intro(2) SYSTEM CALLS intro(2)
NAME
intro - introduction to system calls and error numbers
SYNOPSIS
#include <errno.h>
DESCRIPTION
This section describes all of the system calls. Most of
these calls have one or more error returns. An error condi-
tion is indicated by an otherwise impossible returned value.
This is almost always -1 or the NULL pointer; the individual
descriptions specify the details. An error number is also
made available in the external variable errno. errno is not
cleared on successful calls, so it should be tested only
after an error has been indicated.
Each system call description attempts to list all possible
error numbers. The following is a complete list of the
error numbers and their names as defined in <errno.h>.
1 EPERM Not super-user
Typically this error indicates an attempt to modify a
file in some way forbidden except to its owner or the
super-user. It is also returned for attempts by ordi-
nary users to do things allowed only to the super-user.
2 ENOENT No such file or directory
A file name is specified and the file should exist but
doesn't, or one of the directories in a path name does
not exist.
3 ESRCH No such process
No process can be found corresponding to that specified
by PID in the kill or ptrace routine.
4 EINTR Interrupted system call
An asynchronous signal (such as interrupt or quit),
which the user has elected to catch, occurred during a
system service routine. If execution is resumed after
processing the signal, it will appear as if the inter-
rupted routine call returned this error condition.
5 EIO I/O error
Some physical I/O error has occurred. This error may
in some cases occur on a call following the one to
which it actually applies.
6 ENXIO No such device or address
I/O on a special file refers to a subdevice which does
not exist, or exists beyond the limit of the device.
It may also occur when, for example, a tape drive is
not on-line or no disk pack is loaded on a drive.
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7 E2BIG Arg list too long
An argument list longer than ARG_MAX bytes is presented
to a member of the exec family of routines. The argu-
ment list limit is the sum of the size of the argument
list plus the size of the environment's exported shell
variables.
8 ENOEXEC Exec format error
A request is made to execute a file which, although it
has the appropriate permissions, does not start with a
valid format [see a.out(4)].
9 EBADF Bad file number
Either a file descriptor refers to no open file, or a
read [respectively, write] request is made to a file
that is open only for writing (respectively, reading).
10 ECHILD No child processes
A wait routine was executed by a process that had no
existing or unwaited-for child processes.
11 EAGAIN No more processes
For example, the fork routine failed because the
system's process table is full or the user is not
allowed to create any more processes, or a system call
failed because of insufficient memory or swap space.
12 ENOMEM Not enough space
During execution of an exec, brk, or sbrk routine, a
program asks for more space than the system is able to
supply. This is not a temporary condition; the maximum
size is a system parameter. The error may also occur
if the arrangement of text, data, and stack segments
requires too many segmentation registers, or if there
is not enough swap space during the fork routine. If
this error occurs on a resource associated with Remote
File Sharing (RFS), it indicates a memory depletion
which may be temporary, dependent on system activity at
the time the call was invoked.
13 EACCES Permission denied
An attempt was made to access a file in a way forbidden
by the protection system.
14 EFAULT Bad address
The system encountered a hardware fault in attempting
to use an argument of a routine. For example, errno
potentially may be set to EFAULT any time a routine
that takes a pointer argument is passed an invalid
address, if the system can detect the condition.
Because systems will differ in their ability to reli-
ably detect a bad address, on some implementations
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passing a bad address to a routine will result in unde-
fined behavior.
15 ENOTBLK Block device required
A non-block file was mentioned where a block device was
required (e.g., in a call to the mount routine).
16 EBUSY Device busy
An attempt was made to mount a device that was already
mounted or an attempt was made to unmount a device on
which there is an active file (open file, current
directory, mounted-on file, active text segment). It
will also occur if an attempt is made to enable
accounting when it is already enabled. The device or
resource is currently unavailable.
17 EEXIST File exists
An existing file was mentioned in an inappropriate con-
text (e.g., call to the link routine).
18 EXDEV Cross-device link
A link to a file on another device was attempted.
