C Program To Calculate Numbers Squared | Arguments Segmentation Fault

Question

I'm a beginner to C, trying to write a program to calculate a square number. At my college we're not allowed to use printf() or scanf() which makes things a bit complicated.

This led me to using arguments to get input. I'm getting a segmentation fault (core dumped) when I try to compile. I think this comes from using argv and indexes but I'm not sure how to fix it.

Do you have some insight that might help? It would be much appreciated!

#include <unistd.h>

int main(int argc, char *argv[]) {
    int number;
    int square;

    while(argc != 0) {
        number = argv[1][square];
        square = number * number;

        write(1, &square, 1);
        square++;
    }

    write(1, "\n", 1);
    return 0;
}

Answer 1

The segmentation fault, if not caused by something else first will be caused here:

number = argv[1][square];//seg fault possible when square becomes 
                         //larger than strlen(argv[1]) + 1

Attempting to access memory that the program process does not own, as above, will invoke undefined behavior.

Also, at the time this expression is executed:

square = number * number;

it is unknown what value was contained in square.

These two values should be initialized:

int number = 0;
int square = 0;//this specifically will cause problems later if not initialized

Also, at the time this is called:

    number = argv[1][square];

argv[1] is a string, and needs to be converted to an integer before using.

number = atoi(argv[1]);

Next, the statement:

number = argv[1][square]; //seg fault possible as noted above. 

will blow up when the value of square becomes larger than then string length of argv[1].

If your intent is squaring the value contained in argv[1] in its entirety as a single numeric value, it must first be converted from a string array, to an integer value, then you can easily get the square as you do in your code:

   int number = atoi(argv[1]);
   square = number * number;

If, as it appears in your code, you are interested in squaring each of the component integers making up the string, then given the input of "1234, to convert each of the digits inargv1from ASCII value to its numeric value. (i.e.val = argv1[x] - '0'==>x`), then square it, then move the the next character in the array and so on.... look at the other part of this answer below.

The following is an adaptation of your original to do this...

int main(int argc, char *argv[]) {
    int number = 0;
    int square = 0;

    if(argc != 2) 
    {
          printf("%s\n","Usage prog.exe <nnn>, where nnn is an integer value.\nProgram will exit.");
    }
    else
    {
        char *ptr = argv[1];    

        while(*ptr != '\0') 
        {
            number = *ptr - '0';
            square = number * number;
            printf("%d\n", square);//write() not available on my system, replace as necessary
            square++;
            ptr++;
        }
    }

    return 0;
}

So, for example given prog.exe "1234", the output is:

Answer 2

The completed code should look like this :

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

int main(int argc, char **argv) {
    int number;
    int square;

    if(argc == 2) {
        number = atoi(argv[1]);
        square = number * number;
        char sq_buffer[12];
        sprintf(sq_buffer , "%d" , square);
        strcat(sq_buffer , "\n");
        write(1, (const void *) sq_buffer, strlen(sq_buffer)); 
    }else{
        char message[100];
        strcpy(message , "Please specify exactly one argument \n");
        write(1 , (const void *)message , strlen(message));
    }
   return 0;
}

Answer 3

When you say that you are not allowed to use 'printf', does that also include 'fprintf' and 'sprintf'? If so, then I would solve this problem like this:

void  main( int argc, char **argv )
{
    int     i;
    int     j;
    int     square;
    int     value;
    char    buffer[256];

    for (i = 1; i < argc; i++) {

        /* Get the next integer from the args to the program */
        value = atoi(argv[i]);

        /* Calculate the square of that value */
        square = value * value;

        /* Now convert it to ASCII */
        itoa(square, buffer, 10);

        /* Write out the data */
        write(1, buffer, strlen(buffer));

        /* You could also use putchar() for it, like this: */
        j = 0;
        while (buffer[j] != '\0')
            putchar(buffer[j++]);
    }
}

If he doesn't want you using itoa(), then you could write your own int to ASCII conversion routine like this:

char  *IntToASCII( int  x )
{
    static char    buffer[256];
    char           *ptr;
    int            neg;
    char           *digits = "0123456789";
    unsigned long  tempX;

    neg = (x < 0);
    tempX = x;
    if (neg) {
        tempX *= -1;
    }

    ptr = buffer + sizeof(buffer) - 1;
    *ptr = '\0';
    do {
        ptr--;
        *ptr = digits[tempX % 10];
        tempX /= 10;
    } while (tempX > 0);

    if (neg)
        *ptr = '-';

    return (ptr);
}

Or you can make 'buffer' an input parameter and remove the static declaration for it.

Answer 4

#define STDOUT_FILENO    1
#define MAX_INT_DIGITS   (10+1)  /*+1 for string null terminating*/
int main(int argc, char *argv[]) 
{
  int number=0, square=0;
  char ret_num[MAX_INT_DIGITS];

  for (int i =1; i < argc; ++i) 
  {
    number = atoi(argv[i])
    square = number * number;
    itoa(square, ret_num, 10);
    write(STDOUT_FILENO, ret_num, strlen(ret_num));
    /*write new line*/
    write(STDOUT_FILENO, '\n', 1);
  }


return 0;
}