intro(2) CLIX intro(2)
NAME
intro - Introduction to system calls and error numbers
SYNOPSIS
#include <errno.h>
DESCRIPTION
This section describes all system calls (usually called functions in this
documentation).
Most of these calls have one or more error returns. An error condition 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 available in the external variable
errno. The variable errno is not cleared on successful calls, so it
should be tested only after an error is 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 owner
This error typically indicates an attempt to modify a file
forbidden except to its owner or the superuser. It is also
returned if an ordinary user attempts an action allowed only to the
superuser.
2 ENOENT No such file or directory
This error occurs when a filename is specified and the file should
exist but does not, or when one of the directories in a pathname
does not exist.
3 ESRCH No such process
No process can be found that corresponds to the process specified
by pid in the kill() or ptrace() functions.
4 EINTR Interrupted system call
An asynchronous signal (such as interrupt or quit), that the user
elected to catch, occurred during a system call. If execution is
resumed after processing the signal, it will appear as if the
interrupted system call returned this error condition.
5 EIO I/O error
Some physical I/O error occurred. This error may 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 that does not exist or
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is beyond the limits of the device. It may also occur when a tape
drive is not online or no disk pack is loaded on a drive.
7 E2BIG Arg list too long
An argument list longer than 5,120 bytes is presented to a member
of the exec() function family.
8 ENOEXEC Exec format error
Execution of a file that, although it has the appropriate
permissions, does not start with a valid magic number (see the
a.out() file format) is requested.
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() function call was executed by a process that had no
existing or unwaited-for child processes.
11 EAGAIN No more processes
A fork() function call failed because the system 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 an exec(), brk(), or sbrk(), a program asks for more space
than the system is able to supply. This may not be a temporary
condition; the maximum space 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 a fork(). This error occurring on a resource
associated with Remote File Sharing (RFS) indicates a memory
depletion that may be temporary, depending 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 system call.
15 ENOTBLK Block device required
A nonblock file was mentioned where a block device was required (as
in the mount() function).
16 EBUSY Device or resource busy
The device or resource is currently unavailable. An attempt was
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made to mount a device already mounted or to dismount a device that
has an active file (open file, current directory, mounted-on file,
active text segment). This error will also occur if an attempt is
made to enable accounting when it is already enabled.
17 EEXIST File exists
An existing file was mentioned in an inappropriate context (see the
link() function).
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 system call to a
device (such as reading a write-only device).
20 ENOTDIR Not a directory
A nondirectory was specified where a directory is required (such as
in a path prefix or as an argument to the chdir() function).
21 EISDIR Is a directory
An attempt was made to write on a directory.
22 EINVAL Invalid argument
An invalid argument (such as dismounting a nonmounted device;
mentioning an undefined signal in the signal() or kill() functions;
or reading or writing a file for which lseek() has generated a
negative pointer). The error is also set by the math functions
described in this manual.
23 ENFILE File table overflow
The system file table is temporarily full, and no more opens can be
accepted.
24 EMFILE Too many open files
No process may have more than NOFILES (default 128) descriptors
open at a time.
25 ENOTTY Not a character device (or) Not a typewriter
An attempt was made to execute the ioctl() function on a file that
is not a special character device.
26 ETXTBSY Text file busy
An attempt was made to execute a pure-procedure program currently
open for writing. Also, an attempt to open for writing or to
remove a pure-procedure program being run.
27 EFBIG File too large
The size of a file exceeded the maximum file size or ULIMIT (see
the ulimit() function).
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28 ENOSPC No space left on device
During a write() to an ordinary file, no free space is left on the
device. In the fcntl() function, the setting or removing of record
locks on a file cannot be accomplished because no more record
entries remain on the system.
29 ESPPE Illegal seek
An attempt to perform an illegal seek by the lseek() function 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 was made to make more than the maximum number of links
(1000) to a file.
32 EPPE Broken pipe
An attempt was made to write on a pipe for which no process to read
the data exists. This condition normally generates a signal; the
error is returned if the signal is ignored.
33 EDOM Math argument
The argument of a function in the math package is out of the
function's domain.
34 ERANGE Result too large
The value of a function in the math package 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 the msgop() function).
36 EIDRM Identifier removed
This error is returned to processes that resume execution due to
the removal of an identifier from the file system's name space (see
the msgctl(), semctl(), and shmctl() functions).
