intro

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 specific
     structure for threads other than the initial thread.  For the initial
     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 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 Undefined 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 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 argument
             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 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 writing (reading).

     10 ECHILD No child processes.  A wait(2) or waitpid(2) 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 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 inappropriate
             context, for instance, as the new link name in a link(2) system
             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 open files
             allowable on the system has been reached and requests for an open
             cannot be satisfied until at least one has been closed.

     24 EMFILE Too many open files.  Maximum number of file descriptors
             allowable in the process has been reached and requests for an open
             cannot be satisfied until at least one has been closed.  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 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 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 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 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 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 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(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 Can't 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 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 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
             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 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
             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 file system) 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 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 authentication
             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
             contain 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.

     89 EBADMSG Bad message.  A corrupted message was detected.

     90 EMULTIHOP Multihop attempted.  This error code is unused, but present
             for compatibility with other systems.

     91 ENOLINK Link has been severed.  This error code is unused, but present
             for compatibility with other systems.

     92 EPROTO Protocol error.  A device or socket encountered an unrecoverable
             protocol error.

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

     94 ECAPMODE Not permitted in capability mode.  The system call or operation
             is not permitted for capability mode processes.

     95 ENOTRECOVERABLE State not recoverable.  The state protected by a robust
             mutex is not recoverable.

     96 EOWNERDEAD Previous owner died.  The owner of a robust mutex terminated
             while holding the mutex lock.

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 the calling
             process's reaper (see procctl(2)), normally init(8).

     Process Group
             Each active process is a member of a process group that is
             identified by a non-negative 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 successful 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 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
             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
             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 normally 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 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 supplementary 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 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 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
             separated 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
             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.  (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
             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)

BSD                             September 8, 2016                            BSD