19 ENODEV No such device
An attempt was made to apply an inappropriate operation
to a device (e.g., read a write-only device).
20 ENOTDIR Not a directory
A non-directory was specified where a directory is
required (e.g., in a path prefix or as an argument to
the chdir routine).
21 EISDIR Is a directory
An attempt was made to write on a directory.
22 EINVAL Invalid argument
An invalid argument was specified (e.g., unmounting a
non-mounted device), mentioning an undefined signal in
a call to the signal or kill routine.
23 ENFILE File table overflow
The system file table is full (i.e., SYS_OPEN files are
open, and temporarily no more files can be opened).
24 EMFILE Too many open files
No process may have more than OPEN_MAX file descriptors
open at a time.
25 ENOTTY Not a typewriter
A call was made to the ioctl routine specifying a file
that is not a special character device.
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26 ETXTBSY Text file busy
An attempt was made to execute a pure-procedure program
that is currently open for writing. Also an attempt to
open for writing or to remove a pure-procedure program
that is being executed.
27 EFBIG File too large
The size of a file exceeded the maximum file size,
FCHR_MAX [see getrlimit].
28 ENOSPC No space left on device
While writing an ordinary file or creating a directory
entry, there is no free space left on the device. In
the fcntl routine, the setting or removing of record
locks on a file cannot be accomplished because there
are no more record entries left on the system.
29 ESPIPE Illegal seek
A call to the lseek routine was issued to a pipe.
30 EROFS Read-only file system
An attempt to modify a file or directory was made on a
device mounted read-only.
31 EMLINK Too many links
An attempt to make more than the maximum number of
links, LINK_MAX, to a file.
32 EPIPE Broken pipe
A write on a pipe for which there is no process to read
the data. This condition normally generates a signal;
the error is returned if the signal is ignored.
33 EDOM Math argument out of domain of func
The argument of a function in the math package (3M) is
out of the domain of the function.
34 ERANGE Math result not representable
The value of a function in the math package (3M) is not
representable within machine precision.
35 ENOMSG No message of desired type
An attempt was made to receive a message of a type that
does not exist on the specified message queue [see
msgop(2)].
36 EIDRM Identifier removed
This error is returned to processes that resume execu-
tion due to the removal of an identifier from the file
system's name space [see msgctl(2), semctl(2), and
shmctl(2)].
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37 ECHRNG Channel number out of range
38 EL2NSYNC Level 2 not synchronized
39 EL3HLT Level 3 halted
40 EL3RST Level 3 reset
41 ELNRNG Link number out of range
42 EUNATCH Protocol driver not attached
43 ENOCSI No CSI structure available
44 EL2HLT Level 2 halted
45 EDEADLK Deadlock condition
A deadlock situation was detected and avoided. This
error pertains to file and record locking.
46 ENOLCK No record locks available
There are no more locks available. The system lock
table is full [see fcntl(2)].
47-49 Reserved
58-59 Reserved
60 ENOSTR Device not a stream
A putmsg or getmsg system call was attempted on a file
descriptor that is not a STREAMS device.
61 ENODATA No data available
62 ETIME Timer expired
The timer set for a STREAMS ioctl call has expired.
The cause of this error is device specific and could
indicate either a hardware or software failure, or
perhaps a timeout value that is too short for the
specific operation. The status of the ioctl operation
is indeterminate.
63 ENOSR Out of stream resources
During a STREAMS open, either no STREAMS queues or no
STREAMS head data structures were available. This is a
temporary condition; one may recover from it if other
processes release resources.
64 ENONET Machine is not on the network
This error is Remote File Sharing (RFS) specific. It
occurs when users try to advertise, unadvertise, mount,
or unmount remote resources while the machine has not
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done the proper startup to connect to the network.
65 ENOPKG Package not installed
This error occurs when users attempt to use a system
call from a package which has not been installed.
66 EREMOTE Object is remote
This error is RFS specific. It occurs when users try
to advertise a resource which is not on the local
machine, or try to mount/unmount a device (or pathname)
that is on a remote machine.
