and test. It works as intended when I use:

printf("C = %s\n\n", &C);

but not when I use:

printf("C = %s\n\n", C);

and I can't understand why.

#include <stdio.h>
#include <string.h>

int main(int argc, char ** argv)
{

    int N = 0;
    char C = '\0';

    sscanf(argv[1], "%d", &N);
    sscanf(argv[2], "%s", &C);

    printf("N = %d\n\n", N);
    printf("C = %s\n\n", &C);

    return 0;

}

`sscanf(argv[2], "%s", &C);` is undefined behaviour : you don't have enough room to store the result whatever it is. It works by mere luck — Jean-François Fabre, Dec 19 '17 at 17:04
`C` is a single character and should be printed out using `%c` not `%s` — Chris Turner, Dec 19 '17 at 17:05
You allocate a variable to hold a `char` and then try to get it to hold a string. — David Schwartz, Dec 19 '17 at 17:06

MarkAsh · Answer 1 · 2017-12-19T17:21:27.940

1

I changed to the code below and seems to be working. I changed char C = '\0' to char C[10] so C is a string and not a character and removed ampersand (&) operator from printf("C = %s\n\n", &C).

#include <stdio.h>
#include <string.h>

int main(int argc, char ** argv)
{

    int N = 0;
    char C[10];

    sscanf(argv[1], "%d", &N);
    sscanf(argv[2], "%s", C);

    printf("N = %d\n\n", N);
    printf("C = %s\n\n", C);

    return 0;

}

edited Dec 19 '17 at 17:21

answered Dec 19 '17 at 17:12

MarkAsh

33
6

1

@Yonlif *"explain to him"* It's the OP. :) – HolyBlackCat Dec 19 '17 at 17:14
I mean that he should add something but just code @HolyBlackCat . – Yonlif Dec 19 '17 at 17:16
are you sure 10 is large enough to hold the string though? That is only big enough to hold 9 characters plus the NUL terminator at the end. – Chris Turner Dec 19 '17 at 17:53
the string was "test", so 10 should be enough – MarkAsh Dec 19 '17 at 20:42

score 0 · Answer 2 · answered Dec 19 '17 at 17:11

As I see it there are several problems in your code and in order to fix it you will need to change it alot. I don't believe that someone will do this for you, and I won't as well because everybody need to learn.

Google is your best friend!

Before asking question in Stack Overflow check how to do what you want to do online.

For example, here you can find a great answer about how to properly get command line parameters.

And here you can see the difference between with or without '&' and what does it mean.

Good luck

C - Using "&variable" in printf()

2 Answers2