37-44 Reserved numbers
45 EDEADLK Deadlock
A deadlock situation was detected and avoided. This error pertains
to file and record locking.
46 ENOLCK No lock
In fcntl(), setting or removing record locks on a file cannot be
accomplished because no more record entries remain on the system.
47 ENOSYS Function not implemented
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An attempt was made to use a function that is not available in this
implementation.
60 ENOSTR Not a stream
A putmsg() or getmsg() system call was attempted on a file
descriptor that is not a STREAMS device.
62 ETIME Stream ioctl timeout
The timer set for a STREAMS ioctl() call expired. The cause of
this error is device specific and could indicate a hardware or
software failure or a timeout value that is too short for the
specific operation. The status of the ioctl() operation is
indeterminate.
63 ENOSR No stream resources
During a STREAMS open(), either no STREAMS queues or no STREAMS
head data structures were available.
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 when the machine did not do the proper startup to connect
to the network.
65 ENOPKG No package
This error occurs when users attempt to use a system call from a
package that is not installed.
66 EREMOTE Resource is remote
This error is RFS-specific. It occurs when users try to advertise
a resource that is not on the local machine, or try to
mount/unmount a device (or pathname) that is on a remote machine.
67 ENOLINK Virtual circuit is gone
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 that has been advertised, try to stop the RFS while
resources are still advertised, or try to force an unmount on a
resource when it is still advertised.
69 ESRMNT Srmount error
This error is RFS-specific. It occurs when users try to stop RFS
while resources are still mounted by remote machines.
70 ECOMM Communication error
This error is RFS-specific. It occurs when users try to send
messages to remote machines, but no virtual circuit can be found.
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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 that are not directly accessible.
77 EBADMSG Bad message
During a read(), getmsg(), or ioctl() I_RECVFD system call to a
STREAMS device, something that cannot be processed has come to the
head of the queue. What it is depends on the system call:
read() Control information or a passed file descriptor.
getmsg() Passed file descriptor.
ioctl() Control or data information.
83 ELIBACC Cannot access a needed shared library
Tried to run the exec() function on an a.out file that requires a
shared library (to be linked in) and the shared library does not
exist or the user does not have permission to use it.
84 ELIBBAD Accessing a corrupted shared library
Tried to run the exec() function on an a.out file that requires a
shared library (to be linked in) and exec() could not load the
shared library. The shared library is probably corrupted.
85 ELIBSCN .lib section in a.out corrupted
Tried to run the exec() function on an a.out file that requires a
shared library (to be linked in) and erroneous data was in the .lib
section of the a.out file. The .lib section tells exec() the
shared libraries needed. The a.out file is probably corrupted.
86 ELIBMAX Attempting to link in more shared libraries than system limit
Tried to run the exec() function on an a.out file that requires
more shared libraries (to be linked in) than allowed on the current
system configuration.
87 ELIBEXEC Cannot exec a shared library directly
Tried to run the exec() function on a shared library directly.
This is not allowed.
88 ENAMETOOLONG Filename too long
The size of a pathname string exceeded PATH_MAX, or a pathname
component was longer than NAME_MAX and _POSIX_NO_TRUNC (see the
<unistd.h> header file) was in effect for that file.
89 ENOTEMPTY Directory not empty
A directory with entries other than ``.'' and ``..'' was supplied
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when an empty directory was expected.
90 EWOULDBLOCK Operation would block
An operation that would cause a process to block was attempted on
an object in nonblocking mode (see the fcntl() function).
91 EINPROGRESS Operation now in progress
An operation that takes a long time to complete (such as a
connect() function) was attempted on a nonblocking object (see
fcntl()).
92 EALREADY Operation already in progress
An operation was attempted on a nonblocking object that had an
operation in progress.
93 ENOTSOCK Socket operation on nonsocket
Self-explanatory.
94 EDESTADDRREQ Destination address required
A required address was omitted from an operation on a socket.
95 EMSGSIZE Message too long
A message sent on a socket was larger than the internal message
buffer or some other network limit.
96 EPROTOTYPE Protocol wrong type for socket
A protocol was specified that does not support the semantics of the
socket type requested.
97 EPROTONOSUPPORT Protocol not supported
The protocol was not configured in the system, or no implementation
exists for it.
