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INTRO(2)			     BSD System Calls Manual				 INTRO(2)

NAME
     intro -- introduction to system calls and error numbers

SYNOPSIS
     #include <sys/errno.h>

DESCRIPTION
     This section provides an overview of the system calls, their error returns, and other common
     definitions and concepts.

DIAGNOSTICS
     Nearly all of the system calls provide an error number in the external variable errno, which
     is defined as:

	   extern int errno

     When a system call detects an error, it returns an integer value indicating failure (usually
     -1) and sets the variable errno accordingly.  <This allows interpretation of the failure on
     receiving a -1 and to take action accordingly.> Successful calls never set errno; once set,
     it remains until another error occurs.  It should only be examined after an error.  Note
     that a number of system calls overload the meanings of these error numbers, and that the
     meanings must be interpreted according to the type and circumstances of the call.

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

     0 Error 0.  Not used.

     1 EPERM Operation not permitted.  An attempt was made to perform an operation limited to
	     processes with appropriate privileges or to the owner of a file or other resources.

     2 ENOENT No such file or directory.  A component of a specified pathname did not exist, or
	     the pathname was an empty string.

     3 ESRCH No such process.  No process could be found corresponding to that specified by the
	     given process ID.

     4 EINTR Interrupted function call.  An asynchronous signal (such as SIGINT or SIGQUIT) was
	     caught by the process during the execution of an interruptible function. If the sig-
	     nal handler performs a normal return, the interrupted function call will seem to
	     have returned the error condition.

     5 EIO Input/output error.	Some physical input or output error occurred.  This error will
	     not be reported until a subsequent operation on the same file descriptor and may be
	     lost (over written) by any subsequent errors.

     6 ENXIO No such device or address.  Input or output on a special file referred to a device
	     that did not exist, or made a request beyond the limits of the device.  This error
	     may also occur when, for example, a tape drive is not online or no disk pack is
	     loaded on a drive.

     7 E2BIG Arg list too long.  The number of bytes used for the argument and environment list
	     of the new process exceeded the limit NCARGS (specified in <sys/param.h>).

     8 ENOEXEC Exec format error.  A request was made to execute a file that, although it has the
	     appropriate permissions, was not in the format required for an executable file.

     9 EBADF Bad file descriptor.  A file descriptor argument was out of range, referred to no
	     open file, or a read (write) request was made to a file that was only open for writ-
	     ing (reading).

     10 ECHILD No child processes.  A wait or waitpid function was executed by a process that had
	     no existing or unwaited-for child processes.

     11 EDEADLK Resource deadlock avoided.  An attempt was made to lock a system resource that
	     would have resulted in a deadlock situation.

     12 ENOMEM Cannot allocate memory.	The new process image required more memory than was
	     allowed by the hardware or by system-imposed memory management constraints.  A lack
	     of swap space is normally temporary; however, a lack of core is not.  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 its
	     file access permissions.

     14 EFAULT Bad address.  The system detected an invalid address in attempting to use an argu-
	     ment of a call.

     15 ENOTBLK Not a block device.  A block device operation was attempted on a non-block device
	     or file.

     16 EBUSY Resource busy.  An attempt to use a system resource which was in use at the time in
	     a manner which would have conflicted with the request.

     17 EEXIST File exists.  An existing file was mentioned in an inappropriate context, for
	     instance, as the new link name in a link function.

     18 EXDEV Improper link.  A hard link to a file on another file system was attempted.

     19 ENODEV Operation not supported by device.  An attempt was made to apply an inappropriate
	     function to a device, for example, trying to read a write-only device such as a
	     printer.

     20 ENOTDIR Not a directory.  A component of the specified pathname existed, but it was not a
	     directory, when a directory was expected.

     21 EISDIR Is a directory.	An attempt was made to open a directory with write mode speci-
	     fied.

     22 EINVAL Invalid argument.  Some invalid argument was supplied. (For example, specifying an
	     undefined signal to a signal or kill function).

     23 ENFILE Too many open files in system.  Maximum number of file descriptors allowable on
	     the system has been reached and a requests for an open cannot be satisfied until at
	     least one has been closed.

     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.

     25 ENOTTY Inappropriate ioctl for device.	A control function (see ioctl(2)) was attempted
	     for a file or special device for which the operation was inappropriate.

     26 ETXTBSY Text file busy.  The new process was a pure procedure (shared text) file which
	     was open for writing by another process, or while the pure procedure file was being
	     executed an open call requested write access.

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

     28 ENOSPC Device out of space.  A write to an ordinary file, the creation of a directory or
	     symbolic link, or the creation of a directory entry failed because no more disk
	     blocks were available on the file system, or the allocation of an inode for a newly
	     created file failed because no more inodes were available on the file system.

     29 ESPIPE Illegal seek.  An lseek function was issued on a socket, pipe or FIFO.

     30 EROFS Read-only file system.  An attempt was made to modify a file or directory was made
	     on a file system that was read-only at the time.

