Language: C

#define TRUE 1
#define FALSE 0
#define N 300

int stack[STACK_SIZE];
int top=0;

int isEmpty(){
  if(stack[top]==0){
    return TRUE;
      }else{
    return FALSE;
      }
}

void push(int a){
  top++;
  stack[top]=a;
}

int pop(){
  if(isEmpty()==TRUE){
    printf("error underflow\n");
  }else{
    return stack[top--];
  }
}

int main(){
  int i;
  int c=0;
  int a=0,b=0;
  char input[N];
  char str[10];
  int num;
  int pop1,pop2;

  while(scanf("%c",&input[c])!=EOF){
    c++;
  }
  for(i=0;i<c;i++){

      if(input[i]==' '){
	b=i;
	if('0'<=input[i-1] && input[i-1]<='9'){
	  strncpy(str,input+a,b-a);
	  str[b-a]='\0';
	  num=atoi(str);
	  printf("%d\n",num);
	  push(num);
	}
	a=b+1;
      }else if(input[i]=='+'){
	pop2=pop();
	pop1=pop();
	push(pop1+pop2);
      }else if(input[i]=='-'){
	pop2=pop();
	pop1=pop();
	push(pop1-pop2);
      }else if(input[i]=='*'){
	pop2=pop();
	pop1=pop();
	push(pop1*pop2);
      }else{
	continue;
      }
  }


  printf("%d\n",pop());
  return 0;
}

Compiler output:
main.c:5:11: error: 'STACK_SIZE' undeclared here (not in a function)
    5 | int stack[STACK_SIZE];
      |           ^~~~~~~~~~
main.c: In function 'pop':
main.c:23:5: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   23 |     printf("error underflow\n");
      |     ^~~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'printf'
  +++ |+#include <stdio.h>
    1 | #define TRUE 1
main.c:23:5: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   23 |     printf("error underflow\n");
      |     ^~~~~~
main.c:23:5: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c: In function 'main':
main.c:38:9: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   38 |   while(scanf("%c",&input[c])!=EOF){
      |         ^~~~~
main.c:38:9: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:38:9: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
main.c:38:9: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:38:32: error: 'EOF' undeclared (first use in this function)
   38 |   while(scanf("%c",&input[c])!=EOF){
      |                                ^~~
main.c:38:32: note: 'EOF' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c:38:32: note: each undeclared identifier is reported only once for each function it appears in
main.c:46:11: error: implicit declaration of function 'strncpy' [-Wimplicit-function-declaration]
   46 |           strncpy(str,input+a,b-a);
      |           ^~~~~~~
main.c:1:1: note: include '<string.h>' or provide a declaration of 'strncpy'
  +++ |+#include <string.h>
    1 | #define TRUE 1
main.c:46:11: warning: incompatible implicit declaration of built-in function 'strncpy' [-Wbuiltin-declaration-mismatch]
   46 |           strncpy(str,input+a,b-a);
      |           ^~~~~~~
main.c:46:11: note: include '<string.h>' or provide a declaration of 'strncpy'
main.c:48:15: error: implicit declaration of function 'atoi' [-Wimplicit-function-declaration]
   48 |           num=atoi(str);
      |               ^~~~
main.c:49:11: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   49 |           printf("%d\n",num);
      |           ^~~~~~
main.c:49:11: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:71:3: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   71 |   printf("%d\n",pop());
      |   ^~~~~~
main.c:71:3: note: include '<stdio.h>' or provide a declaration of 'printf'