build error in code block for function gets

size can have any value when the array marks is allocated because it is not initialized. The array might be smaller than the entered size and so marks are stored in non-allocated memory, giving you the error.

This is a possible solution, but it doesn't compile with strict ISO C90. Presumably your CodeBlocks uses GCC that accepts variable length arrays and mixed declarations and code.

#include <stdio.h>

int main(void) {
    int size;
    printf("enter size: ");
    scanf("%d",&size);
    int marks[size];
    int counter;
    for (counter = 0; counter < size; counter++) {
        scanf("%d", &marks[counter]);
    }
    for (counter = 0; counter < size; counter++) {
        printf("%d: %d\n", counter, marks[counter]);
    }
    return 0;
}

BTW, please don't say "build error" if you have a runtime error. ;-)

Please don't use gets. It's dangerous.

As for your error in the scanf example, the first problem is the line

int size,counter,marks[size];

which declares marks with the uninitialized size value. Try initializing size first, then declaring the marks array.

Your second problem is scanf formatting string. Use scanf to read formatted input, not output a prompt. Use puts or printf for that.

Here's a full example:

#include <stdio.h>

int main(void) {  
    int size;
    printf("Enter a size value: ");
    scanf("%d", &size);
    int marks[size];

    for (int i = 0; i < size; i++) {
        printf("Enter element %d: ", i);
        scanf("%d", &marks[i]);
    }

    printf("You entered: ");

    for (int i = 0; i < size; i++) {
        printf("%d ", marks[i]);
    }

    puts("");
    return 0;
}

Here's a sample run:

Enter a size value: 4
Enter element 0: 88
Enter element 1: 77
Enter element 2: 66
Enter element 3: 55
You entered: 88 77 66 55

If you're writing ANSI C-compatible code you can use dynamic memory with malloc:

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

int main(void) {  
    int i, size, *marks;
    printf("Enter a size value: ");
    scanf("%d", &size);

    if (size < 1) {
        fprintf(stderr, "Invalid size specified\n");
        exit(1);
    }

    marks = malloc(size * sizeof(int));

    if (!marks) {
        fprintf(stderr, "malloc failed\n");
        exit(1);
    }

    for (i = 0; i < size; i++) {
        printf("Enter element %d: ", i);
        scanf("%d", &marks[i]);
    }

    printf("You entered: ");

    for (i = 0; i < size; i++) {
        printf("%d ", marks[i]);
    }

    free(marks);
    puts("");
    return 0;
}

size must have a defined value, for example:

#include <stdio.h>
int main()
{
    int size;
    size = 5; // size must be constant
    int counter, marks[size];
    for (counter = 0; counter < size; counter++)
    {
        scanf("%d", &marks[counter]);
    }
    //Printing it returns correct values:
    for (counter = 0; counter < size; counter++)
    {
        printf("%d\n", marks[counter]);
    }
}

You can instead input it's value from the user if you want.

However, if for some reason, size is to be defined after the array is declared, use pointers:

#include <stdio.h>
#include "stdlib.h"
int main()
{
    int size;
    int counter, *marks;
    size = 5; //declared after the array
    marks = (int *)malloc(size * sizeof(int));
    for (counter = 0; counter < size; counter++)
    {
        scanf("%d", &marks[counter]);
    }
    //Printing it returns correct values:
    for (counter = 0; counter < size; counter++)
    {
        printf("%d\n", marks[counter]);
    }
    //Don't forget to free the array in the end
    free(marks);
}