This code don't run on other compiler except vscode - c

Question

The giver code is for converting infix to postfix using stack. I tried it everywhere no errors or warning were shown except for one time there was a segmentation fault on my school's computer. If anyone can explain the correct way to write the code it'll be very helpful as I'm unable to find.
In code we scan the infix expression and check if it's an operator and then if it is an operator we push it to stack and character or digit go to an array. We then pop the operator from the stack if we find another operator of more precedence than the previous pushed operator and we keep popping till we get less precedence operator in stack and append it to the array that is also our output.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include<ctype.h>
struct stack
{
int size;
int top;
char *arr;
};
int stacktop(struct stack *sp)
{
return sp->arr[sp->top];
}
void push(struct stack *ptr, char val)
{
if (ptr->top == ptr->size - 1)
{
printf("Stack is full\n");
}
else
{
ptr->top++;
ptr->arr[ptr->top] = val;
}
}
char pop(struct stack *ptr)
{
int a;
if (ptr->top == -1)
{
printf("\n");
}
else
{
a = ptr->arr[ptr->top];
ptr->top--;
return a;
}
}
int isoperator(char symbol)
{
if (symbol == '^' || symbol == '*' || symbol == '/' || symbol == '+' || symbol == '-')
{
return 1;
}
else
{
return 0;
}
}
int precedence(char symbol)
{
if (symbol == '^')
{
return (3);
}
else if (symbol == '*' || symbol == '/')
{
return (2);
}
else if (symbol == '-' || symbol == '+')
{
return (1);
}
else
{
return (0);
}
}
char *infix_to_postfix(char *infix)
{
struct stack *sp = (struct stack *)malloc(sizeof(struct stack));
sp->size = 50;
sp->top = -1;
sp->arr = (char *)malloc(sp->size * sizeof(char));
char *postfix = (char *)malloc(strlen((infix) + 1) * sizeof(char));
int i = 0, j = 0, x = 0;
while (infix[i] != '\0')
{
if (infix[i] == '(')
{
push(sp, infix[i]);
}
else if( isdigit(infix[i]) || isalpha(infix[i])){
postfix[j]=infix[i];
j++;
}
else if (isoperator(infix[i])==1)
{
x=pop(sp);
while(isoperator(x)==1 && precedence(x)>=precedence(infix[i])){
postfix[j]=x;
j++;
x=pop(sp);
}
push(sp, x);
push(sp, infix[i]);
}
else if (infix[i] == ')')
{
x = pop(sp);
while (x != '(')
{
postfix[j] = x;
j++;
x = pop(sp);
}
}
i++;
}
while (!(sp->top == -1))
{
postfix[j] = pop(sp);
j++;
}
postfix[j] = '\0';
return postfix;
}
int main()
{
char *infix;
printf("Enter your expression\n");
scanf("%s",infix);
printf("Postfix is %s", infix_to_postfix(infix));
return 0;
}

Answer 1

The pointer variable char *infix doesn't point to anything so it's not about VSCode and other compilers. It's complete luck that it's working on VSCode.
You should allocate some memory either statically char infix[100] or dynamically char *infix = (char *)malloc(100).
Also, you have a problem with pop function. You shouöd always return a char from this function not only under certain conditions.