Test the getpid() function

getpid.c

#include <stdio.h>
#include <sys/types.h>

int main(void){
  pid_t this_process;
  this_process = getpid();
  printf("this process value is %d and hex value if 0x%x\n", this_process,this_process);
}

it gives me a warning:

getpid.c:6:17: warning: implicit declaration of function 'getpid' is invalid in
C99 [-Wimplicit-function-declaration]
this_process = getpid();
^
1 warning generated.