Infix to Postfix absurd value

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I have written a program below to convert Infix expression to Postfix expression.

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

typedef struct Stack{
    int top;
    int capacity;
    char *array;
}Stack;

int isEmpty(Stack *s){
    return s->top == -1;
}
char pop(Stack *s){
    char c;
    if(isEmpty(s))
        return -1;
    c = s->array[s->top];
    s->array[s->top--] = '\0';
    return c;
}

void push(Stack *s,char c){
    s->array[++s->top] = c;
}

char peek(Stack *s){
    return s->array[s->top];
}

char *toString(char c){
    char *str = (char *)malloc(sizeof(char)*2);
    str[0] = c;
    str[1] = '\0';
    return str;
}

int hasGreaterPriority(char c1,char c2){
    if(c1 == '*' && (c2 == '+' || c2 == '-'))
        return 1;
    else if(c1 == '/' && (c2 == '+' || c2 == '-'))
        return 1;
    else
        return 0;
}

void infixtopostfix(char exp[]){
    int len = strlen(exp),i;
    char *out = (char *)malloc(sizeof(char));
    out[0] = '\0';

    Stack *stack = (Stack *)malloc(sizeof(Stack));
    stack->capacity = len;
    stack->top = -1;
    stack->array = malloc(sizeof(sizeof(char)*len));

    for(i=0; i<len; i++){
        if(exp[i] >= 'a' && exp[i] <= 'z')
            strcat(out,toString(exp[i]));
        else if(exp[i] == ')'){
            while(peek(stack) != '(')
                strcat(out,toString(pop(stack)));
            pop(stack);
        }
        else{
            if(isEmpty(stack) || exp[i] == '(')
                push(stack,exp[i]);
            else if(hasGreaterPriority(peek(stack),exp[i])){
                strcat(out,toString(pop(stack)));
                push(stack,exp[i]);
            }
            else
                push(stack,exp[i]);
       }
    }
    /* UNTIL HERE */
    while(!isEmpty(stack))
        strcat(out,toString(pop(stack)));

    strcat(out,"\0");
    printf("Postfix Expression: %s\n",out);
}

int main(void) {
    char exp[] = "a+b*(c/d-e)/(f+g*h)-i\0";
    printf("Infix Expression: %s\n",exp);
    infixtopostfix(exp);
    return 0;
}

Problem is as follows:

In infixtopostfix method, prior to while indicated by "UNTIL HERE" out variable value seems to be correct. I checked with the help of gdb. After 2 iterations of while loop also the value in out variable seems to be fine. In the last iteration i.e. while adding + into out,the value becomes as follows

abcd/e-fgh*+/i-*\377\377\377\377\025+

How did \377\377\377\377\025+ get added to out variable?

Expected value would be abcd/e-fgh*+/i-*+

asked 55 secs ago

user3409814

50