Process ID (PID)¶

Each process has a unique non-negative integer identifier that is assigned when the process is created using fork(2). A process can obtain its PID using getpid(2). A PID is represented using the type pid_t (defined in <sys/types.h>).

PIDs are used in a range of system calls to identify the process affected by the call, for example: kill(2), ptrace(2), setpriority(2) setpgid(2), setsid(2), sigqueue(2), and waitpid(2).

A process's PID is preserved across an execve(2).

Parent Process ID (PPID)¶

A process's parent process ID identifies the process that created this process using fork(2). A process can obtain its PPID using getppid(2). A PPID is represented using the type pid_t.

A process's PPID is preserved across an execve(2).

Process Group ID and Session ID¶

Each process has a session ID and a process group ID, both represented using the type pid_t. A process can obtain its session ID using getsid(2), and its process group ID using getpgrp(2).

A child created by fork(2) inherits its parent's session ID and process group ID. A process's session ID and process group ID are preserved across an execve(2).

Sessions and process groups are abstractions devised to support shell job control. A process group (sometimes called a "job") is a collection of processes that share the same process group ID; the shell creates a new process group for the process(es) used to execute single command or pipeline (e.g., the two processes created to execute the command "ls | wc" are placed in the same process group). A process's group membership can be set using setpgid(2). The process whose process ID is the same as its process group ID is the process group leader for that group.

A session is a collection of processes that share the same session ID. All of the members of a process group also have the same session ID (i.e., all of the members of a process group always belong to the same session, so that sessions and process groups form a strict two-level hierarchy of processes.) A new session is created when a process calls setsid(2), which creates a new session whose session ID is the same as the PID of the process that called setsid(2). The creator of the session is called the session leader.

User and Group Identifiers¶

Each process has various associated user and groups IDs. These IDs are integers, respectively represented using the types uid_t and gid_t (defined in <sys/types.h>).

On Linux, each process has the following user and group identifiers:

• Real user ID and real group ID. These IDs determine who owns the process. A process can obtain its real user (group) ID using getuid(2) (getgid(2)).
• Effective user ID and effective group ID. These IDs are used by the kernel to determine the permissions that the process will have when accessing shared resources such as message queues, shared memory, and semaphores. On most Unix systems, these IDs also determine the permissions when accessing files. However, Linux uses the file system IDs described below for this task. A process can obtain its effective user (group) ID using geteuid(2) (getegid(2)).
• Saved set-user-ID and saved set-group-ID. These IDs are used in set-user-ID and set-group-ID programs to save a copy of the corresponding effective IDs that were set when the program was executed (see execve(2)). A set-user-ID program can assume and drop privileges by switching its effective user ID back and forth between the values in its real user ID and saved set-user-ID. This switching is done via calls to seteuid(2), setreuid(2), or setresuid(2). A set-group-ID program performs the analogous tasks using setegid(2), setregid(2), or setresgid(2). A process can obtain its saved set-user-ID (set-group-ID) using getresuid(2) (getresgid(2)).
• File system user ID and file system group ID (Linux-specific). These IDs, in conjunction with the supplementary group IDs described below, are used to determine permissions for accessing files; see path_resolution(7) for details. Whenever a process's effective user (group) ID is changed, the kernel also automatically changes the file system user (group) ID to the same value. Consequently, the file system IDs normally have the same values as the corresponding effective ID, and the semantics for file-permission checks are thus the same on Linux as on other Unix systems. The file system IDs can be made to differ from the effective IDs by calling setfsuid(2) and setfsgid(2).
• Supplementary group IDs. This is a set of additional group IDs that are used for permission checks when accessing files and other shared resources. On Linux kernels before 2.6.4, a process can be a member of up to 32 supplementary groups; since kernel 2.6.4, a process can be a member of up to 65536 supplementary groups. The call sysconf(_SC_NGROUPS_MAX) can be used to determine the number of supplementary groups of which a process may be a member. A process can obtain its set of supplementary group IDs using getgroups(2), and can modify the set using setgroups(2).

A child process created by fork(2) inherits copies of its parent's user and groups IDs. During an execve(2), a process's real user and group ID and supplementary group IDs are preserved; the effective and saved set IDs may be changed, as described in execve(2).

Aside from the purposes noted above, a process's user IDs are also employed in a number of other contexts:

• when determining the permissions for sending signals — see kill(2);
• when determining the permissions for setting process-scheduling parameters (nice value, real time scheduling policy and priority, CPU affinity, I/O priority) using setpriority(2), sched_setaffinity(2), sched_setscheduler(2), sched_setparam(2), and ioprio_set(2);
• when checking resource limits; see getrlimit(2);
• when checking the limit on the number of inotify instances that the process may create; see inotify(7).