67 ENOLINK Link has been severed
This error is RFS specific. It occurs when the link
(virtual circuit) connecting to a remote machine is
gone.
68 EADV Advertise error
This error is RFS specific. It occurs when users try
to advertise a resource which has been advertised
already, or try to stop RFS while there are resources
still advertised, or try to force unmount a resource
when it is still advertised.
69 ESRMNT Srmount error
This error is RFS specific. It occurs when an attempt
is made to stop RFS while resources are still mounted
by remote machines, or when a resource is readvertised
with a client list that does not include a remote
machine that currently has the resource mounted.
70 ECOMM Communication error on send
This error is RFS specific. It occurs when the current
process is waiting for a message from a remote machine,
and the virtual circuit fails.
71 EPROTO Protocol error
Some protocol error occurred. This error is device
specific, but is generally not related to a hardware
failure.
74 EMULTIHOP Multihop attempted
This error is RFS specific. It occurs when users try
to access remote resources which are not directly
accessible.
76 EDOTDOT Error 76
This error is RFS specific. A way for the server to
tell the client that a process has transferred back
from mount point.
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77 EBADMSG Not a data message
During a read, getmsg, or ioctl IRECVFD system call to
a STREAMS device, something has come to the head of the
queue that can't be processed. That something depends
on the system call:
read: control information or a passed file descrip-
tor.
getmsg: passed file descriptor.
ioctl: control or data information.
78 ENAMETOOLONG File name too long
The length of the path argument exceeds PATHMAX, or
the length of a path component exceeds NAMEMAX while
POSIXNOTRUNC is in effect; see limits(4).
79 EOVERFLOW
Value too large for defined data type.
80 ENOTUNIQ Name not unique on network
Given log name not unique.
81 EBADFD File descriptor in bad state
Either a file descriptor refers to no open file or a
read request was made to a file that is open only for
writing.
82 EREMCHG Remote address changed
83 ELIBACC Cannot access a needed shared library
Trying to exec an a.out that requires a static shared
library and the static shared library doesn't exist or
the user doesn't have permission to use it.
84 ELIBBAD Accessing a corrupted shared library
Trying to exec an a.out that requires a static shared
library (to be linked in) and exec could not load the
static shared library. The static shared library is
probably corrupted.
85 ELIBSCN .lib section in a.out corrupted
Trying to exec an a.out that requires a static shared
library (to be linked in) and there was erroneous data
in the .lib section of the a.out. The .lib section
tells exec what static shared libraries are needed.
The a.out is probably corrupted.
86 ELIBMAX Attempting to link in more shared libraries
than system limit
Trying to exec an a.out that requires more static
shared libraries than is allowed on the current confi-
guration of the system. See the System Administrator's
Guide.
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87 ELIBEXEC Cannot exec a shared library directly
Attempting to exec a shared library directly.
88 EILSEQ Error 88
Illegal byte sequence. Handle multiple characters as a
single character.
89 ENOSYS Operation not applicable
90 ELOOP Number of symbolic links encountered during path
name traversal exceeds MAXSYMLINKS
91 ESTART Error 91
Interrupted system call should be restarted.
92 ESTRPIPE Error 92
Streams pipe error (not externally visible).
93 ENOTEMPTY Directory not empty
94 EUSERS Too many users
Too many users.
95 ENOTSOCK Socket operation on non-socket
Self-explanatory.
96 EDESTADDRREQ Destination address required
A required address was omitted from an operation on a
transport endpoint. Destination address required.
97 EMSGSIZE Message too long
A message sent on a transport provider was larger than
the internal message buffer or some other network
limit.
98 EPROTOTYPE Protocol wrong type for socket
A protocol was specified that does not support the
semantics of the socket type requested.
99 ENOPROTOOPT Protocol not available
A bad option or level was specified when getting or
setting options for a protocol.
120 EPROTONOSUPPORT Protocol not supported
The protocol has not been configured into the system or
no implementation for it exists.
