获取进程标识符
#include <unistd.h>
pid_t getpid(void);
//Returns: process ID of calling process
pid_t getppid(void);
//Returns: parent process ID of calling process
uid_t getuid(void);
//Returns: real user ID of calling process
uid_t geteuid(void);
//Returns: effective user ID of calling process
gid_t getgid(void);
//Returns: real group ID of calling process
gid_t getegid(void);
//Returns: effective group ID of calling process