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

NAME
     intro — introduction to system calls and error numbers

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <errno.h>

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

RETURN VALUES
     Nearly all of the system calls provide an error number referenced via the
     external identifier errno.	 This identifier is defined in <sys/errno.h>
     as

	   extern int * __error();
	   #define errno (* __error())

     The __error() function returns a pointer to a field in the thread spe‐
     cific structure for threads other than the initial thread.	 For the ini‐
     tial thread and non-threaded processes, __error() returns a pointer to a
     global errno variable that is compatible with the previous definition.

     When a system call detects an error, it returns an integer value indicat‐
     ing 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 Undefined error: 0.  Not used.

     1 EPERM Operation not permitted.  An attempt was made to perform an oper‐
	     ation 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 system call.  An asynchronous signal (such as SIGINT
	     or SIGQUIT) was caught by the process during the execution of an
	     interruptible function.  If the signal handler performs a normal
	     return, the interrupted system 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 Device not configured.  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 Argument list too long.  The number of bytes used for the argu‐
	     ment and environment list of the new process exceeded the current
	     limit (NCARGS 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 for‐
	     mat 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 writing (reading).

     10 ECHILD No child processes.  A wait(2) or waitpid(2) function was exe‐
	     cuted by a process that had no existing or unwaited-for child
	     processes.

     11 EDEADLK Resource deadlock avoided.  An attempt was made to lock a sys‐
	     tem 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 mem‐
	     ory 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 argument of a call.

     15 ENOTBLK Block device required.	A block device operation was attempted
	     on a non-block device or file.

     16 EBUSY Device 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 inappropri‐
	     ate context, for instance, as the new link name in a link(2) sys‐
	     tem call.

     18 EXDEV Cross-device 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 specified.

     22 EINVAL Invalid argument.  Some invalid argument was supplied.  (For
	     example, specifying an undefined signal to a signal(3) function
	     or a kill(2) system call).

     23 ENFILE Too many open files in system.  Maximum number of file descrip‐
	     tors 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.)  The getdtablesize(2) system call
	     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(2) call
	     requested write access.

     27 EFBIG File too large.  The size of a file exceeded the maximum.

     28 ENOSPC No space left on device.	 A write(2) 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(2) system call was issued on a socket,
	     pipe or FIFO.

     30 EROFS Read-only file system.  An attempt was made to modify a file or
	     directory 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 Result too large.  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 condi‐
	     tion 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 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 Protocol not available.  A bad option or level was speci‐
	     fied 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 connec‐
	     tion 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 should not 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 create a socket with an address not on this
	     machine.

     50 ENETDOWN Network is down.  A socket operation encountered a dead net‐
	     work.

     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 con‐
	     nected 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(2) request was made on
	     an already connected socket; or, a sendto(2) or sendmsg(2)
	     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 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(2) or send(2) request failed
	     because the connected party did not properly respond after a
	     period of time.  (The timeout period is dependent on the communi‐
	     cation 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 for‐
	     eign host.

     62 ELOOP Too many levels of symbolic links.  A path name lookup involved
	     more than 32 (MAXSYMLINKS) symbolic links.

     63 ENAMETOOLONG File name too long.  A component of a path name exceeded
	     {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX}
	     characters.  (See also the description of _PC_NO_TRUNC in
	     pathconf(2).)

     64 EHOSTDOWN Host is down.	 A socket operation failed because the desti‐
	     nation 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 supplied 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(2) to an ordinary file, the cre‐
	     ation 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 file system) which is now unavailable as refer‐
	     enced 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 unsuccess‐
	     ful.

     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 regis‐
	     tered on the remote host.

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

     76 EPROCUNAVAIL Bad procedure for program.	 An RPC call was attempted for
	     a procedure which does not 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 authentica‐
	     tion ticket to mount a NFS file system.

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

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

     83 ENOMSG No message of desired type.  An IPC message queue does not con‐
	     tain a message of the desired type, or a message catalog does not
	     contain the requested message.

     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 ECANCELED Operation canceled.  The scheduled operation was canceled.

     86 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.

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

     88 EDOOFUS Programming error.  A function or API is being abused in a way
	     which could only be detected at run-time.

     93 ENOTCAPABLE Capabilities insufficient.	An operation on a capability
	     file descriptor requires greater privilege than the capability
	     allows.

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 99999.

     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(8).

     Process Group
	     Each active process is a member of a process group that is iden‐
	     tified by a non-negative integer called the process group ID.
	     This is the process ID of the group leader.  This grouping per‐
	     mits 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 successful call to setsid(2), which causes the call‐
	     er 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 controlling 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 con‐
	     trolling 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 contending 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(8), 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 definition.

     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 Group Access List
	     Access to system resources is governed by two values: the effec‐
	     tive 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 supplemen‐
	     tary group IDs, and it is unspecified whether the effective group
	     ID is a member 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 con‐
	     vention, 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 deter‐
	     mining resource accessibility.  Access checks are performed as
	     described below in ``File Access Permissions''.

     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 values 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.

     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 {NAME_MAX} characters may be used to
	     name an ordinary file, special file, or directory.

	     These characters may be arbitrary eight-bit values, excluding NUL
	     (ASCII 0) and the ‘/’ character (slash, ASCII 47).

	     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 sep‐
	     arated by slashes, optionally followed by a file name.  The total
	     length of a path name must be less than {PATH_MAX} characters.
	     (On some systems, this limit may be infinite.)

	     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 contains 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 per‐
	     mission 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.
	     (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 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 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 com‐
	     munications 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)

BSD			       February 27, 1995			   BSD
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