121 ESOCKTNOSUPPORT Socket type not supported
The support for the socket type has not been configured
into the system or no implementation for it exists.
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122 EOPNOTSUPP Operation not supported on transport end-
point
For example, trying to accept a connection on a
datagram transport endpoint.
123 EPFNOSUPPORT Protocol family not supported
The protocol family has not been configured into the
system or no implementation for it exists. Used for
the Internet protocols.
124 EAFNOSUPPORT Address family not supported by protocol
family
An address incompatible with the requested protocol was
used.
125 EADDRINUSE Address already in use
User attempted to use an address already in use, and
the protocol does not allow this.
126 EADDRNOTAVAIL Cannot assign requested address
Results from an attempt to create a transport endpoint
with an address not on the current machine.
127 ENETDOWN Network is down
Operation encountered a dead network.
128 ENETUNREACH Network is unreachable
Operation was attempted to an unreachable network.
129 ENETRESET Network dropped connection because of reset
The host you were connected to crashed and rebooted.
130 ECONNABORTED Software caused connection abort
A connection abort was caused internal to your host
machine.
131 ECONNRESET Connection reset by peer
A connection was forcibly closed by a peer. This nor-
mally results from a loss of the connection on the
remote host due to a timeout or a reboot.
132 ENOBUFS No buffer space available
An operation on a transport endpoint or pipe was not
performed because the system lacked sufficient buffer
space or because a queue was full.
133 EISCONN Transport endpoint is already connected
A connect request was made on an already connected
transport endpoint; or, a sendto or sendmsg request on
a connected transport endpoint specified a destination
when already connected.
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134 ENOTCONN Transport endpoint is not connected
A request to send or receive data was disallowed
because the transport endpoint is not connected and
(when sending a datagram) no address was supplied.
143 ESHUTDOWN Cannot send after transport endpoint shut-
down
A request to send data was disallowed because the tran-
sport endpoint has already been shut down.
144 ETOOMANYREFS Too many references: cannot splice
145 ETIMEDOUT Connection timed out
A connect or send request failed because the connected
party did not properly respond after a period of time.
(The timeout period is dependent on the communication
protocol.)
146 ECONNREFUSED Connection refused
No connection could be made because the target machine
actively refused it. This usually results from trying
to connect to a service that is inactive on the remote
host.
147 EHOSTDOWN Host is down
A transport provider operation failed because the des-
tination host was down.
148 EHOSTUNREACH No route to host
A transport provider operation was attempted to an
unreachable host.
149 EALREADY Operation already in progress
An operation was attempted on a non-blocking object
that already had an operation in progress.
150 EINPROGRESS Operation now in progress
An operation that takes a long time to complete (such
as a connect) was attempted on a non-blocking object.
151 ESTALE Stale NFS file handle
DEFINITIONS
Background Process Group
Any process group that is not the foreground process group
of a session that has established a connection with a con-
trolling terminal.
Controlling Process
A session leader that established a connection to a control-
ling terminal.
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Controlling Terminal
A terminal that is associated with a session. Each session
may have, at most, one controlling terminal associated with
it and a controlling terminal may be associated with only
one session. Certain input sequences from the controlling
terminal cause signals to be sent to process groups in the
session associated with the controlling terminal; see ter-
mio(7).
Directory
Directories organize files into a hierarchical system where
directories are the nodes in the hierarchy. A directory is
a file that catalogues the list of files, including direc-
tories (sub-directories), that are directly beneath it in
the hierarchy. Entries in a directory file are called
links. A link associates a file identifier with a filename.
By convention, a directory contains at least two links, .
(dot) and .. (dot-dot). The link called dot refers to the
directory itself while dot-dot refers to its parent direc-
tory. The root directory, which is the top-most node of the
hierarchy, has itself as its parent directory. The pathname
of the root directory is / and the parent directory of the
root directory is /.
Downstream
In a stream, the direction from stream head to driver.
Driver
In a stream, the driver provides the interface between peri-
pheral hardware and the stream. A driver can also be a
pseudo-driver, such as a multiplexor or log driver [see
log(7)], which is not associated with a hardware device.
