INTRO(2)	    UNIX Programmer's Manual		 INTRO(2)


NAME
     intro - introduction to system calls and error numbers

SYNOPSIS
     #include <sys/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 return value.
     This is almost always -1; the individual descriptions
     specify the details.  Note that a number of system calls
     overload the meanings of these error numbers, and that the
     meanings must be interpreted according to the type and cir-
     cumstances of the call.

     As with normal arguments, all return codes and values from
     functions are of type integer unless otherwise noted.  An
     error number is also made available in the external variable
     errno, which is not cleared on successful calls.  Thus errno
     should be tested only after an error has occurred.

     The following is a complete list of the errors and their
     names as given in <sys/errno.h>.

     0	     Error 0
	  Unused.

     1	EPERM  Not owner
	  Typically this error indicates an attempt to modify a
	  file in some way forbidden except to its owner or
	  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
	  This error occurs when a file name is specified and the
	  file should exist but doesn't, or when one of the
	  directories in a path name does not exist.

     3	ESRCH  No such process
	  The process or process group whose number was given
	  does not exist, or any such process is already dead.

     4	EINTR  Interrupted system call
	  An asynchronous signal (such as interrupt or quit) that
	  the user has elected to catch occurred during a system
	  call.  If execution is resumed after processing the
	  signal and the system call is not restarted, it will
	  appear as if the interrupted system call returned this
	  error condition.


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     5	EIO  I/O error
	  Some physical I/O error occurred during a read or
	  write.  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 that does
	  not exist, or beyond the limits of the device.  It may
	  also occur when, for example, an illegal tape drive
	  unit number is selected or a disk pack is not loaded on
	  a drive.

     7	E2BIG  Arg list too long
	  An argument list longer than 20480 bytes (or the
	  current limit, NCARGS in <sys/param.h>) is presented to
	  execve.

     8	ENOEXEC  Exec format error
	  A request is made to execute a file that, although it
	  has the appropriate permissions, does not start with a
	  valid magic number, (see a.out(5)).

     9	EBADF  Bad file number
	  Either a file descriptor refers to no open file, or a
	  read (resp. write) request is made to a file that is
	  open only for writing (resp. reading).

     10  ECHILD  No children
	  Wait and the process has no living or unwaited-for
	  children.

     11  EAGAIN  No more processes
	  In a fork, the system's process table is full or the
	  user is not allowed to create any more processes.

     12  ENOMEM  Not enough memory
	  During an execve or break, a program asks for more core
	  or swap space than the system is able to supply, or a
	  process size limit would be exceeded.  A lack of swap
	  space is normally a temporary condition; however, a
	  lack of core is not a temporary condition; the maximum
	  size of the text, data, and stack segments is a system
	  parameter.  Soft limits may be increased to their
	  corresponding hard limits.

     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 access the arguments of a system call.


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     15  ENOTBLK  Block device required
	  A plain file was mentioned where a block device was
	  required, e.g., in mount.

     16  EBUSY	Device busy
	  An attempt to mount a device that was already mounted
	  or an attempt was made to dismount a device on which
	  there is an active file (open file, current directory,
	  mounted-on file, or active text segment).  A request
	  was made to an exclusive access device that was already
	  in use.

     17  EEXIST  File exists
	  An existing file was mentioned in an inappropriate con-
	  text, e.g., link.

     18  EXDEV	Cross-device link
	  A hard 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, e.g., to read a write-only device, or
	  the device is not configured by the system.

     20  ENOTDIR  Not a directory
	  A non-directory was specified where a directory is
	  required, for example, in a path name or as an argument
	  to chdir.

     21  EISDIR  Is a directory
	  An attempt to write on a directory.

     22  EINVAL  Invalid argument
	  Some invalid argument: dismounting a non-mounted dev-
	  ice, mentioning an unknown signal in signal, or some
	  other argument inappropriate for the call.  Also set by
	  math functions, (see math(3)).

     23  ENFILE  File table overflow
	  The system's table of open files is full, and tem-
	  porarily no more opens can be accepted.

     24  EMFILE  Too many open files
	  As released, the limit on the number of open files per
	  process is 64.  Getdtablesize(2) will obtain the
	  current limit.  Customary configuration limit on most
	  other UNIX systems is 20 per process.

     25  ENOTTY  Inappropriate ioctl for device
	  The file mentioned in an ioctl is not a terminal or one
	  of the devices to which this call applies.


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     26  ETXTBSY  Text file busy
	  An attempt to execute a pure-procedure program that is
	  currently open for writing.  Also an attempt to open
	  for writing a pure-procedure program that is being exe-
	  cuted.

     27  EFBIG	File too large
	  The size of a file exceeded the maximum (about 2.1E9
	  bytes).

     28  ENOSPC  No space left on device
	  A write to an ordinary file, the creation of a direc-
	  tory or symbolic link, or the creation of a directory
	  entry failed because no more disk blocks are available
	  on the file system, or the allocation of an inode for a
	  newly created file failed because no more inodes are
	  available on the file system.

