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While loop with scanf get stack overflow after 8 number typed. When less amount, everything is ok. What's the problem?

 #include <stdio.h>

main ()
{
    int x=0;
    int j;
    int a [x];

    while (scanf("%d\n", &a[x++]) == 1);

    for (j = 0; j < x; j++)
    printf ("%d element of array is \t%d\n\n", j, a[j]);

    return 0;
}

How does code in this topic work? I mean the most accepted answer.

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6
  • 3
    int a [x]; meant int a [0]; a has no space to store the value. Commented Aug 5, 2017 at 14:45
  • So in this way stack overflow is after 14 inputs Commented Aug 5, 2017 at 15:00
  • 1
    it is random as it is an UB. it can happen instantly or after 50zilions iterations See my answer Commented Aug 5, 2017 at 15:06
  • Arrays in C do not "grow" automatically. You either need to allocate a fixed size array that is big enough for the worst case input, or use dynamic memory allocation (malloc/realloc) to create a variable sized array. Commented Aug 5, 2017 at 15:33
  • How does code in this topic work? I mean the most accepted answer. (the link is edited in question) Commented Aug 5, 2017 at 15:37

2 Answers 2

Reset to default
3

This code causes undefined behavior:

int a [x];

Here, a[] is a variable length array, and x has been initialized to 0. In §6.7.6.2 ¶5 about array declarators of the C11 Draft Standard, can be found:

If the size is an expression that is not an integer constant expression: if it occurs in a declaration at function prototype scope, it is treated as if it were replaced by *; otherwise, each time it is evaluated it shall have a value greater than zero.

Since x evaluates to 0 here, in violation of a "shall" outside of the constraints, this is undefined behavior: anything could happen. That anything could happen includes the possibility that the code appears to work for small array indices. But, as it is, this is not a valid C program.

Note that, even in the absence of the first problem, there is another issue. If, for example, x is instead initialized to 1, then a[] is an array of one int. In this case, the line:

while (scanf("%d\n", &a[x++]) == 1);

leads to undefined behavior, since a[x] is already an out of bounds access with x == 1 when input begins.

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2 Comments

It's the same when my x is 1, nothing changes
Well, that is a different question. Then you have undefined behavior because you are accessing out of array bounds in the input loop. I have added some comments about further undefined behavior in the posted code.
-1

You are lucky - after 9. Should be instant.

int a[x] - and x is equal zero. so no storage at all.

if size has to be a variable.

int main (void)
{
    int x=0;
    int y = 10;
    int j;
    int a [y];

    while (x < y )
        read_array_item(&a[x++]);

    //for (j = 0; j < y; j++)  // or x - it does not matter
        //printf ("%d element of array is \t%d\n\n", j, a[j]);

    return 0;
}
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10 Comments

I might be reading this wrong, but your "or x" should really use x, I think. You declare a as an array of y int without initialization, and if x is less than y - less integers read - then you're passing those indeterminate values to printf.
Hmm, also if there's a read failure before x == y, then the value in x - 1 is also unusable, right?
@Ilja Everilä I do not understand your comments. My changes were only to avoid writing and reading from the unallocated memory - avoiding segfaults. So I just added check if scanf will not write anything outside the array. Anything else in your comment is completely not about my answer - to make it clearer I commented unrelated lines - which were just copied from the original post
No, it is not. You fixed it and broke it at the same time, is how I see it. The comment is yours, so's the potentially broken bounds check in the print loop. It seems you're impervious to criticism.
Well, no. Now it's just terrible. I hope your hissy fit function writes some default in case of error, so that the whole array is initialized. Answers should be useful to future readers as well, not just OP (and writing proper examples is useful to OP too). Broken as designed code just spreads bad habbits. If you're answering about bounds checking, why not do it properly?
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