98 ESOCKTNOSUPPORT Socket type not supported
The support for the socket type was not configured in the system,
or no implementation exists for it.
99 EOPNOTSUPP Operation not supported on socket
Self-explanatory.
100 EPFNOSUPPORT Protocol family not supported
The protocol family was not configured in the system, or no
implementation exists for it.
101 EAFNOSUPPORT Address family not supported by protocol family
An address was used that is incompatible with the requested
protocol.
102 EADDRINUSE Address already in use
Only one use of each address is normally permitted.
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103 EADDRNOTAVAIL Can't assign requested address
This normally results from an attempt to create a socket with an
address that is not on this machine.
104 ENETDOWN Network is down
A socket operation encountered a dead network.
105 ENETUNREACH Network is unreachable
A socket operation attempted to an unreachable network.
106 ENETRESET Network dropped connection on reset
The host that was connected to crashed and rebooted.
107 ECONNABORTED Software caused connection abort
A connection abort occurred and was caused the host machine
software.
108 ECONNRESET Connection reset be peer
A connection was forcibly closed by a peer. This normally results
from a loss of the connection on the remote socket due to a timeout
or a reboot.
109 ENOBUFS No buffer space available
An operation on a socket was not performed because the system
lacked sufficient buffer space, or because a queue was full.
110 EISCONN Socket is already connected
A connect() request was made on an already connected socket. This
also occurs if a sendto() request on a connected socket specified a
destination when it was already connected.
111 ENOTCONN Socket is not connected
An request to send or receive data was disallowed because the
socket is not connected and (when sending on a datagram socket) no
address was supplied.
112 ESHUTDOWN Can't send after socket shutdown
A request to send data was disallowed because the socket was
shutdown with a previous shutdown() call.
114 ETIMEDOUT Connection timed out
A socket operation timed out. The timeout period depends on the
communication protocol.
115 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 foreign host.
116 EHOSTDOWN Host is down
A socket operation failed because the destination host was down.
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117 EHOSTUNREACH No route to host
A socket operation was attempted to an unreachable host.
118 ENOPROTOOPT Protocol not available
A bad option or level was specified in a getsockopt() call.
Definitions
Process ID
Each active process in the system is uniquely identified by a
positive integer called a process ID. The range of this ID is from
1 to 30,000.
Parent Process ID
A new process is created by a currently active process (see fork()
). The parent process ID of a process is the process ID of its
creator.
Process Group ID
Each active process is a member of a process group 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() ).
Tty Group ID
Each active process can be a member of a terminal group identified
by a positive integer called the tty group ID. This grouping is
used to terminate a group of related processes when one of the
processes in the group is terminated (see exit() and signal() ).
Real User ID and Real Group ID
Each user allowed on the system is identified by a positive integer
(0 to 65535) called a real user ID.
Each user is also a member of a group. The group is identified by
a positive integer called the real group ID.
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 process creation.
Effective User ID and Effective Group ID
An active process has an effective user ID and an effective group
ID used to determine file access permissions. 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() ).
Superuser
A process is recognized as a superuser process and is granted
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special privileges, such as immunity from file permissions, if its
effective user ID is 0.
Special Processes
The processes with a process ID of 0 and a process ID of 1 are
special processes and are referred to as proc0 and proc1.
Proc0 is the 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.
File Descriptor
A file descriptor is a small integer used to perform I/O on a file.
The value of a file descriptor is from 0 to (NOFILE - 1). A
process may have no more than NOFILE (defined in master.d) 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().
Filename
Names consisting of 1 to 14 characters may be used to name an
ordinary file, special file, or directory.
These characters may be selected from the set of all character
values excluding \0 (null) and the ASCII code for / (slash).
It is generally unwise to use *, ?, [, or ] as part of filenames
because of the special meaning attached to these characters by the
shell. (See the command sh().) Although permitted, using
unprintable characters in filenames should be avoided.
Pathname and Path Prefix
A pathname is a null-terminated character string starting with an
optional slash (/), followed by zero or more directory names
separated by slashes, and optionally followed by a filename.
If a pathname 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 pathname is treated
as if it named a nonexistent file.
Directory
Directory entries are called links. By convention, a directory
contains at least two links, ``.'' and ``..'', referred to as dot
and dot-dot, respectively. Dot is the directory itself and dot-dot
is its parent directory.