     29  ESPIPE  Illegal seek
	  An lseek was issued to a socket or pipe.  This error
	  may also be issued for other non-seekable devices.

     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 32767 hard links to a
	  file.

     32  EPIPE	Broken pipe
	  A write on a pipe or socket for which there is no pro-
	  cess to read the data.  This condition normally gen-
	  erates a signal; the error is returned if the signal is
	  caught or ignored.

     33  EDOM  Argument too large
	  The argument of a function in the math package (3M) is
	  out of the domain of the function.

     34  ERANGE  Result too large
	  The value of a function in the math package (3M) is
	  unrepresentable within machine precision.

     35  EWOULDBLOCK  Operation would block
	  An operation that would cause a process to block was
	  attempted on an object in non-blocking mode (see
	  fcntl(2)).

     36  EINPROGRESS  Operation now in progress
	  An operation that takes a long time to complete (such
	  as a connect(2)) was attempted on a non-blocking object


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	  (see fcntl(2)).

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

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

     39  EDESTADDRREQ  Destination address required
	  A required address was omitted from an operation on a
	  socket.

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

     41  EPROTOTYPE  Protocol wrong type for socket
	  A protocol was specified that does not support the
	  semantics of the socket type requested. For example,
	  you cannot use the ARPA Internet UDP protocol with type
	  SOCK_STREAM.

     42  ENOPROTOOPT  Option not supported by protocol
	  A bad option or level was specified in a getsockopt(2)
	  or setsockopt(2) call.

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

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

     45  EOPNOTSUPP  Operation not supported on socket
	  For example, trying to accept a connection on a
	  datagram socket.

     46  EPFNOSUPPORT  Protocol family not supported
	  The protocol family has not been configured into the
	  system or no implementation for it exists.

     47  EAFNOSUPPORT  Address family not supported by protocol
	  family
	  An address incompatible with the requested protocol was
	  used.  For example, you shouldn't necessarily expect to
	  be able to use NS addresses with ARPA Internet proto-
	  cols.

     48  EADDRINUSE  Address already in use
	  Only one usage of each address is normally permitted.


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     49  EADDRNOTAVAIL	Can't assign requested address
	  Normally results from an attempt to create a socket
	  with an address not on this machine.

     50  ENETDOWN  Network is down
	  A socket operation encountered a dead network.

     51  ENETUNREACH  Network is unreachable
	  A socket operation was attempted to an unreachable net-
	  work.

     52  ENETRESET  Network dropped connection on reset
	  The host you were connected to crashed and rebooted.

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

     54  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 socket due to a timeout or a reboot.

     55  ENOBUFS  No buffer space available
	  An operation on a socket or pipe was not performed
	  because the system lacked sufficient buffer space or
	  because a queue was full.

     56  EISCONN  Socket is already connected
	  A connect request was made on an already connected
	  socket; or, a sendto or sendmsg request on a connected
	  socket specified a destination when already connected.

     57  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.

     58  ESHUTDOWN  Can't send after socket shutdown
	  A request to send data was disallowed because the
	  socket had already been shut down with a previous shut-
	  down(2) call.

     59  unused

     60  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.)


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     61  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.

     62  ELOOP	Too many levels of symbolic links
	  A path name lookup involved more than 8 symbolic links.

     63  ENAMETOOLONG  File name too long
	  A component of a path name exceeded 255 (MAXNAMELEN)
	  characters, or an entire path name exceeded 1023
	  (MAXPATHLEN-1) characters.

     64  EHOSTDOWN  Host is down
	  A socket operation failed because the destination host
	  was down.

     65  EHOSTUNREACH  Host is unreachable
	  A socket operation was attempted to an unreachable
	  host.

     66  ENOTEMPTY  Directory not empty
	  A directory with entries other than "." and ".." was
	  supplied to a remove directory or rename call.

     69  EDQUOT  Disc quota exceeded
	  A write to an ordinary file, the creation of a direc-
	  tory or symbolic link, or the creation of a directory
	  entry failed because the user's quota of disk blocks
	  was exhausted, or the allocation of an inode for a
	  newly created file failed because the user's quota of
	  inodes was exhausted.

DEFINITIONS
     Process ID
	  Each active process in the system is uniquely identi-
	  fied by a positive integer called a process ID.  The
	  range of this ID is from 0 to 30000.

     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.

     Process Group ID
	  Each active process is a member of a process group that
	  is identified by a positive integer called the process
	  group ID.  This is the process ID of the group leader.
	  This grouping permits the signaling of related
	  processes (see killpg(2)) and the job control mechan-
	  isms of csh(1).


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     Tty Group ID
	  Each active process can be a member of a terminal group
	  that is identified by a positive integer called the tty
	  group ID.  This grouping is used to arbitrate between
	  multiple jobs contending for the same terminal; (see
	  csh(1) and tty(4)).

     Real User ID and Real Group ID
	  Each user on the system is identified by a positive
	  integer termed the real user ID.

