·#include <unistd.h>
pid_t getpid(void);
Returns: process ID of calling process
pid_t 四字节，见如下
#sys/_types.h
typedef __int32_t __pid_t; /* process [group] */
#sys/types.h
typedef __pid_t pid_t; /* user id */
pid_t getppid(void);
Returns: parent process ID of calling process
uid_t getuid(void);
sys/_types.h
typedef __uint32_t __uid_t;
#sys/types.h
typedef __uid_t uid_t; /* user id */
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
