Warning in C when strict ISO standards enabled in Code Blocks

This is the program i wrote and got the warning, what in this is strictly not a part of C?
#include<stdio.h>
#include<stdlib.h>
#include<stdbool.h>
#include"stackADTs.h"
bool pity_lessOReq(char *c,char ca)
{
if((ca=='*'||ca=='/')&&(*c=='+'||*c=='-'))
return false;
else
return true;

}
int main()
{
int i=0;
STACK *stack;
char q[20]={0},ch,*chP1,*chP2,*chP3;
FILE *p;
stack=create_stack();
p=fopen("boo1.txt","w");
if(!p)
{
printf("ERROR, could not find the file\n");
exit(0);
}
printf("Enter a balanced infix expression\n");
while((ch=getchar())!=EOF)
fputc(ch,p);
fclose(p);
p=fopen("boo1.txt","r");
while((ch=fgetc(p))!=EOF)
{
if(ch=='(')
continue;
else if(ch=='a'||ch=='b'||ch=='c'||ch=='x'||ch=='y'||ch=='z')
q[i++]=ch;
else if(ch=='*'||ch=='/'||ch=='+'||ch=='-')
{
chP1=(char *)malloc(sizeof(char));
*chP1=ch;
if(!empty_stack(stack))
{
chP2=(char*)pop_stack(stack);
if(pity_lessOReq(chP2,ch))
{
push_stack(stack,chP1);
push_stack(stack,chP2);
}
else
{
push_stack(stack,chP2);
push_stack(stack,chP1);
}
}
else
push_stack(stack,chP1);
}
if(ch==')')
{
chP3=(char*)pop_stack(stack);
q[i++]=*chP3;
}
}
printf("The PostFix expression is : %s",q);
destroy_stack(stack);
return 0;
}