	  Each user is also a member of one or more groups. One
	  of these groups is distinguished from others and used
	  in implementing accounting facilities.  The positive
	  integer corresponding to this distinguished group is
	  termed the real group ID.

	  All processes have a real user ID and real group ID.
	  These are initialized from the equivalent attributes of
	  the process that created it.

     Effective User Id, Effective Group Id, and Access Groups
	  Access to system resources is governed by three values:
	  the effective user ID, the effective group ID, and the
	  group access list.

	  The effective user ID and effective group ID are ini-
	  tially the process's real user ID and real group ID
	  respectively.  Either may be modified through execution
	  of a set-user-ID or set-group-ID file (possibly by one
	  its ancestors) (see execve(2)).

	  The group access list is an additional set of group
	  ID's used only in determining resource accessibility.
	  Access checks are performed as described below in
	  ``File Access Permissions''.

     Super-user
	  A process is recognized as a super-user process and is
	  granted special privileges if its effective user ID is
	  0.

     Special Processes
	  The processes with a process ID's of 0, 1, and 2 are
	  special.  Process 0 is the scheduler.  Process 1 is the
	  initialization process init, and is the ancestor of
	  every other process in the system.  It is used to con-
	  trol the process structure.  Process 2 is the paging
	  daemon.

     Descriptor
	  An integer assigned by the system when a file is


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	  referenced by open(2) or dup(2), or when a socket is
	  created by pipe(2), socket(2) or socketpair(2), which
	  uniquely identifies an access path to that file or
	  socket from a given process or any of its children.

     File Name
	  Names consisting of up to 255 (MAXNAMELEN) characters
	  may be used to name an ordinary file, special file, or
	  directory.

	  These characters may be selected from the set of all
	  ASCII character excluding 0 (null) and the ASCII code
	  for / (slash).  (The parity bit, bit 8, must be 0.)

	  Note that it is generally unwise to use *, ?, [ or ] as
	  part of file names because of the special meaning
	  attached to these characters by the shell.

     Path Name
	  A path name is a null-terminated character string
	  starting with an optional slash (/), followed by zero
	  or more directory names separated by slashes, option-
	  ally followed by a file name.  The total length of a
	  path name must be less than 1024 (MAXPATHLEN) charac-
	  ters.

	  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.  A null pathname
	  refers to the current directory.

     Directory
	  A directory is a special type of file that contains
	  entries that are references to other files.  Directory
	  entries are called links.  By convention, a directory
	  contains at least two links, . and .., referred to as
	  dot and dot-dot respectively.  Dot refers to the direc-
	  tory itself and dot-dot refers to its parent directory.

     Root Directory and Current Working Directory
	  Each process has associated with it a concept of a root
	  directory and a current working directory for the pur-
	  pose of resolving path name searches.  A process's root
	  directory need not be the root directory of the root
	  file system.

     File Access Permissions
	  Every file in the file system has a set of access per-
	  missions.  These permissions are used in determining
	  whether a process may perform a requested operation on
	  the file (such as opening a file for writing).  Access


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	  permissions are established at the time a file is
	  created.  They may be changed at some later time
	  through the chmod(2) call.

	  File access is broken down according to whether a file
	  may be: read, written, or executed.  Directory files
	  use the execute permission to control if the directory
	  may be searched.

	  File access permissions are interpreted by the system
	  as they apply to three different classes of users: the
	  owner of the file, those users in the file's group,
	  anyone else.	Every file has an independent set of
	  access permissions for each of these classes.  When an
	  access check is made, the system decides if permission
	  should be granted by checking the access information
	  applicable to the caller.

	  Read, write, and execute/search permissions on a file
	  are granted to a process if:

	  The process's effective user ID is that of the super-
	  user.

	  The process's effective user ID matches the user ID of
	  the owner of the file and the owner permissions allow
	  the access.

	  The process's effective user ID does not match the user
	  ID of the owner of the file, and either the process's
	  effective group ID matches the group ID of the file, or
	  the group ID of the file is in the process's group
	  access list, and the group permissions allow the
	  access.

	  Neither the effective user ID nor effective group ID
	  and group access list of the process match the
	  corresponding user ID and group ID of the file, but the
	  permissions for ``other users'' allow access.

	  Otherwise, permission is denied.

     Sockets and Address Families

	  A socket is an endpoint for communication between
	  processes.  Each socket has queues for sending and
	  receiving data.

	  Sockets are typed according to their communications
	  properties.  These properties include whether messages
	  sent and received at a socket require the name of the
	  partner, whether communication is reliable, the format


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	  used in naming message recipients, etc.

	  Each instance of the system supports some collection of
	  socket types; consult socket(2) for more information
	  about the types available and their properties.

	  Each instance of the system supports some number of
	  sets of communications protocols.  Each protocol set
	  supports addresses of a certain format.  An Address
	  Family is the set of addresses for a specific group of
	  protocols.  Each socket has an address chosen from the
	  address family in which the socket was created.

SEE ALSO
     intro(3), perror(3)


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