intro.2

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operation, and
.I msg_ctime
is the time of the last 
.PN msgctl 
operation that changed a member of the above structure.
.TP 5
Parent process ID
.br
A new process is created by a currently active process.  For further
information, see 
.MS fork 2 .
The parent process ID of a process is the process ID of its creator.
.TP 5
Pathname
.br
A pathname is a null-terminated character string containing an 
optional slash (/), followed by zero or more directory names separated
by slashes.  This sequence can optionally be followed by another slash
and a filename.
The total length of a pathname must be less
than {PATHNAME_MAX} characters.
.IP
If a pathname begins with a slash, the path search begins at the
.PN root
directory.
Otherwise, the search begins from the current working directory.
A slash by itself names the 
.PN root 
directory.  A null
pathname refers to the current directory.
.TP 5
Process ID
.br
Each active process in the system is uniquely identified by a positive
integer called a process ID.  The range of this ID is from
0 to {PROC_MAX}.
.TP 5
Process Group ID
.br
Each active process is a member of a process group that is identified by
a positive integer called the process group ID.  This is the process
ID of the group leader.  This grouping permits the signaling of related
processes.  For more information, see 
.MS killpg 2
and the job control mechanisms described in 
.MS csh 1 .
.TP 5
Real User ID and Real Group ID
.br
Each user on the system is identified by a positive integer
called the real user ID.
.IP
Each user is also a member of one or more groups. 
One of these groups is distinguished from others and
used in implementing accounting facilities.  The positive
integer corresponding to this group is called
the real group ID.
.IP
All processes have a real user ID and real group ID.
These are initialized from the equivalent attributes
of the parent process. 
.TP 5
Root Directory and Current Working Directory
.br
Each process has associated with it a concept of a root directory
and a current working directory for the purpose of resolving path
name searches.  A process's root directory does not need to be
the root directory of the root file system.
.TP 5
Semaphore Identifier
.IP
A semaphore identifier (semid) is a unique positive integer
created by a 
.PN semget 
system call.  Each semid has a set of semaphores and a
data structure associated with it.  The data structure
is referred to as
.I semid_ds
and contains the following members:
.EX 0
struct  ipc_perm sem_perm; /*operation permission struct*/
ushort  sem_nsems;         /*number of sems in set */
time_t  sem_otime;         /*last operation time*/
time_t  sem_ctime;         /*last change time*/
                           /*Times measured in secs since*/
                           /*00:00:00 GMT, Jan. 1, 1970*/
.EE
The
.I sem_perm
is an 
.I ipc_perm 
structure that specifies the semaphore
operation permission.  This structure includes
the following members:
.EX 0
ushort cuid;  /*creator user id*/
ushort cgid;  /*creator group id*/
ushort uid;   /*user id*/
ushort gid;   /*group id*/
ushort mode;  /*r/a permission*/
.EE
The value of
.I sem_nsems
is equal to the number of semaphores in the set.  Each
semaphore in the set is referenced by a positive integer
referred to as a
.I sem_num.
The
.I sem_num 
values run sequentially from 0 to the value of
sem_nsems minus 1.
The
.I sem_otime
member is the time of the last 
.PN semop 
operation, and
.I sem_ctime
is the time of the last 
.PN semctl 
operation that changed a member of the above structure.
.IP
A semaphore is a data structure that contains the following
members:
.EX 0
ushort  semval;  /*semaphore value*/
short   sempid;  /*pid of last operation*/
ushort  semncnt; /*# awaiting semval > cval*/
ushort  semzcnt; /*# awaiting semval = 0*/
.EE
The
.I semval
member is a non-negative integer.
The
.I sempid
member is equal to the process ID of the last
process that performed a semaphore operation on this
semaphore.
The
.I semncnt
member is a count of the number of processes that are currently
suspended awaiting this semaphore's semval to become greater
than its current value.
The
.I semzcnt
member is a count of the number of processes that are currently suspended
awaiting this semaphore's semval to become zero.
.TP 5
Semaphore Operation Permissions
.IP
In the 
.MS semop 2
and
.MS semctl 2
system call descriptions, the permission required for an operation
is specified as {token}.  The token argument is the type of permission
needed and it is interpreted as follows:
.EX
00400   Read by user
00200   Alter by user
00060   Read, Alter by group
00006   Read, Alter by others
.EE
Read and alter permissions on a semid are granted to a process
if one or more of the following are true:
.RS 10
.IP \(bu 5
The effective user ID of the process is superuser.
.IP \(bu
The effective user ID of the process matches
.I sem_perm.[c]uid
in the data structure associated with
semid
and the appropriate bit of the user portion (0600) of
.I sem_perm.mode
is set.
.IP \(bu
The effective user ID of the process does not match
.I sem_perm.[c]uid,
but the effective group ID of the process matches
.I sem_perm.[c]gid
and the appropriate bit of the group portion (060) of
.I sem_perm.mode
is set.