     31 EMLINK Too many links.	Maximum allowable hard links to a single file has been exceeded
	     (limit of 32767 hard links per file).

     32 EPIPE Broken pipe.  A write on a pipe, socket or FIFO for which there is no process to
	     read the data.

     33 EDOM Numerical argument out of domain.	A numerical input argument was outside the
	     defined domain of the mathematical function.

     34 ERANGE Numerical result out of range.  A numerical result of the function was too large
	     to fit in the available space (perhaps exceeded precision).

     35 EAGAIN Resource temporarily unavailable.  This is a temporary condition and later calls
	     to the same routine may complete normally.

     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 (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 opera-
	     tion on a socket.

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

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

     42 ENOPROTOOPT Protocol not available.  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.  The attempted operation is not supported for the
	     type of object referenced.  Usually this occurs when a file descriptor refers to a
	     file or socket that cannot support this operation, 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 protocols.

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

     49 EADDRNOTAVAIL Cannot assign requested address.	Normally results from an attempt to cre-
	     ate 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
	     network.

     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
	     normally 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 destina-
	     tion when already connected.

     57 ENOTCONN Socket is not connected.  An request to send or receive data was disallowed
	     because the socket was not connected and (when sending on a datagram socket) no
	     address was supplied.

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

     60 ETIMEDOUT Operation 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.)

     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 sym-
	     bolic 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 No route to host.	A socket operation was attempted to an unreachable host.

     66 ENOTEMPTY Directory not empty.	A directory with entries other than '.' and '..' was sup-
	     plied to a remove directory or rename call.

     67 EPROCLIM Too many processes.

     68 EUSERS Too many users.	The quota system ran out of table entries.

     69 EDQUOT Disc quota exceeded.  A write to an ordinary file, the creation of a directory 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.

     70 ESTALE Stale NFS file handle.  An attempt was made to access an open file (on an NFS
	     filesystem) which is now unavailable as referenced by the file descriptor.  This may
	     indicate the file was deleted on the NFS server or some other catastrophic event
	     occurred.

     72 EBADRPC RPC struct is bad.  Exchange of RPC information was unsuccessful.

     73 ERPCMISMATCH RPC version wrong.  The version of RPC on the remote peer is not compatible
	     with the local version.

     74 EPROGUNAVAIL RPC prog. not avail.  The requested program is not registered on the remote
	     host.

     75 EPROGMISMATCH Program version wrong.  The requested version of the program is not avail-
	     able on the remote host (RPC).

     76 EPROCUNAVAIL Bad procedure for program.  An RPC call was attempted for a procedure which
	     doesn't exist in the remote program.

     77 ENOLCK No locks available.  A system-imposed limit on the number of simultaneous file
	     locks was reached.

     78 ENOSYS Function not implemented.  Attempted a system call that is not available on this
	     system.

     79 EFTYPE Inappropriate file type or format.  The file was the wrong type for the operation,
	     or a data file had the wrong format.

     80 EAUTH Authentication error.  Attempted to use an invalid authentication ticket to mount
	     an NFS file system.

     81 ENEEDAUTH Need authenticator.  An authentication ticket must be obtained before the given
	     NFS file system may be mounted.

     82 EPWROFF Device power is off.  The device power is off.

     83 EDEVERR Device error.  A device error has occurred, e.g. a printer running out of paper.

     84 EOVERFLOW Value too large to be stored in data type.  A numerical result of the function
	     was too large to be stored in the caller provided space.

     85 EBADEXEC Bad executable (or shared library).  The executable or shared library being ref-
	     erenced was malformed.

     86 EBADARCH Bad CPU type in executable.  The executable in question does not support the
	     current CPU.

     87 ESHLIBVERS Shared library version mismatch.  The version of the shared library on the
	     system does not match the version which was expected.

     88 EBADMACHO Malformed Mach-o file.  The Mach object file is malformed.

     89 ECANCELED Operation canceled.  The scheduled operation was canceled.

     90 EIDRM Identifier removed.  An IPC identifier was removed while the current process was
	     waiting on it.

     91 ENOMSG No message of desired type.  An IPC message queue does not contain a message of
	     the desired type, or a message catalog does not contain the requested message.

     92 EILSEQ Illegal byte sequence.  While decoding a multibyte character the function came
	     along an invalid or an incomplete sequence of bytes or the given wide character is
	     invalid.

     93 ENOATTR Attribute not found.  The specified extended attribute does not exist.

DEFINITIONS
     Process ID.
	     Each active process in the system is uniquely identified by a non-negative integer
	     called a process ID.  The range of this ID is from 0 to 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 initially the process ID of its creator.  If the creating
	     process exits, the parent process ID of each child is set to the ID of a system
	     process, init.

     Process Group
	     Each active process is a member of a process group that is identified by a non-nega-
	     tive integer called the process group ID.	This is the process ID of the group
	     leader.  This grouping permits the signaling of related processes (see termios(4))
	     and the job control mechanisms of csh(1).

