Error if i declare constant with #define or const in my C program

Question

I use gcc version 4.3.2 (Debian 4.3.2-1.1). I wrote a simple program in C to implement and test a integer stack. Stack is implemented by the STACK structure. I used a constant named STACKSIZE to define the STACK's size. My program code looks like this:

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

#define STACKSIZE 10;

typedef struct {
    int size;
    int items[STACKSIZE];
} STACK;

void push(STACK *ps, int x)
{
    if (ps->size == STACKSIZE) {
        fputs("Error: stack overflow\n", stderr);
        abort();
    } else
        ps->items[ps->size++] = x;
}

int pop(STACK *ps)
{
    if (ps->size == 0){
        fputs("Error: stack underflow\n", stderr);
        abort();
    } else
    return ps->items[--ps->size];
}

int main() {
    STACK st;
    st.size = 0;
    int i;
    for(i=0; i < STACKSIZE + 1; i++) {
        push(&st, i);
    }
    while(st.size != 0)
        printf("%d\n", pop(&st));
    printf("%d\n", pop(&st));
    return 0;
}

when i used #define STACKSIZE 10; gcc would return following errors:

ex_stack1.c:8: error: expected ‘]’ before ‘;’ token
ex_stack1.c:9: warning: no semicolon at end of struct or union
ex_stack1.c: In function ‘push’:
ex_stack1.c:13: error: expected ‘)’ before ‘;’ token
ex_stack1.c:17: error: ‘STACK’ has no member named ‘items’
ex_stack1.c: In function ‘pop’:
ex_stack1.c:26: error: ‘STACK’ has no member named ‘items’
ex_stack1.c: In function ‘main’:
ex_stack1.c:33: error: expected ‘)’ before ‘;’ token

when i used

const int STACKSIZE=10;

gcc would return following error:

ex_stack1.c:8: error: variably modified ‘items’ at file scope

when i used

enum {STACKSIZE=10};

gcc would compile my program successfully.

What happenned? How should i modify my program to use

#define STACKSIZE 10;

or

const int STACKSIZE=10;

Answer 1

Drop the semicolon, it's wrong

#define STACKSIZE 10;
                    ^

If you keep it the preprocessor will translate int items[STACKSIZE]; into the obviously wrong int items[10;];.

For the const bit, there is a C FAQ.

The value of a const-qualified object is not a constant expression in the full sense of the term, and cannot be used for array dimensions, case labels, and the like.

Answer 2

For future reference, you can view the results of the preprocessor by making use of the -E option to gcc. That is,

gcc -E ex_stack1.c -o ex_stack1.i 

Examination of the resulting ex_stack1.i file would have made the problem more obvious.

Answer 3

#define does textual replacement. Since you have:

#define STACKSIZE 10;

then

typedef struct {
    int size;
    int items[STACKSIZE];
} STACK;

becomes:

typedef struct {
    int size;
    int items[10;];
} STACK;

In C, const is used to declare variables that can't (easily) be modified. They are not compile-time constants.

enum, however, does define a compile-time constant. In general, you should prefer enum over const int over #define where possible. (See Advantage and disadvantages of #define vs. constants? ).