Language: C

void push(int x)
{
  if(isFull() == 0){
  top++;
  S[top] = x;
  }
  else fprintf(stderr, "error\n");
}

int  pop(void)
{
  if(isEmpty() == 0){
  top--;
  return S[top+1];
  }
  else fprintf(stderr, "error\n");
}

int isFull( void )
{
  if(top > N -1) return 1;
  else return 0;
}

int isEmpty( void )
{
  if(top == 0) return 1;
  else return 0;
}

int main()
{
  int a, b, n;
  char C[N];
  while( scanf("%s", C) != EOF ){
    if( C[0] == '+' ){
      a = pop();
      b = pop();
      push( a + b );
    }
    else if( C[0] == '-' ){
      a = pop(); 
      b = pop();
      push( b - a );
    }
    else if( C[0] == '*' ){
      a = pop();
      b = pop();
      push( a * b );
    }
    else push( atoi(C) );
  }    

  printf("%d\n", pop());

  return 0;
}

Compiler output:
main.c: In function 'push':
main.c:3:6: error: implicit declaration of function 'isFull' [-Wimplicit-function-declaration]
    3 |   if(isFull() == 0){
      |      ^~~~~~
main.c:4:3: error: 'top' undeclared (first use in this function)
    4 |   top++;
      |   ^~~
main.c:4:3: note: each undeclared identifier is reported only once for each function it appears in
main.c:5:3: error: 'S' undeclared (first use in this function)
    5 |   S[top] = x;
      |   ^
main.c:7:8: error: implicit declaration of function 'fprintf' [-Wimplicit-function-declaration]
    7 |   else fprintf(stderr, "error\n");
      |        ^~~~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'fprintf'
  +++ |+#include <stdio.h>
    1 | void push(int x)
main.c:7:8: warning: incompatible implicit declaration of built-in function 'fprintf' [-Wbuiltin-declaration-mismatch]
    7 |   else fprintf(stderr, "error\n");
      |        ^~~~~~~
main.c:7:8: note: include '<stdio.h>' or provide a declaration of 'fprintf'
main.c:7:16: error: 'stderr' undeclared (first use in this function)
    7 |   else fprintf(stderr, "error\n");
      |                ^~~~~~
main.c:7:16: note: 'stderr' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c: In function 'pop':
main.c:12:6: error: implicit declaration of function 'isEmpty' [-Wimplicit-function-declaration]
   12 |   if(isEmpty() == 0){
      |      ^~~~~~~
main.c:13:3: error: 'top' undeclared (first use in this function); did you mean 'pop'?
   13 |   top--;
      |   ^~~
      |   pop
main.c:14:10: error: 'S' undeclared (first use in this function)
   14 |   return S[top+1];
      |          ^
main.c:16:8: warning: incompatible implicit declaration of built-in function 'fprintf' [-Wbuiltin-declaration-mismatch]
   16 |   else fprintf(stderr, "error\n");
      |        ^~~~~~~
main.c:16:8: note: include '<stdio.h>' or provide a declaration of 'fprintf'
main.c:16:16: error: 'stderr' undeclared (first use in this function)
   16 |   else fprintf(stderr, "error\n");
      |                ^~~~~~
main.c:16:16: note: 'stderr' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c: In function 'isFull':
main.c:21:6: error: 'top' undeclared (first use in this function); did you mean 'pop'?
   21 |   if(top > N -1) return 1;
      |      ^~~
      |      pop
main.c:21:12: error: 'N' undeclared (first use in this function)
   21 |   if(top > N -1) return 1;
      |            ^
main.c: In function 'isEmpty':
main.c:27:6: error: 'top' undeclared (first use in this function); did you mean 'pop'?
   27 |   if(top == 0) return 1;
      |      ^~~
      |      pop
main.c: In function 'main':
main.c:34:10: error: 'N' undeclared (first use in this function)
   34 |   char C[N];
      |          ^
main.c:35:10: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   35 |   while( scanf("%s", C) != EOF ){
      |          ^~~~~
main.c:35:10: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:35:10: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
main.c:35:10: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:35:28: error: 'EOF' undeclared (first use in this function)
   35 |   while( scanf("%s", C) != EOF ){
      |                            ^~~
main.c:35:28: note: 'EOF' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c:51:16: error: implicit declaration of function 'atoi' [-Wimplicit-function-declaration]
   51 |     else push( atoi(C) );
      |                ^~~~
main.c:54:3: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   54 |   printf("%d\n", pop());
      |   ^~~~~~
main.c:54:3: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:54:3: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:54:3: note: include '<stdio.h>' or provide a declaration of 'printf'