Effective User ID and Effective Group ID
An active process has an effective user ID and an effective
group ID that are used to determine file access permissions
(see below). The effective user ID and effective group ID
are equal to the process's real user ID and real group ID
respectively, unless the process or one of its ancestors
evolved from a file that had the set-user-ID bit or set-
group ID bit set [see exec(2)].
File Access Permissions
Read, write, and execute/search permissions on a file are
granted to a process if one or more of the following are
true:
The effective user ID of the process is super-user.
The effective user ID of the process matches the user
ID of the owner of the file and the appropriate access
bit of the ``owner'' portion (0700) of the file mode is
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set.
The effective user ID of the process does not match the
user ID of the owner of the file, but either the effec-
tive group ID or one of the supplementary group IDs of
the process match the group ID of the file and the
appropriate access bit of the ``group'' portion (0070)
of the file mode is set.
The effective user ID of the process does not match the
user ID of the owner of the file, and neither the
effective group ID nor any of the supplementary group
IDs of the process match the group ID of the file, but
the appropriate access bit of the ``other'' portion
(0007) of the file mode is set.
Otherwise, the corresponding permissions are denied.
File Descriptor
A file descriptor is a small integer used to do I/O on a
file. The value of a file descriptor is from 0 to
(NOFILES-1). A process may have no more than NOFILES file
descriptors open simultaneously. A file descriptor is
returned by system calls such as open, or pipe. The file
descriptor is used as an argument by calls such as read,
write, ioctl, and close.
File Name
Names consisting of 1 to NAMEMAX characters may be used to
name an ordinary file, special file or directory.
These characters may be selected from the set of all charac-
ter values excluding \0 (null) and the ASCII code for /
(slash).
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 [see sh(1)]. Although per-
mitted, the use of unprintable characters in file names
should be avoided.
A file name is sometimes referred to as a pathname com-
ponent. The interpretation of a pathname component is
dependent on the values of NAMEMAX and POSIXNOTRUNC
associated with the path prefix of that component. If any
pathname component is longer than NAMEMAX and
POSIXNOTRUNC is in effect for the path prefix of that
component [see fpathconf(2) and limits(4)], it shall be con-
sidered an error condition in that implementation. Other-
wise, the implementation shall use the first NAMEMAX bytes
of the pathname component.
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Foreground Process Group
Each session that has established a connection with a con-
trolling terminal will distinguish one process group of the
session as the foreground process group of the controlling
terminal. This group has certain privileges when accessing
its controlling terminal that are denied to background pro-
cess groups.
Message
In a stream, one or more blocks of data or information, with
associated STREAMS control structures. Messages can be of
several defined types, which identify the message contents.
Messages are the only means of transferring data and commun-
icating within a stream.
Message Queue
In a stream, a linked list of messages awaiting processing
by a module or driver.
Message Queue Identifier
A message queue identifier (msqid) is a unique positive
integer created by a msgget system call. Each msqid has a
message queue and a data structure associated with it. The
data structure is referred to as msqidds and contains the
following members:
struct ipcperm msgperm;
struct msg *msgfirst;
struct msg *msglast;
ushort msgcbytes;
ushort msgqnum;
ushort msgqbytes;
pidt msglspid;
pidt msglrpid;
timet msgstime;
timet msgrtime;
timet msgctime;
Here are descriptions of the fields of the msqidds struc-
ture:
msgperm is an ipcperm structure that specifies the
message operation permission (see below). This struc-
ture includes the following members:
uidt cuid; /* creator user id */
gidt cgid; /* creator group id */
uidt uid; /* user id */
gidt gid; /* group id */
modet mode; /* r/w permission */
ushort seq; /* slot usage sequence # */
keyt key; /* key */
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*msgfirst is a pointer to the first message on the
queue.
