Failing to implement a very simple stack array in C - c

Question

#include <stdio.h>
#include <stdlib.h>
static int top = 0;
static char stack[100];
void push(char thing2push)
{
if (top == 100){
fprintf(stderr, "Too many things in the stack");
exit(1);
}else{
stack[top] = thing2push;
top++;
}
}
then in the main I have:
extern void push(int);
push(1);
but that results in "segmentation fault". My guess that it has something to do with memory violations, but I have no idea on how to fix it.
EDIT Here's the full code:
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
extern int pop();
extern void push(int);
void readTag(){
int tagVal = getchar();
int poppedVal;
if (getchar() == '/'){
poppedVal = pop();
if (poppedVal != tagVal){
printf("NOT Valid");
exit(1);
}
}else{
push(1);
}
}
int main(int argc, char * argv[])
{
int ch;
while ((ch = getchar()) != EOF) {
if (!(isalpha(ch) || ch == '<'))
continue;
readTag();
}
printf("Valid");
exit(0);
}
and here's the stack:
#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 stack[top+1];
}
void push(char thing2push)
{
if (top == 100){
fprintf(stderr, "Too many things in the stack");
exit(1);
}else{
stack[top] = thing2push;
top++;
}
}

Answer 1

The top variable always indicates the next entry in the stack (which obviously does not contain a valid element, as far as your program concerns). So you're not supposed to read the value at stack[top].
The segmentation fault occurs in function pop, when top reaches 100:
top--; // top == 99
return stack[top+1]; // return stack[100]
You should write into stack[top], but read from stack[top-1].
You can leave function push as is, and change only function pop:
top--;
return stack[top];