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 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 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 ordinary 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 interrupted 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.

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 argument 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.

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 reliably detect a bad address, on some implementations passing a bad address to a routine will result in undefined behavior. 

15  ENOTBLK   Block device required
A non-block file was mentioned where a block device was required (for example, 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 context (for example, 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 (for example, read a write-only device).

20  ENOTDIR   Not a directory
A non-directory was specified where a directory is required (for example, 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 (for example, 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 (that is, 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. 

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 execution due to the removal of an identifier from the file system’s name space [see msgctl(2), semctl(2), and shmctl(2)]. 

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 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 path-name) 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. 

77  EBADMSG   Not a data message
During a read, getmsg, or ioctl I_RECVFD 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 descriptor. 
  getmsg: passed file descriptor. 
  ioctl: control or data information. 

78  ENAMETOOLONG   File name too long
The length of the path argument exceeds PATH_MAX, or the length of a path component exceeds NAME_MAX while _POSIX_NO_TRUNC 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 configuration of the system. 

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).

158  ENOTEMPTY   Directory not empty

160  EUSERS   Too many users
Too many users.

130  ENOTSOCK   Socket operation on non-socket
Self-explanatory.

131  EDESTADDRREQ   Destination address required
A required address was omitted from an operation on a transport endpoint. Destination address required.

132  EMSGSIZE   Message too long
A message sent on a transport provider was larger than the internal message buffer or some other network limit.

133  EPROTOTYPE   Protocol wrong type for socket
A protocol was specified that does not support the semantics of the socket type requested.

134  ENOPROTOOPT   Protocol not available
A bad option or level was specified when getting or setting options for a protocol.

135  EPROTONOSUPPORT   Protocol not supported
The protocol has not been configured into the system or no implementation for it exists.

136  ESOCKTNOSUPPORT   Socket type not supported
The support for the socket type has not been configured into the system or no implementation for it exists.

137  EOPNOTSUPP   Operation not supported on transport endpoint
For example, trying to accept a connection on a datagram transport endpoint.

138  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.

139  EAFNOSUPPORT   Address family not supported by protocol family
An address incompatible with the requested protocol was used.

140  EADDRINUSE   Address already in use
User attempted to use an address already in use, and the protocol does not allow this.

141  EADDRNOTAVAIL   Cannot assign requested address
Results from an attempt to create a transport endpoint with an address not on the current machine.

142  ENETDOWN   Network is down
Operation encountered a dead network.

143  ENETUNREACH   Network is unreachable
Operation was attempted to an unreachable network.

144  ENETRESET   Network dropped connection because of reset
The host you were connected to crashed and rebooted.

145  ECONNABORTED   Software caused connection abort
A connection abort was caused internal to your host machine.

146  ECONNRESET   Connection reset by peer
A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote host due to a timeout or a reboot.

147  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.

148  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. 

149  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.

150  ESHUTDOWN   Cannot send after transport endpoint shutdown
A request to send data was disallowed because the transport endpoint has already been shut down.

151  ETOOMANYREFS   Too many references: cannot splice

152  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.)

153  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.

156  EHOSTDOWN   Host is down
A transport provider operation failed because the destination host was down.

157  EHOSTUNREACH   No route to host
A transport provider operation was attempted to an unreachable host.

129  EALREADY   Operation already in progress
An operation was attempted on a non-blocking object that already had an operation in progress.

128  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. 

162  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 controlling terminal. 

Controlling Process

A session leader that established a connection to a controlling terminal. 

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 termio(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 directories (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 file-name.  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 directory.  The root directory, which is the top-most node of the hierarchy, has itself as its parent directory.  The path-name 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 peripheral 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 set. 

The effective user ID of the process does not match the user ID of the owner of the file, but either the effective 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 NAME_MAX 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). 

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 permitted, the use of unprintable characters in file-names should be avoided. 

A file-name is sometimes referred to as a path-name component.  The interpretation of a path-name component is dependent on the values of NAME_MAX and _POSIX_NO_TRUNC associated with the path prefix of that component.  If any path-name component is longer than NAME_MAX and _POSIX_NO_TRUNC is in effect for the path prefix of that component [see fpathconf(2) and limits(4)], it shall be considered an error condition in that implementation.  Otherwise, the implementation shall use the first NAME_MAX bytes of the path-name component. 

Foreground Process Group

Each session that has established a connection with a controlling 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 process 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 communicating 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 msqid_ds and contains the following members:

struct  ipc_perm msg_perm;
struct  msg ∗msg_first;
struct  msg ∗msg_last;
ulong        msg_cbytes;
ulong        msg_qnum;
ulong        msg_qbytes;
pid_t        msg_lspid;
pid_t        msg_lrpid;
time_t       msg_stime;
long         msg_susec;
time_t       msg_rtime;
long         msg_rusec;
time_t       msg_ctime;
long         msg_cusec;

Here are descriptions of the fields of the msqid_ds structure:

msg_perm is an ipc_perm structure that specifies the message operation permission (see below).  This structure includes the following members:

uid_t	cuid;	/∗ creator user id ∗/
gid_t	cgid;	/∗ creator group id ∗/
uid_t	uid;	/∗ user id ∗/
gid_t	gid;	/∗ group id ∗/
mode_t	mode;	/∗ r/w permission ∗/
ushort	seq;	/∗ slot usage sequence # ∗/
key_t	key;	/∗ key ∗/

∗msg_first is a pointer to the first message on the queue. 