*msglast is a pointer to the last message on the
queue.
msgcbytes is the current number of bytes on the queue.
msgqnum is the number of messages currently on the
queue.
msgqbytes is the maximum number of bytes allowed on
the queue.
msglspid is the process ID of the last process that
performed a msgsnd operation.
msglrpid is the process id of the last process that
performed a msgrcv operation.
msgstime is the time of the last msgsnd operation.
msgrtime is the time of the last msgrcv operation
msgctime is the time of the last msgctl operation that
changed a member of the above structure.
Message Operation Permissions
In the msgop and msgctl system call descriptions, the per-
mission required for an operation is given as {token}, where
token is the type of permission needed, interpreted as fol-
lows:
00400 READ by user
00200 WRITE by user
00040 READ by group
00020 WRITE by group
00004 READ by others
00002 WRITE by others
Read and write permissions on a msqid are granted to a pro-
cess if one or more of the following are true:
The effective user ID of the process is super-user.
The effective user ID of the process matches
msgperm.cuid or msgperm.uid in the data structure
associated with msqid and the appropriate bit of the
``user'' portion (0600) of msgperm.mode is set.
The effective group ID of the process matches
msgperm.cgid or msgperm.gid and the appropriate bit
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of the ``group'' portion (060) of msgperm.mode is set.
The appropriate bit of the ``other'' portion (006) of
msgperm.mode is set.
Otherwise, the corresponding permissions are denied.
Module
A module is an entity containing processing routines for
input and output data. It always exists in the middle of a
stream, between the stream's head and a driver. A module is
the STREAMS counterpart to the commands in a shell pipeline
except that a module contains a pair of functions which
allow independent bidirectional (downstream and upstream)
data flow and processing.
Multiplexor
A multiplexor is a driver that allows streams associated
with several user processes to be connected to a single
driver, or several drivers to be connected to a single user
process. STREAMS does not provide a general multiplexing
driver, but does provide the facilities for constructing
them and for connecting multiplexed configurations of
streams.
Orphaned Process Group
A process group in which the parent of every member in the
group is either itself a member of the group, or is not a
member of the process group's session.
Path Name
A path name is a null-terminated character string starting
with an optional slash (/), followed by zero or more direc-
tory names separated by slashes, optionally followed by a
file name.
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.
Unless specifically stated otherwise, the null path name is
treated as if it named a non-existent file.
Process ID
Each process in the system is uniquely identified during its
lifetime by a positive integer called a process ID. A pro-
cess ID may not be reused by the system until the process
lifetime, process group lifetime and session lifetime ends
for any process ID, process group ID and session ID equal to
that process ID.
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Parent Process ID
A new process is created by a currently active process [see
fork(2)]. The parent process ID of a process is the process
ID of its creator.
Privilege
Having appropriate privilege means having the capability to
override system restrictions.
Process Group
Each process in the system is a member of a process group
that is identified by a process group ID. Any process that
is not a process group leader may create a new process group
and become its leader. Any process that is not a process
group leader may join an existing process group that shares
the same session as the process. A newly created process
joins the process group of its parent.
Process Group Leader
A process group leader is a process whose process ID is the
same as its process group ID.
Process Group ID
Each active process is a member of a process group and is
identified by a positive integer called the process group
ID. This ID is the process ID of the group leader. This
grouping permits the signaling of related processes [see
kill(2)].
Process Lifetime
A process lifetime begins when the process is forked and
ends after it exits, when its termination has been ack-
nowledged by its parent process. See wait(2).
Process Group Lifetime
A process group lifetime begins when the process group is
created by its process group leader, and ends when the life-
time of the last process in the group ends or when the last
process in the group leaves the group.
Read Queue
In a stream, the message queue in a module or driver con-
taining messages moving upstream.
Real User ID and Real Group ID
Each user allowed on the system is identified by a positive
integer (0 to MAXUID) called a real user ID.
Each user is also a member of a group. The group is identi-
fied by a positive integer called the real group ID.
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An active process has a real user ID and real group ID that
are set to the real user ID and real group ID, respectively,
of the user responsible for the creation of the process.
