C Program, printf() and strlen() argument format for output

Question

Why does my terminal crash when I attempt to run this C Program?

Here are the instructions for the programming exercise.

#include <stdio.h>
#include <string.h>
 
int main(void)
{

char fname[20], lname[20];

printf("Please enter your first name:\n");
scanf("%s", fname);
printf("Please enter your last name:\n");
scanf("%s", lname);


printf("%s %s\n", fname, lname);
printf("%*d %*d\n", strlen(fname), strlen(lname));

printf("\n%s %s\n", fname, lname);
printf("%-*d %-*d", strlen(fname), strlen(lname));

return 0;
}

But when I run it like this it works fine? I do not understand why there are 4 arguments after the control statement which has two format specifiers.

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

int main(void)
{

char fname[20], lname[20];

printf("Please enter your first name:\n");
scanf("%s", fname);
printf("Please enter your last name:\n");
scanf("%s", lname);


printf("%s %s\n", fname, lname);
printf("%*d %*d\n", strlen(fname), strlen(fname), strlen(lname), 
strlen(lname));

printf("\n%s %s\n", fname, lname);
printf("%-*d %-*d", strlen(fname), strlen(lname), strlen(lname), 
strlen(lname));

return 0;
}

Answer 1

Salut, (Why can't I say "hello" at the beginning of my message?)

Why does my terminal crash when I attempt to run this C Program?

Because you don't provide enough argument in your printf function. Suddenly printf tries to read a forbidden memory area

I do not understand why there are 4 arguments

Maybe this example can help you to understand :

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

int main(void)
{

char fname[20], lname[20];

int val = 9;
    printf("%d   stop\n", val);


val = 1;
    printf("%-3d stop\n", val);
    /*printf("%-6d stop\n", val);
    printf("%-9d stop\n", val);
    printf("%-12d stop\n", val);*/

val = 2;
    printf("%-*d stop\n", 3, val);
    /*printf("%-*d stop\n", 6, val);
    printf("%-*d stop\n", 9, val);
    printf("%-*d stop\n", 12, val);*/

char *fill = "##############################";
    printf("3%.*s stop\n", 3, fill);// 3 != 3-1
    /*printf("3%.*s stop\n", 6, fill);
    printf("3%.*s stop\n", 9, fill);
    printf("3%.*s stop\n", 12, fill);*/


return 0;
}

output:

9   stop
1   stop
2   stop
3### stop

PS : What causes stack overflow errors?