创建进程
在 Linux 中每个进程都是父进程创建的，Linux 启动时会创建init 进程，这是系统的第一个进程，其 PID 为 1。 在 C 语言中，我们可以用fork函数创建新的进程：
#include <unistd.h>
pid_t fork(void);
- If fork() returns a negative value, the creation of a child process was unsuccessful.
- fork() returns a zero to the newly created child process.
- fork() returns a positive value, the process ID of the child process, to the parent. The returned process ID is of type pid_t defined in sys/types.h. Normally, the process ID is an integer. Moreover, a process can use function getpid() to retrieve the process ID assigned to this process.
也就是：
On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno is set to indicate the error.
也就是：
- 返回负数，创建失败
- 返回 0 ，就是子进程，它永远返回 0
- 返回大于 0 的整数，这是父进程，它会返回子进程的pid