.IP \(bu
The effective user ID of the process does not match
.I sem_perm.[c]uid
and the effective group ID of the process does not match
.I sem_perm.[c]gid,
but the appropriate bit of the other portion (06) of
.I sem_perm.mode 
is set.
.RE
.IP 
If none of the previous conditions are true, the read and alter
permissions are denied.
.TP 5
Session
.IP
Each process group is a member of a session. A process is considered
to be a member of the session of which its process group is a member.
Typically there is one session per login.
.TP 5
Shared Memory Identifier
.IP
A shared memory identifier (shmid) is a unique positive
integer created by a 
.PN shmget 
system call.  Each shmid has a segment of memory (referred
to as a shared memory segment) and a data structure associated
with it.  The data structure is referred to as
.PN shmid_ds
and contains the following members:
.EX 0
struct  ipc_perm shm_perm;  /*operation permission struct*/
int     shm_segsz;          /*size of segment*/
ushort  shm_cpid;           /*creator pid*/
ushort  shm_lpid;           /*pid of last operation*/
short   shm_nattch;         /*number of current attaches*/
time_t  shm_atime;          /*last attach time*/
time_t  shm_dtime;          /*last detach time*/
time_t  shm_ctime;          /*last change time*/
                            /*Times measured in secs since*/
                            /*00:00:00 GMT, Jan. 1, 1970*/
.EE
The
.I shm_perm
member is an 
.I ipc_perm
structure that specifies the shared
memory operation permission.  This structure
includes the following members:
.EX 0
ushort  cuid;  /*creator user id*/
ushort  cgid;  /*creator group id*/
ushort  uid;   /*user id*/
ushort  gid;   /*group id*/
ushort  mode;  /*r/w permission*/
.EE
The
.I shm_segz
member specifies the size of the shared memory segment.
The
.I shm_cpid
member is the process ID of the process that created the shared
memory identifier.
The
.I shm_lpid
member is the process ID of the last process that performed
a 
.PN shmop 
operation.
The
.I shm_nattch
member is the number of processes that currently have this segment
attached.  
The
.I shm_atime
member is the time of the last
.PN shmat
operation,
.I shm_dtime
is the time of the last
.PN shmdt
operation, and
.I shm_ctime
is the time of the last 
.PN shmctl 
operation that changed one of the members of the above
structure.
.TP 5
Shared Memory Operation Permissions
.IP
In the 
.MS shmop 2 
and
.MS shmctl 2
system call descriptions, the permission required for an
operation is given as {token}.  The token argument is the
type of permission needed and it is interpreted as follows:
.EX
00400  Read by user
00200  Write by user
00060  Read, Write by group
00006  Read, Write by others
.EE
Read and write permissions on a shmid are granted to a
process if one or more of the following are true:
.RS 10
.IP \(bu 5
The effective user ID of the process is superuser.
.IP \(bu 5
The effective user ID of the process matches
.I shm_perm.[c]uid
in the data structure associated with shmid
and the appropriate bit of the
user portion (0600) of
.I shm_perm.mode 
is set.
.IP \(bu 5
The effective user ID of the process does not match
.I shm_perm.[c]uid,
but the effective group ID of the process matches
.I shm_perm.[c]gid
and the appropriate bit of the group portion (060)
of
.I shm_perm.mode
is set.
.IP \(bu 5
The effective user ID of the process does not match
.I shm_perm.[c]uid
and the effective group ID of the process does not match
.I shm_perm.[c]gid,
but the appropriate bit of the other portion (06) of
.I shm_perm.mode
is set.
.RE
.IP
If none of the previous conditions are true, the read and write 
permissions are denied.
.TP 5
Sockets and Address Families
.IP
A socket is an endpoint for communication between processes.
Each socket has queues for sending and receiving data.
.IP
Sockets are typed according to their communications properties.
These properties determine whether messages sent and received
at a socket require the name of the partner, if communication
is reliable, and if the format is used in naming message
recipients.
.IP
Each instance of the system supports some
collection of socket types.  See
.MS socket 2
for more information about the types available and
their properties.
.IP
Each instance of the system supports some number of sets of
communications protocols.  Each protocol set supports addresses
of a certain format.  An Address Family is the set of addresses
for a specific group of protocols.  Each socket has an address
chosen from the address family in which the socket was created.
.TP 5
Special Processes
.br
Those processes that have a process ID of 0, 1, and 2 are considered
special processes.  Process 0 is the scheduler.  Process 1 is the 
initialization process 
.IR init ,
and is the ancestor of every other process in the system.
It controls the process structure.
Process 2 is the paging daemon.
.TP 5
Superuser
.br
A process is recognized as a superuser process and is granted
special privileges if its effective user ID is 0.
.TP 5
tty Group ID
.br
Each active process can be a member of a terminal group
that is identified by a positive integer called the tty group ID.  
This grouping is used to arbitrate between multiple jobs contending 
for the same terminal.  For more information, see 
.MS csh 1  
and 
.MS tty 4 .
.SH See Also
cc(1), csh(1), tty(4), intro(3), perror(3)