I'm trying to create a stack in c using structures but my push function doesn't work

Question

I'm trying to create a stack in C using structures but the push() function I wrote is acting strangely. I'm sure it is something obvious that I'm missing but I just couldn't figure out what.

#include <stdio.h>

#define STACK_SIZE 50

typedef struct stack
{
    int top;
    int items[STACK_SIZE];
}
STACK;

void push(STACK* st, int newitem)
{
    st->top++;
    st->items[st->top] = newitem;
    printf("%d", st->items[st->top]);
}


int main()
{
    int n = 1;
    STACK* st;

    printf("test 1\n");
    
    st->top = -1;

    push(st, n);
    
    printf("test 2\n");
    
    return 0;
}

DevCpp only compiles but doesn't execute the code. OnlineGDB runs it but only prints the first test.

Answer 1

This is because your variable STACK* st; was never initialized properly.

Some Important Points:

• Don't assign -1 to the length ( top ), 0 would be better
• STACK* st; should be just STACK st;
• Your function void push(STACK* st, int newitem) should be declared with static linkage.
• Write st->top++
• Pass st variable by address to the push() function
• Instead of using bare return 0; , use return EXIT_SUCCESS; , which is defined in the header file stdlib.h .
• As your total STACK_SIZE is only 50 so, int will be sufficient. But as your STACK_SIZE grows use size_t for your length( top ).
• use int main(void) { } , instead of int main() { }
• NOTE: If STACK_SIZE and top becomes equal means your array is filled completely then further addition of data will lead to Undefined Behavior .

Final Code

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

#define STACK_SIZE 50

typedef struct stack
{
    int top;
    int items[STACK_SIZE];
}
STACK;

static void push(STACK* st, int newitem)
{
    if(st->top == STACK_SIZE)
    {
        fprintf(stderr, "stack size reached maximum length\n");
        exit(EXIT_FAILURE);
    }
    st->items[st->top++] = newitem;
    printf("%d\n", st->items[st->top - 1]); // we added +1 to `top` in the above line
}


int main(void)
{
    int n = 1;
    STACK st;
    printf("test 1\n");
    st.top = 0;
    push(&st, n); //pass by address

    return EXIT_SUCCESS;
}