C - scanf has gone ROGUE

Question

I have this c program where I am inputing a number N followed by N more numbers. For example, I'll enter 100 followed by 100 more numbers. For some reason, after so many inputs the scanf function will stop working properly. It's as if it has stopped taking input and will just continue one with whatever value is in size.

The use case I came up with is 100 1 2 3 4 5 6 7 8 9 10... (repeated ten times). then after three or four times of that I'll type in 100 10 9 8 7 6 5 4 3 2 1... (repeated ten times) and then there will be an infinite loop of print statements.

int main(int argc, const char * argv[]) {
  int histogram[10000];
  int i;
  while (1) {
    int *rectPtr = histogram;
    int size;
    scanf("%d", &size);
    if (!size) return 0;
    for (i = 0; i < size; ++i) {
        scanf("%d", rectPtr);
        rectPtr++;
    }
    printf("%d", 1);
    printf("\n");
  }
  return 0;
}

Answer 1

IMO, you need to check the return value of scanf() for proper operation. Please check the below code. I have added some modifications.

To exit from the program, you need to press CTRL+ D which will generate the EOF. Alternatively, upon entering some invalid input [like a char instead of int] wiil also cause the program to beak out of while() llop and terminate.

I have put the sequence to check first scanf(). All others need to be checked, too.

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

int main(int argc, const char * argv[]) {
        int histogram[10000] = {0};
        int i;

        int *rectPtr = histogram;
        int size = 0;
        int retval = 0;


        printf("Enter the number of elements \n");
        while (  (retval = scanf("%d", &size)) != EOF && (retval == 1)) {
                rectPtr = histogram;
                if (!size) return 0;
                printf("Enter %d elements\n", size);
                for (i = 0; i < size; ++i) {
                        scanf("%d", rectPtr);   //check in a simmilar way to above
                        rectPtr++;
                }
                printf("%d\n", 1111111);

                printf("Enter the number of elements: \n");
        }
        return 0;
}

The output of a sample run

[sourav@broadsword temp]$ ./a.out 
Enter the number of elements: 2
Enter 2 elements
1
2
1111111
Enter the number of elements: 3
Enter 3 elements
1
2
3
1111111
Enter the number of elements: 9
Enter 9 elements
0
9
8
7
6
5
4
3
2
1111111
Enter the number of elements: r   
[sourav@broadsword temp]$

Answer 2

Distrust infinite loops.

In a series of comments, I said:

You're not testing the return value from scanf(), so you don't know whether it is working. The pair of printf() statements is odd; why not write printf("%d\n", 1); or even puts("1");?

Your code does not test or capture the return value from scanf(), so you do not know whether scanf() is reporting a problem. As a general rule, test the return value of input functions to make sure what you thought happened did in fact happen. You could also print out the values read just after you read them:

if (scanf("%d", rectPtr) != 1)
{
    fprintf(stderr, "scanf() failed\n");
    return 1;
}
printf("--> %d\n", *rectPtr);
rectPtr++;

Similarly when inputting size. Also consider if (size <= 0) return 0;. And using fgets() plus `sscanf() can make reporting errors easier.

j.will commented:

It is great to know if scanf fails, but I want to know why it fails and prevent it from failing. How do I do that?

I responded:

I understand you'd like to know. With scanf(), the best you can do after a failure is usually to read all the characters that follow up to a newline or EOF, and if you want to know what went wrong, then you print those characters too, because scanf() leaves the last character that it read in the input buffer ready for the next input operation.

void gobble(void)
{
    printf("Error at: <<");
    int c;
    while ((c = getchar()) != EOF && c != '\n')
        putchar(c);
    puts(">>");
    if (c == EOF)
        puts("<<EOF>>");
}

The first character in the output is what caused the failure.

See also How to use sscanf() in loops?

---

Hacking your code to match this:

#include <stdio.h>

static void gobble(void)
{
    printf("Error at: <<");
    int c;
    while ((c = getchar()) != EOF && c != '\n')
        putchar(c);
    puts(">>");
    if (c == EOF)
        puts("<<EOF>>");
}

int main(void)
{
    enum { MAX_VALUES = 10000 };
    int histogram[MAX_VALUES];
    int size;
    while (printf("Number of items: ") > 0 && scanf("%d", &size) == 1 &&
           size > 0 && size <= MAX_VALUES)
    {
        int *rectPtr = histogram;
        for (int i = 0; i < size; ++i)
        {
            if (scanf("%d", rectPtr) != 1)
            {
                gobble();
                return 1;
            }
            rectPtr++;
        }
        printf("size %d items read\n", size);
    }
    return 0;
}

Answer 3

histogram is declared to have size 10000. You say you do 100 1 2 3 ... repeated 10 times. If I correctly understand that uses 1000 slots in histogram.

If you repeat the test more than 10 times, you exhaust histogram and begin to write past the end of array causing undefined behaviour.

So you must either :

• reset recPtr = histogram at each iteration
• control recPtr - histogram + size <= sizeof(histogram) after reading size (IMHO better)

And as other said, you should always control input operations : anything can happen outside of your program ...