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. The root directory of a pro-
cess need not be the root directory of the root file system.
Saved User ID and Saved Group ID
The saved user ID and saved group ID are the values of the
effective user ID and effective groupID prior to an exec of
a file whose set user or set group file mode bit has been
set [see exec(2)].
Semaphore Identifier
A semaphore identifier (semid) is a unique positive integer
created by a semget system call. Each semid has a set of
semaphores and a data structure associated with it. The
data structure is referred to as semidds and contains the
following members:
struct ipcperm semperm; /* operation permission struct */
struct sem *sembase; /* ptr to first semaphore in set */
ushort semnsems; /* number of sems in set */
timet semotime; /* last operation time */
timet semctime; /* last change time */
/* Times measured in secs since */
/* 00:00:00 GMT, Jan. 1, 1970 */
Here are descriptions of the fields of the semidds struc-
ture:
semperm is an ipcperm structure that specifies the
semaphore operation permission (see below). This
structure includes the following members:
uidt uid; /* user id */
gidt gid; /* group id */
uidt cuid; /* creator user id */
gidt cgid; /* creator group id */
modet mode; /* r/a permission */
ushort seq; /* slot usage sequence number */
keyt key; /* key */
semnsems is equal to the number of semaphores in the
set. Each semaphore in the set is referenced by a non-
negative integer referred to as a semnum. semnum
values run sequentially from 0 to the value of
semnsems minus 1.
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intro(2) SYSTEM CALLS intro(2)
semotime is the time of the last semop operation.
semctime is the time of the last semctl operation that
changed a member of the above structure.
A semaphore is a data structure called sem that contains the
following members:
ushort semval; /* semaphore value */
pidt sempid; /* pid of last operation */
ushort semncnt; /* # awaiting semval > cval */
ushort semzcnt; /* # awaiting semval = 0 */
semval is a non-negative integer that is the actual
value of the semaphore.
sempid is equal to the process ID of the last process
that performed a semaphore operation on this semaphore.
semncnt is a count of the number of processes that are
currently suspended awaiting this semaphore's semval to
become greater than its current value.
semzcnt is a count of the number of processes that are
currently suspended awaiting this semaphore's semval to
become 0.
Semaphore Operation Permissions
In the semop and semctl system call descriptions, the per-
mission required for an operation is given as {token}, where
token is the type of permission needed interpreted as fol-
lows:
00400 READ by user
00200 ALTER by user
00040 READ by group
00020 ALTER by group
00004 READ by others
00002 ALTER by others
Read and alter permissions on a semid are granted to a pro-
cess if one or more of the following are true:
The effective user ID of the process is super-user.
The effective user ID of the process matches
semperm.cuid or semperm.uid in the data structure
associated with semid and the appropriate bit of the
``user'' portion (0600) of semperm.mode is set.
The effective group ID of the process matches
semperm.cgid or semperm.gid and the appropriate bit
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intro(2) SYSTEM CALLS intro(2)
of the ``group'' portion (060) of semperm.mode is set.
The appropriate bit of the ``other'' portion (06) of
semperm.mode is set.
Otherwise, the corresponding permissions are denied.
Session
A session is a group of processes identified by a common ID
called a session ID, capable of establishing a connection
with a controlling terminal. Any process that is not a pro-
cess group leader may create a new session and process
group, becoming the session leader of the session and pro-
cess group leader of the process group. A newly created
process joins the session of its creator.
Session ID
Each session in the system is uniquely identified during its
lifetime by a positive integer called a session ID, the pro-
cess ID of its session leader.
Session Leader
A session leader is a process whose session ID is the same
as its process and process group ID.
Session Lifetime
A session lifetime begins when the session is created by its
session leader, and ends when the lifetime of the last pro-
cess that is a member of the session ends, or when the last
process that is a member in the session leaves the session.
