sys/type.h in some head files in C/C++

I met a problem with including a header file which has `sys/type.h`. When I compile using `g++` or `gcc filename.cpp` it always show me the error: `sys/type.h, no such a file or directory`. very confused. Can someone help me? ``` #include<stdio.h> #include<sys/type.h> #include<unistd.h> int main() { pid_t child_pid; printf("the main program ID is %d\n", (int)getpid()); child_pid = fork(); if (child_pid != 0) { printf("This is the parent process, with id %d\n", (int)getpid()); printf("the child's process ID is %d\n", (int)child_pid); } else { printf("this is the child process, with id %d\n", (int)getpid()); } } ```