pid_t, uid_t, gid_t, id_t - process/user/group identifier

pid_t is a type used for storing process IDs, process group IDs, and session IDs.  It is a signed integer
       type.

       uid_t is a type used to hold user IDs.  It is an integer type.

       gid_t is a type used to hold group IDs.  It is an integer type.

       id_t  is  a  type used to hold a general identifier.  It is an integer type that can be used to contain a
       pid_t, uid_t, or gid_t.

       POSIX.1-2001.

       Standard C library (libc)

       pid_t, uid_t, gid_t, id_t - process/user/group identifier

       The following headers also provide  pid_t:  <fcntl.h>,  <sched.h>,  <signal.h>,  <spawn.h>,  <sys/msg.h>,
       <sys/sem.h>, <sys/shm.h>, <sys/wait.h>, <termios.h>, <time.h>, <unistd.h>, and <utmpx.h>.

       The  following  headers  also provide uid_t: <pwd.h>, <signal.h>, <stropts.h>, <sys/ipc.h>, <sys/stat.h>,
       and <unistd.h>.

       The following headers  also  provide  gid_t:  <grp.h>,  <pwd.h>,  <signal.h>,  <stropts.h>,  <sys/ipc.h>,
       <sys/stat.h>, and <unistd.h>.

       The following header also provides id_t: <sys/resource.h>.

chown(2),  fork(2),  getegid(2),  geteuid(2), getgid(2), getgroups(2), getpgid(2), getpid(2), getppid(2),
       getpriority(2),  getpwnam(3),  getresgid(2),  getresuid(2),  getsid(2),  gettid(2),  getuid(2),  kill(2),
       pidfd_open(2), sched_setscheduler(2), waitid(2), getgrnam(3), sigqueue(3), credentials(7)

Linux man-pages 6.9.1                              2024-05-02                                        id_t(3type)

       POSIX.1-2008.

#include<sys/types.h>typedef /* ... */ pid_t;typedef /* ... */ uid_t;typedef /* ... */ gid_t;typedef /* ... */ id_t;