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Root Directory and Current Working Directory
Each process has associated with it a concept of a root directory
and a current working directory for resolving pathname searches.
The root directory of a process need not be the root directory of
the root file system.
File Access Permissions
Read, write, and execute/search permissions on a file are granted
to a process if one or more of the following is true:
⊕ The effective user ID of the process is superuser.
⊕ The effective user ID of the process matches the user ID of the
file owner and the appropriate access bit of the ``owner''
portion (0700) of the file mode is set.
⊕ The effective user ID of the process does not match the user ID
of the file owner, the effective group ID of the process matches
the group of the file, and the appropriate access bit of the
file mode ``group'' portion (0070) is set.
⊕ The effective user ID of the process does not match the user ID
of the file owner, the effective group ID of the process does
not match the group ID of the file, and the appropriate access
bit of the file mode ``other'' portion (0007) is set.
Otherwise, the corresponding permissions are denied.
Message Queue Identifier
A message queue identifier (msqid) is a unique positive integer
created by a msgget() system call. Each msqid has an associated
message queue and data structure. The data structure is referred
to as msqid_ds and contains the following members:
struct ipc_perm msg_perm;
struct msg *msg_first;
struct msg *msg_last;
ushort msg_cbytes;
ushort msg_qnum;
ushort msg_qbytes;
ushort msg_lspid;
ushort msg_lrpid;
time_t msg_stime;
time_t msg_rtime;
time_t msg_ctime;
msg_perm
An ipc_perm structure that specifies the message operation
permission (see below). This structure includes the
following members:
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ushort cuid; /* creator user ID */
ushort cgid; /* creator group ID */
ushort uid; /* user ID */
ushort gid; /* group ID */
ushort mode; /* r/w permission */
ushort seq; /* slot usage sequence # */
key_t key; /* key */
msg *msg_first
A pointer to the first message on the queue.
msg *msg_last
A pointer to the last message on the queue.
msg_cbytes
The current number of bytes on the queue.
msg_qnum
The number of messages currently on the queue.
msg_qbytes
The maximum number of bytes allowed on the queue.
msg_lspid
The process ID of the last process that performed a msgsnd
(message send) operation.
msg_lrpid
The process ID of the last process that performed a msgrcv
(message receive) operation.
msg_stime
The time of the last msgsnd operation.
msg_rtime
The time of the last msgrcv operation.
msg_ctime
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
permission required for an operation is given as {token}, where
token is the type of permission needed, interpreted as follows:
00400 Read by user
00200 Write by user
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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 process if
one or more of the following is true:
⊕ The effective user ID of the process is superuser.
⊕ The effective user ID of the process matches msg_perm.cuid or
msg_perm.uid in the data structure associated with msqid and the
appropriate bit of the ``user'' portion (0600) of msg_perm.mode
is set.
⊕ The effective group ID of the process matches msg_perm.cgid or
msg_perm.gid and the appropriate bit of the ``group'' portion
(060) of msg_perm.mode is set.
⊕ The appropriate bit of the ``other'' portion (006) of
msg_perm.mode is set.
Otherwise, the corresponding permissions are denied.
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 semid_ds and contains the following members:
struct ipc_perm sem_perm; /* operation permission struct */
struct sem *sem_base; /* ptr to first semaphore in set */
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 */
sem_perm
An ipc_perm structure that specifies the semaphore operation
permission. This structure includes the following members:
ushort uid; /* user ID */
ushort gid; /* group ID */
ushort cuid; /* creator user ID */
ushort cgid; /* creator group ID */
ushort mode; /* r/a permission */
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ushort seq; /* slot usage sequence number */
key_t key; /* key */
sem_nsems
Equal to the number of semaphores in the set. Each
semaphore in the set is referenced by a positive integer
referred to as a sem_num. Values for sem_num run
sequentially from 0 to the value of sem_nsems minus 1.
sem_otime
The time of the last semop() operation.
sem_ctime
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 */
short sempid; /* pid of last operation */
ushort semncnt; /* # awaiting semval > cval */
ushort semzcnt; /* # awaiting semval = 0 */
semval A non-negative integer that is the actual value of the
semaphore.
sempid Equal to the process ID of the last process that performed a
semaphore operation on this semaphore.
semncnt
A count of the number of processes currently suspended and
awaiting this semaphore's semval to become greater than its
current value.
semzcnt
A count of the number of processes currently suspended and
awaiting this semaphore's semval to become 0.
