This piece of code evaluates an post-fix expression using Stack. Highly undocumented! Ask if in Doubt!

EvalPostfix.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <math.h>

struct node{
    float data;
    struct node *next;
};
struct node *head = NULL;

void push(float data){
    if (head == NULL) {
        head = (struct node *)malloc(sizeof(struct node));
        if (head == NULL) {
            printf("Memory Underflow! Insertion failed!");
            return;
        }
        head->data = data;
        head->next = NULL;
        return;
    }

    struct node *newNode = (struct node *)malloc(sizeof(struct node));
    
    if (newNode == NULL) {
        printf("Memory Underflow! Insertion failed!");
        return;
    }
    else {
        struct node *ptr = head;
        while(ptr->next != NULL){  
            ptr = ptr->next;  
        }
        newNode->data = data;
        newNode->next = NULL;  
        ptr->next = newNode; 
    }
}

float pop(){
    if (head == NULL) {
        printf("List underflow! Unable to delete!");
        return INT_MIN;
    }

    struct node *ptr = head, *ptr_prev;
    while(ptr->next != NULL){
        ptr_prev = ptr; 
        ptr = ptr->next;
    }
    
    ptr_prev->next = NULL;  

    float popped = ptr->data;
    free(ptr);  
    return popped;
}

int isOperator(char ch) {
    char optlist[] = "+-*/^";
    for(int i=0; i<strlen(optlist); i++)
    {
        if(ch == optlist[i])
            return 1;
    }
    return 0;
}

int isOperand(char ch) {
   if(ch >= '0' && ch <= '9')
      return 1;
   return 0;   
}

float operation(float a, float b, char op) 
{
    switch(op)
    {
        case '+':   return b+a;      break;
        case '-':   return b-a;      break;
        case '*':   return b*a;      break;
        case '/':   return b/a;      break;
        case '^':   return pow(b,a); break;
        default:    return INT_MIN;
    }
}

float EvalPostfix(char* postfix) 
{
    float a, b;
    push(INT_MIN);

    for(int i=0; i<strlen(postfix); i++) 
    {
        if(isOperator(postfix[i])) 
        {
          a = pop();
          b = pop();
          push(operation(a, b, postfix[i]));
        }

        else if(isOperand(postfix[i])) 
        {
            int j = i, k = 0;
            char str_num[10];
            while(postfix[j] != ',')
            {
                str_num[k] = postfix[j];
                k++;
                j++;
            }
            str_num[k] = '\0';
            int num = atoi(str_num);
            push(num);
            i = j;
        }
        else if(postfix[i] == ',')
            continue;
    }

    float res = pop();
    return res;
}

int main()
{
    char postfix[] = "5,6,2,+,*,12,4,/,-";
    float ans = EvalPostfix(postfix);
    printf("Answer: %lf\n", ans);
    return 0;
}