*** glibc detected *** invalid pointer: 0x00000031bee21188 - c

Question

I've looked through similar questions on stackoverflow, but I'm still not sure how to fix it.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
extern char * pop();
extern void push(char *);
int i;
int j=0;
//Resize the array to 1.1 it's size
void reSize(char* tag){
char *temp = malloc(1.1*sizeof(tag));
for (i=0;i<(sizeof(tag)/sizeof(tag[0]));i++){
*(temp+i) = *(tag+i);
}
free(tag);
tag = temp;
}
int compare(char* tag, char* popVal){
i=0;
while (i<sizeof(tag)/sizeof(tag[0])){
if (*(tag+i) == *(popVal+i)){
i++;
}else{
return 0;
}
}
return 1;
}
void dothis(){
int ch;
int n=0;
char *tag = malloc(10* sizeof(char));
char *popVal;
while ((ch = getchar()) != '>'){
tag[n] = ch;
n++;
if (n > (sizeof(tag)/sizeof(tag[0]))-1 ){
reSize(tag);
}
}
if (*tag == '/'){
popVal = malloc(sizeof(tag));
popVal = pop();
j--;
if (!(compare(tag,popVal))){ // Compare will return 1 if the same
printf("Invalid");
exit(1);
}
}else{
push(tag);
j++;
}
free(tag);
free(popVal);
}
int main(int argc, char * argv[])
{
int ch;
while ((ch = getchar()) != EOF) {
if (!(isalpha(ch) || ch == '<'))
continue;
dothis();
}
if (j != 0){
printf("Invalid\n");
exit(1);
}
printf("Valid\n");
exit(0);
}
then the external methods:
#include <stdio.h>
#include <stdlib.h>
static int top = 0;
static char * stack[100];
int isEmpty()
{
return !(top);
}
char * pop()
{
if (isEmpty()){
fprintf(stderr, "Stack is empty");
exit(1);
}
top--;
return (char *) stack[top];
}
void push(char * thing2push)
{
if (top == 100){
fprintf(stderr, "Too many things in the stack");
exit(1);
}else{
stack[top] = thing2push;
top++;
}
}
In a previous question, the selected answer was "passing a pointer to memory you haven't allocated with malloc will definitely not do good things.", but I"m pretty sure I allocated everything

Answer 1

Here's a bug:
popVal = malloc(sizeof(tag));
popVal = pop();
You malloc an area and then immediately lose that value, replacing it with something from pop().
This is most definitely a bug:
while ((ch = getchar()) != '>'){
tag[n] = ch;
n++;
if (n > (sizeof(tag)/sizeof(tag[0]))-1 ){
You assign to tag[n] before checking the range of n. When you do check the range of n after the fact you use sizeof(tag). tag is a pointer. It's size is 4 (32 bit) or 8 (64 bit). Neither size has anything to do with how big n can be before tag[n] writes into invalid memory.
Another bug:
char * pop()
{
if (isEmpty()){
fprintf(stderr, "Stack is empty");
exit(1);
}
top--;
return (char *) stack[top];
}
If you're a beginning C programmer, never cast a pointer. Because I doubt that you have learned enough yet to know if this is a good or bad idea.
The type system exists for good reasons and if it complains about some types not matching, it is far more likely to be right than you are.