     Session
	     A session is a set of one or more process groups.	A session is created by a suc-
	     cessful call to setsid(2), which causes the caller to become the only member of the
	     only process group in the new session.

     Session leader
	     A process that has created a new session by a successful call to setsid(2), is known
	     as a session leader.  Only a session leader may acquire a terminal as its control-
	     ling terminal (see termios(4)).

     Controlling process
	     A session leader with a controlling terminal is a controlling process.

     Controlling terminal
	     A terminal that is associated with a session is known as the controlling terminal
	     for that session and its members.

     Terminal Process Group ID
	     A terminal may be acquired by a session leader as its controlling terminal.  Once a
	     terminal is associated with a session, any of the process groups within the session
	     may be placed into the foreground by setting the terminal process group ID to the ID
	     of the process group.  This facility is used to arbitrate between multiple jobs con-
	     tending for the same terminal; (see csh(1) and tty(4)).

     Orphaned Process Group
	     A process group is considered to be orphaned if it is not under the control of a job
	     control shell.  More precisely, a process group is orphaned when none of its members
	     has a parent process that is in the same session as the group, but is in a different
	     process group.  Note that when a process exits, the parent process for its children
	     is changed to be init, which is in a separate session.  Not all members of an
	     orphaned process group are necessarily orphaned processes (those whose creating
	     process has exited).  The process group of a session leader is orphaned by defini-
	     tion.

     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 distin-
	     guished from others and used in implementing accounting facilities.  The positive
	     integer corresponding to this distinguished group is termed the real group ID.

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

     Effective User Id, Effective Group Id, and Group Access List
	     Access to system resources is governed by two values: the effective user ID, and the
	     group access list.  The first member of the group access list is also known as the
	     effective group ID.  (In POSIX.1, the group access list is known as the set of sup-
	     plementary group IDs, and it is unspecified whether the effective group ID is a mem-
	     ber of the list.)

	     The effective user ID and effective group ID are initially the process's real user
	     ID and real group ID respectively.  Either may be modified through execution of a
	     set-user-ID or set-group-ID file (possibly by one its ancestors) (see execve(2)).
	     By convention, the effective group ID (the first member of the group access list) is
	     duplicated, so that the execution of a set-group-ID program does not result in the
	     loss of the original (real) group ID.

	     The group access list is a set of group IDs used only in determining resource acces-
	     sibility.	Access checks are performed as described below in ``File Access Permis-
	     sions''.

     Saved Set User ID and Saved Set Group ID
	     When a process executes a new file, the effective user ID is set to the owner of the
	     file if the file is set-user-ID, and the effective group ID (first element of the
	     group access list) is set to the group of the file if the file is set-group-ID.  The
	     effective user ID of the process is then recorded as the saved set-user-ID, and the
	     effective group ID of the process is recorded as the saved set-group-ID.  These val-
	     ues may be used to regain those values as the effective user or group ID after
	     reverting to the real ID (see setuid(2)).	(In POSIX.1, the saved set-user-ID and
	     saved set-group-ID are optional, and are used in setuid and setgid, but this does
	     not work as desired for the super-user.)

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

     Special Processes
	     The processes with process IDs of 0, 1, and 2 are special.  Process 0 is the sched-
	     uler.  Process 1 is the initialization process init, and is the ancestor of every
	     other process in the system.  It is used to control the process structure.  Process
	     2 is the paging daemon.

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

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

	     These characters may be selected from the set of all ASCII character excluding 0
	     (NUL) 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.

     Path Name
	     A path name is a NUL-terminated character string starting with an optional slash
	     '/', followed by zero or more directory names separated by slashes, optionally fol-
	     lowed by a file name.  The total length of a path name must be less than 1024
	     (MAXPATHLEN) characters.

	     If a path name begins with a slash, the path search begins at the root directory.
	     Otherwise, the search begins from the current working directory.  A slash by itself
	     names the root directory.	An empty pathname refers to the current directory.

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

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

     File Access Permissions
	     Every file in the file system has a set of access permissions.  These permissions
	     are used in determining whether a process may perform a requested operation on the
	     file (such as opening a file for writing).  Access permissions are established at
	     the time a file is created.  They may be changed at some later time through the
	     chmod(2) call.

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

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

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

	     The process's effective user ID is that of the super-user. (Note: even the super-
	     user cannot execute a non-executable file.)

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

	     The process's effective user ID does not match the user ID of the owner of the file,
	     and either the process's effective group ID matches the group ID of the file, or the
	     group ID of the file is in the process's group access list, and the group permis-
	     sions 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 permis-
	     sions 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 part-
	     ner, whether communication is reliable, the format used in naming message recipi-
	     ents, 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 proto-
	     cols.  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
     perror(3)

HISTORY
     An intro manual page appeared in Version 6 AT&T UNIX.

4th Berkeley Distribution		December 11, 1993		4th Berkeley Distribution
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