Shared Memory Identifier
A shared memory identifier (shmid) is a unique positive
integer created by a shmget system call. Each shmid has a
segment of memory (referred to as a shared memory segment)
and a data structure associated with it. (Note that these
shared memory segments must be explicitly removed by the
user after the last reference to them is removed.) The data
structure is referred to as shmidds and contains the fol-
lowing members:
struct ipcperm shmperm; /* operation permission struct */
int shmsegsz; /* size of segment */
struct region *shmreg; /* ptr to region structure */
char pad[4]; /* for swap compatibility */
pidt shmlpid; /* pid of last operation */
pidt shmcpid; /* creator pid */
ushort shmnattch; /* number of current attaches */
ushort shmcnattch; /* used only for shminfo */
timet shmatime; /* last attach time */
timet shmdtime; /* last detach time */
timet shmctime; /* last change time */
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intro(2) SYSTEM CALLS intro(2)
/* Times measured in secs since */
/* 00:00:00 GMT, Jan. 1, 1970 */
Here are descriptions of the fields of the shmidds struc-
ture:
shmperm is an ipcperm structure that specifies the
shared memory operation permission (see below). This
structure includes the following members:
uidt cuid; /* creator user id */
gidt cgid; /* creator group id */
uidt uid; /* user id */
gidt gid; /* group id */
modet mode; /* r/w permission */
ushort seq; /* slot usage sequence # */
keyt key; /* key */
shmsegsz specifies the size of the shared memory seg-
ment in bytes.
shmcpid is the process ID of the process that created
the shared memory identifier.
shmlpid is the process ID of the last process that
performed a shmop operation.
shmnattch is the number of processes that currently
have this segment attached.
shmatime is the time of the last shmat operation [see
shmop(2)].
shmdtime is the time of the last shmdt operation [see
shmop(2)].
shmctime is the time of the last shmctl operation that
changed one of the members of the above structure.
Shared Memory Operation Permissions
In the shmop and shmctl system call descriptions, the per-
mission required for an operation is given as {token}, where
token is the type of permission needed interpreted as fol-
lows:
00400 READ by user
00200 WRITE by user
00040 READ by group
00020 WRITE by group
00004 READ by others
00002 WRITE by others
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intro(2) SYSTEM CALLS intro(2)
Read and write permissions on a shmid are granted to a pro-
cess if one or more of the following are true:
The effective user ID of the process is super-user.
The effective user ID of the process matches
shmperm.cuid or shmperm.uid in the data structure
associated with shmid and the appropriate bit of the
``user'' portion (0600) of shmperm.mode is set.
The effective group ID of the process matches
shmperm.cgid or shmperm.gid and the appropriate bit
of the ``group'' portion (060) of shmperm.mode is set.
The appropriate bit of the ``other'' portion (06) of
shmperm.mode is set.
Otherwise, the corresponding permissions are denied.
Special Processes
The process with ID 0 and the process with ID 1 are special
processes referred to as proc0 and proc1; see kill(2).
proc0 is the process scheduler. proc1 is the initialization
process (init); proc1 is the ancestor of every other process
in the system and is used to control the process structure.
STREAMS
A set of kernel mechanisms that support the development of
network services and data communication drivers. It defines
interface standards for character input/output within the
kernel and between the kernel and user level processes. The
STREAMS mechanism is composed of utility routines, kernel
facilities and a set of data structures.
Stream
A stream is a full-duplex data path within the kernel
between a user process and driver routines. The primary
components are a stream head, a driver and zero or more
modules between the stream head and driver. A stream is
analogous to a shell pipeline except that data flow and pro-
cessing are bidirectional.
Stream Head
In a stream, the stream head is the end of the stream that
provides the interface between the stream and a user pro-
cess. The principle functions of the stream head are pro-
cessing STREAMS-related system calls, and passing data and
information between a user process and the stream.
Super-user
A process is recognized as a super-user process and is
granted special privileges, such as immunity from file
21
intro(2) SYSTEM CALLS intro(2)
permissions, if its effective user ID is 0.
Upstream
In a stream, the direction from driver to stream head.
Write Queue
In a stream, the message queue in a module or driver con-
taining messages moving downstream.
22