∗msg_last is a pointer to the last message on the queue. 

msg_cbytes is the current number of bytes on the queue. 

msg_qnum is the number of messages currently on the queue. 

msg_qbytes is the maximum number of bytes allowed on the queue. 

msg_lspid is the process ID of the last process that performed a msgsnd operation. 

msg_lrpid is the process id of the last process that performed a msgrcv operation. 

msg_stime and msg_susec are the seconds and microseconds respectively, of the time of the last msgsnd operation. 

msg_rtime and msg_rusec are the seconds and microseconds respectively, of the time of the last msgrcv operation. 

msg_ctime and msg_cusec are the seconds and microseconds respectively, of 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
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 are true:

The effective user ID of the process is super-user. 

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. 

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 directory 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 process 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. 

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 acknowledged 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 lifetime 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 containing 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 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 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 process 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 group ID 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 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 ∗/
char           sem_pad[2];
ushort         sem_nsems;    /∗ # of sems in set ∗/
time_t         sem_otime;    /∗ last semop time ∗/
long           sem_ousec;    /∗ in secs and microsecs. ∗/
time_t         sem_ctime;    /∗ last change time ∗/
long           sem_cusec     /∗ in secs and microsecs. ∗/

Here are descriptions of the fields of the semid_ds structure:

sem_perm is an ipc_perm structure that specifies the semaphore operation permission (see below).  This structure includes the following members:

uid_t	uid;	/∗ user id ∗/
gid_t	gid;	/∗ group id ∗/
uid_t	cuid;	/∗ creator user id ∗/
gid_t	cgid;	/∗ creator group id ∗/
mode_t	mode;	/∗ r/a permission ∗/
ushort	seq;	/∗ slot usage sequence number ∗/
key_t	key;	/∗ key ∗/

sem_nsems is equal to the number of semaphores in the set.  Each semaphore in the set is referenced by a nonnegative integer referred to as a sem_num.  sem_num values run sequentially from 0 to the value of sem_nsems minus 1. 

sem_otime and sem_ousec are the seconds and microseconds respectively, of the time of the last semop operation. 

sem_ctime and sem_cusec are the seconds and microseconds respectively, of 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 ∗/
pid_t	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 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
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 are true:

The effective user ID of the process is super-user. 

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 (06) of sem_perm.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 process group leader may create a new session and process group, becoming the session leader of the session and process 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 process 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 process 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 shmid_ds and contains the following members:

struct ipc_perm    shm_perm;    /∗ operation permission struct ∗/
int                 shm_segsz;   /∗ size of segment in bytes ∗/
struct  anon_map   ∗shm_amp;    /∗ segment anon_map pointer∗/
pid_t               shm_lpid;    /∗ pid of last operation ∗/
pid_t               shm_cpid;    /∗ pid of creator ∗/
ulong               shm_nattch;  /∗ used only for shminfo ∗/
ulong               shm_cnattch; /∗ used only for shminfo ∗/
time_t              shm_atime;   /∗ last shmat time ∗/
long                shm_ausec;   /∗ in secs and microsecs.∗/
time_t              shm_dtime;   /∗ last shmdt time ∗/
long                shm_cusec;   /∗ in secs and microsecs. ∗/
time_t              shm_ctime    /∗ last change time ∗/
long                shm_cusec    /∗ in secs and microsecs. ∗/

Here are descriptions of the fields of the shmid_ds structure:

shm_perm is an ipc_perm structure that specifies the shared memory operation permission (see below).  This structure includes the following members:

uid_t	cuid;	/∗ creator user id ∗/
gid_t	cgid;	/∗ creator group id ∗/
uid_t	uid;	/∗ user id ∗/
gid_t	gid;	/∗ group id ∗/
mode_t	mode;	/∗ r/w permission ∗/
ushort	seq;	/∗ slot usage sequence # ∗/
key_t	key;	/∗ key ∗/

shm_segsz specifies the size of the shared memory segment in bytes. 

shm_cpid is the process ID of the process that created the shared memory identifier. 

shm_lpid is the process ID of the last process that performed a shmop operation. 

shm_nattch is the number of processes that currently have this segment attached. 

shm_otime and shm_ausec are the seconds and microseconds respectively, of the time of the last shmat operation [see shmop(2)]. 

shm_dtime and shm_dusec are the seconds and microseconds respectively, of the time of the last shmdt operation [see shmop(2)]. 

shm_ctime and shm_cusec are the seconds and microseconds respectively, of the time of the last shmctl operation that changed members of the above structure. 

Shared Memory Operation Permissions

In the shmop and shmctl 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
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 are true:

The effective user ID of the process is super-user. 

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. 

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 processing 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 process.  The principal functions of the stream head are processing 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 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 containing messages moving downstream.