Language: C

int isFull(void);
void push(int x);
int pop(void);
int top;
int S[MAX];
int main(){
  int ans=0;
  char x;
  initialize();
  while(1){
    if(scanf(" %c",&x)!=1) break;
    if(x>='0' && x<='9'){
      push((int)x);
      printf("%c\n",x);
  }
    
    else if ( x == '+' ){
      ans=pop()+pop()-96;
    } else if ( x == '-' ){
      ans=pop()-pop();
    } else if( x == '*' ){
      ans=pop()*pop();
    }
  }
  printf("%d\n",ans);
  return 0;
}
void initialize(void){
  top=0;
}

int isEmpty(void){
  if(top==0) return 1;
  else return 0;
}
int isFull(void){
  if(top>=MAX-1) return 1;
  return 0;
}
void push(int x){
  
  if(isFull()==1){
    fprintf(stderr,"??¨??????????????????????????????)\n");
    exit(1);
  }
  top++;
  S[top]=x;
}
int pop(void){
  
  if(isEmpty()==1){
    fprintf(stderr,"??¨???????????¢?????????????????????\n");
    exit(1);
    return 0;
  }
  top--;
  return S[top+1];
}

Compiler output:
main.c:5:7: error: 'MAX' undeclared here (not in a function)
    5 | int S[MAX];
      |       ^~~
main.c: In function 'main':
main.c:9:3: error: implicit declaration of function 'initialize' [-Wimplicit-function-declaration]
    9 |   initialize();
      |   ^~~~~~~~~~
main.c:11:8: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   11 |     if(scanf(" %c",&x)!=1) break;
      |        ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | int isFull(void);
main.c:11:8: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   11 |     if(scanf(" %c",&x)!=1) break;
      |        ^~~~~
main.c:11:8: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:14:7: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   14 |       printf("%c\n",x);
      |       ^~~~~~
main.c:14:7: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:14:7: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:14:7: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:25:3: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   25 |   printf("%d\n",ans);
      |   ^~~~~~
main.c:25:3: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c: At top level:
main.c:28:6: warning: conflicting types for 'initialize'; have 'void(void)'
   28 | void initialize(void){
      |      ^~~~~~~~~~
main.c:9:3: note: previous implicit declaration of 'initialize' with type 'void(void)'
    9 |   initialize();
      |   ^~~~~~~~~~
main.c: In function 'push':
main.c:43:5: error: implicit declaration of function 'fprintf' [-Wimplicit-function-declaration]
   43 |     fprintf(stderr,"??¨??????????????????????????????)\n");
      |     ^~~~~~~
main.c:43:5: note: include '<stdio.h>' or provide a declaration of 'fprintf'
main.c:43:5: warning: incompatible implicit declaration of built-in function 'fprintf' [-Wbuiltin-declaration-mismatch]
main.c:43:5: note: include '<stdio.h>' or provide a declaration of 'fprintf'
main.c:43:13: error: 'stderr' undeclared (first use in this function)
   43 |     fprintf(stderr,"??¨??????????????????????????????)\n");
      |             ^~~~~~
main.c:43:13: note: 'stderr' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c:43:13: note: each undeclared identifier is reported only once for each function it appears in
main.c:43:52: warning: trigraph ??) ignored, use -trigraphs to enable [-Wtrigraphs]
   43 |     fprintf(stderr,"??¨??????????????????????????????)\n");
main.c:44:5: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
   44 |     exit(1);
      |     ^~~~
main.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
  +++ |+#include <stdlib.h>
    1 | int isFull(void);
main.c:44:5: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
   44 |     exit(1);
      |     ^~~~
main.c:44:5: note: include '<stdlib.h>' or provide a declaration of 'exit'
main.c: In function 'pop':
main.c:52:5: warning: incompatible implicit declaration of built-in function 'fprintf' [-Wbuiltin-declaration-mismatch]
   52 |     fprintf(stderr,"??¨???????????¢?????????????????????\n");
      |     ^~~~~~~
main.c:52:5: note: include '<stdio.h>' or provide a declaration of 'fprintf'
main.c:52:13: error: 'stderr' undeclared (first use in this function)
   52 |     fprintf(stderr,"??¨???????????¢?????????????????????\n");
      |             ^~~~~~
main.c:52:13: note: 'stderr' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c:53:5: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
   53 |     exit(1);
      |     ^~~~
main.c:53:5: note: include '<stdlib.h>' or provide a declaration of 'exit'