When a system call detects an error,
it returns an integer value
indicating failure (usually -1)
and sets the variable
errno
accordingly.
(This allows interpretation of the failure on receiving
a -1 and to take action accordingly.)
Successful calls never set
errno
once set, it remains until another error occurs.
It should only be examined after an error.
Note that a number of system calls overload the meanings of these
error numbers, and that the meanings must be interpreted according
to the type and circumstances of the call.
The following is a complete list of the errors and their
names as given in
In sys/errno.h .
- Process ID
-
Each active process in the system is uniquely identified by a non-negative
integer called a process ID.
The range of this ID is from 0 to 99999.
- Parent process ID
-
A new process is created by a currently active process (see
fork(2)).
The parent process ID of a process is initially the process ID of its creator.
If the creating process exits,
the parent process ID of each child is set to the ID of a system process,
init(8).
- Process Group
-
Each active process is a member of a process group that is identified by
a non-negative integer called the process group ID.
This is the process
ID of the group leader.
This grouping permits the signaling of related
processes (see
termios(4))
and the job control mechanisms of
csh(1).
- Session
-
A session is a set of one or more process groups.
A session is created by a successful call to
setsid(2),
which causes the caller to become the only member of the only process
group in the new session.
- Session leader
-
A process that has created a new session by a successful call to
setsid(2),
is known as a session leader.
Only a session leader may acquire a terminal as its controlling terminal (see
termios(4)).
- Controlling process
-
A session leader with a controlling terminal is a controlling process.
- Controlling terminal
-
A terminal that is associated with a session is known as the controlling
terminal for that session and its members.
- "Terminal Process Group ID"
-
A terminal may be acquired by a session leader as its controlling terminal.
Once a terminal is associated with a session, any of the process groups
within the session may be placed into the foreground by setting
the terminal process group ID to the ID of the process group.
This facility is used
to arbitrate between multiple jobs contending for the same terminal;
(see
csh(1)
and
tty(4)).
- "Orphaned Process Group"
-
A process group is considered to be
orphaned
if it is not under the control of a job control shell.
More precisely, a process group is orphaned
when none of its members has a parent process that is in the same session
as the group,
but is in a different process group.
Note that when a process exits, the parent process for its children
is changed to be
init(8),
which is in a separate session.
Not all members of an orphaned process group are necessarily orphaned
processes (those whose creating process has exited).
The process group of a session leader is orphaned by definition.
- "Real User ID and Real Group ID"
-
Each user on the system is identified by a positive integer
termed the real user ID.
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 distinguished group is termed
the real group ID.
All processes have a real user ID and real group ID.
These are initialized from the equivalent attributes
of the process that created it.
- "Effective User Id, Effective Group Id, and Group Access List"
-
Access to system resources is governed by two values:
the effective user ID, and the group access list.
The first member of the group access list is also known as the
effective group ID.
(In POSIX.1, the group access list is known as the set of supplementary
group IDs, and it is unspecified whether the effective group ID is
a member of the list.)
The effective user ID and effective group ID are initially the
process's real user ID and real group ID respectively.
Either
may be modified through execution of a set-user-ID or set-group-ID
file (possibly by one its ancestors) (see
execve(2)).
By convention, the effective group ID (the first member of the group access
list) is duplicated, so that the execution of a set-group-ID program
does not result in the loss of the original (real) group ID.
The group access list is a set of group IDs
used only in determining resource accessibility.
Access checks
are performed as described below in ``File Access Permissions''.
- "Saved Set User ID and Saved Set Group ID"
-
When a process executes a new file, the effective user ID is set
to the owner of the file if the file is set-user-ID, and the effective
group ID (first element of the group access list) is set to the group
of the file if the file is set-group-ID.
The effective user ID of the process is then recorded as the saved set-user-ID,
and the effective group ID of the process is recorded as the saved set-group-ID.
These values may be used to regain those values as the effective user
or group ID after reverting to the real ID (see
setuid(2)).
(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
and are used in setuid and setgid, but this does not work as desired
for the super-user.)
- Super-user
-
A process is recognized as a
super-user
process and is granted special privileges if its effective user ID is 0.
- Descriptor
-
An integer assigned by the system when a file is referenced
by
open(2)
or
dup(2),
or when a socket is created by
pipe(2),
socket(2)
or
socketpair(2),
which uniquely identifies an access path to that file or socket from
a given process or any of its children.
- File Name
-
Names consisting of up to
Br q Dv NAME_MAX
characters may be used to name
an ordinary file, special file, or directory.
These characters may be arbitrary eight-bit values,
excluding
NUL
( ASCII
0) and the
`/'
character (slash,
ASCII
47).
Note that it is generally unwise to use
`*'
,
`?'
,
`['
or
`]'
as part of
file names because of the special meaning attached to these characters
by the shell.
- Path Name
-
A path name is a
NUL -terminated
character string starting with an
optional slash
`/'
,
followed by zero or more directory names separated
by slashes, optionally followed by a file name.
The total length of a path name must be less than
Br q Dv PATH_MAX
characters.
(On some systems, this limit may be infinite.)
If a path name begins with a slash, the path search begins at the
root
directory.
Otherwise, the search begins from the current working directory.
A slash by itself names the root directory.
An empty
pathname refers to the current directory.
- Directory
-
A directory is a special type of file that contains entries
that are references to other files.
Directory entries are called links.
By convention, a directory
contains at least two links,
`.'
and
`..'
,
referred to as
dot
and
dot-dot
respectively.
Dot refers to the directory itself and
dot-dot refers to its parent directory.
- "Root Directory and Current Working Directory"
-
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 need not be the root
directory of the root file system.
- File Access Permissions
-
Every file in the file system has a set of access permissions.
These permissions are used in determining whether a process
may perform a requested operation on the file (such as opening
a file for writing).
Access permissions are established at the
time a file is created.
They may be changed at some later time
through the
chmod(2)
call.
File access is broken down according to whether a file may be: read,
written, or executed.
Directory files use the execute
permission to control if the directory may be searched.
File access permissions are interpreted by the system as
they apply to three different classes of users: the owner
of the file, those users in the file's group, anyone else.
Every file has an independent set of access permissions for
each of these classes.
When an access check is made, the system
decides if permission should be granted by checking the access
information applicable to the caller.
Read, write, and execute/search permissions on
a file are granted to a process if:
The process's effective user ID is that of the super-user.
(Note:
even the super-user cannot execute a non-executable file.)
The process's effective user ID matches the user ID of the owner
of the file and the owner permissions allow the access.
The process's effective user ID does not match the user ID of the
owner of the file, and either the process's effective
group ID matches the group ID
of the file, or the group ID of the file is in
the process's group access list,
and the group permissions allow the access.
Neither the effective user ID nor effective group ID
and group access list of the process
match the corresponding user ID and group ID of the file,
but the permissions for ``other users'' allow access.
Otherwise, permission is denied.
- Sockets and Address Families
-
A socket is an endpoint for communication between processes.
Each socket has queues for sending and receiving data.
Sockets are typed according to their communications properties.
These properties include whether messages sent and received
at a socket require the name of the partner, whether communication
is reliable, the format used in naming message recipients, etc.
Each instance of the system supports some
collection of socket types; consult
socket(2)
for more information about the types available and
their properties.
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.