Semaphore Operation Permissions
In the semop() and semctl() descriptions, the permission required
for an operation is given as {token}, where token is the type of
permission needed, interpreted as follows:
00400 Read by user
00200 Alter by user
00040 Read by group
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00020 Alter by group
00004 Read by others
00002 Alter by others
Read and alter permissions on a semid are granted to a process if
one or more of the following is true:
⊕ The effective user ID of the process is superuser.
⊕ The effective user ID of the process matches sem_perm.cuid or
sem_perm.uid in the data structure associated with semid and the
appropriate bit of the ``user'' portion (0600) of sem_perm.mode
is set.
⊕ The effective group ID of the process matches sem_perm.cgid or
sem_perm.gid and the appropriate bit of the ``group'' portion
(060) of sem_perm.mode is set.
⊕ The appropriate bit of the ``other'' portion (006) of
sem_perm.mode is set.
Otherwise, the corresponding permissions are denied.
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 an associated
data structure. (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 shmid_ds and contains the
following members:
struct ipc_perm shm_perm; /* operation permission struct */
int shm_segsz; /* size of segment */
struct region *shm_reg; /* ptr to region structure */
char pad[4]; /* for swap compatibility */
ushort shm_lpid; /* pid of last operation */
ushort shm_cpid; /* creator pid */
ushort shm_nattch; /* number of current attaches */
ushort shm_cnattch; /* used only for shminfo */
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 */
shm_perm An ipc_perm structure that specifies the shared memory
operation permission. This structure includes the
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following members:
ushort cuid; /* creator user ID */
ushort cgid; /* creator group ID */
ushort uid; /* user ID */
ushort gid; /* group ID */
ushort mode; /* r/w permission */
ushort seq; /* slot usage sequence # */
key_t key; /* key */
shm_segsz The size of the shared memory segment in bytes.
shm_cpid The process ID of the process that created the shared
memory identifier.
shm_lpid The process ID of the last process that performed a
shmop() operation.
shm_nattch The number of processes that currently have this
segment attached.
shm_atime The time of the last shmat() operation.
shm_dtime The time of the last shmdt() operation.
shm_ctime The time of the last shmctl() operation that changed a
member of the above structure.
Shared Memory Operation Permissions
In the shmop() and shmctl() descriptions, the permission required
for an operation is given as {token}, where token is the type of
permission needed, interpreted as follows:
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 shmid are granted to a process if
one or more of the following is true:
⊕ The effective user ID of the process is superuser.
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⊕ The effective user ID of the process matches shm_perm.cuid or
shm_perm.uid in the data structure associated with shmid and the
appropriate bit of the ``user'' portion (0600) of shm_perm.mode
is set.
⊕ The effective group ID of the process matches shm_perm.cgid or
shm_perm.gid and the appropriate bit of the ``group'' portion
(060) of shm_perm.mode is set.
⊕ The appropriate bit of the ``other'' portion (06) of
shm_perm.mode is set.
Otherwise, the corresponding permissions are denied.
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 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 processing are bidirectional.
Stream Head
The end of the stream that provides the interface between the
stream and a user process. The principle functions of the stream
head are processing STREAMS-related system calls and passing data
and information between a user process and the stream.
Driver The interface between peripheral hardware and the stream. A driver
can also be a pseudo-driver, such as a multiplexor or log driver
(see the log() file), that is not associated with a hardware
device.
Module 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 that allow independent bidirectional (downstream and
upstream) data flow and processing.
Downstream
The direction from stream head to driver.
Upstream
The direction from driver to stream head.
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intro(2) CLIX intro(2)
Message
One or more blocks of data or information with associated STREAMS
control structures. Messages can be of several defined types, that
identify the message contents. Messages are the only means of
transferring data and communicating within a stream.
Message Queue
A linked list of messages awaiting processing by a module or
driver.
Read Queue
The message queue in a module or driver containing messages moving
upstream.
Write Queue
The message queue in a module or driver containing messages moving
downstream.
Multiplexor
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 provides the facilities for
constructing them and for connecting multiplexed streams
configurations.
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, and so on.
Each instance of the system supports some collection of socket
types; consult socket() 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.
RELATED INFORMATION
Functions: intro(3)
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