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

     intro, errno -- introduction to system calls and error numbers

     #include <errno.h>

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

     Nearly all of the system calls provide an error number in the external
     variable 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 has been reported, because otherwise a leftover value from some
     previous error may be found instead.  (Many library functions that are
     not system calls also set errno on return, in the same fashion.  In these
     cases a nonzero value may be left in errno even upon successful return if
     some internal action failed.)

     The manual page for each system call will list some of the common errno
     codes that system call can return, but that should not be considered an
     exhaustive list, i.e.  a properly written program should be able to
     gracefully recover from any error that a system call might return.
     Documenting all the error codes that a system call can return in a more
     specification-like manner would take more resources than this project has

     Note also that a number of system calls overload the meanings of these
     error numbers, and that in these cases the meanings must be interpreted
     according to the type and circumstances of the call.

     The following is a list of the errors and their names as given in

     0 OK Error 0.  Not used.  (The symbol "OK" is only used inside the kernel

     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.  It Er 4 EINTR Em "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 signal 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 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 Arg list too long.  The number of bytes used for the argument and
             environment list of the new process exceeded the current limit of
             262144 bytes (ARG_MAX in <limits.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(2) (or write(2))
             request was made to a file that was only open for writing (or

     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

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

     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.  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.  The reliable detection
             of this error cannot be guaranteed and when not detected may
             result in the generation of a signal, indicating an address
             violation, which is sent to the process.

     15 ENOTBLK Block device required.  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

     17 EEXIST File exists.  An existing file was mentioned in an
             inappropriate context, for instance, as the new link name in a
             link(2) function.

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

     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

     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) or kill(2)

     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.> The getrlimit(2) call with the
             RLIMIT_NOFILE resource 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.  Attempt to execute a program
             that is open for writing.  Obsolete under MINIX 3.

     27 EFBIG File too large.  The size of a file exceeded the maximum.  (The
             system-wide maximum file size is 2147483648 (2GB) bytes.  Each
             file system may impose a lower limit for files contained within

     28 ENOSPC Device out of space.  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) 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

     31 EMLINK Too many links.  The number of hard links to a single file has
             exceeded the maximum.  (The system-wide maximum number of hard
             links is 32767.  Each file system may impose a lower limit for
             files contained within it).

     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 or too small.  The result of the function is
             too large or too small to be represented in the available space.

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

     36 ENAMETOOLONG File name too long.  A component of a path name exceeded
             (NAME_MAX) characters, or an entire path name exceeded 255
             (PATH_MAX) characters.

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

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

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

     40 ELOOP Too many levels of symbolic links.  A path name lookup involved
             more than 16 (SYMLOOP_MAX) symbolic links.

     41 ERESTART Service restarted.

     43 ERESTART Identifier removed.  An IPC identifier was removed while the
             current process was waiting on it.

     44 EILSEQ Illegal byte sequence.  A wide character/multibyte character
             encoding error occurred.

     45 EFTYPE Inappropriate file type or format.  Attempted a file operation
             on a file of a type for which it was invalid.

     46 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.

     50 EPACKSIZE Invalid packet size.

     51 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.

     53 EBADMODE Bad mode in ioctl.

     54 EWOULDBLOCK Would block.

     55 ENETUNREACH Network is unreachable.  A socket operation was attempted
             to an unreachable network.

     56 EHOSTUNREACH No route to host.  A socket operation was attempted to an
             unreachable host.

     57 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.

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

     59 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.

     60 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.

     61 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).

     62 EURG Urgent data present.

     63 ENOURG No urgent data present.

     64 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.

     65 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.

     66 ENOCONN No such connection.

     67 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.

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

     69 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.

     70 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)).

     71 EADDRNOTAVAIL Cannot assign requested address.  Normally results from
             an attempt to create a socket with an address not on this

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

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

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

     75 ENOPROTOOPT Protocol option not available.  A bad option or level was
             specified in a getsockopt(2) or setsockopt(2) call.

     76 EOPNOTSUPP Operation not supported (has alias ENOTSUP).  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.

     77 ENETDOWN Network is down.  A socket operation encountered a dead

     78 EPFNOSUPPORT Protocol family not supported.  A socket operation
             specified an unsupported protocol family.

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

     80 EHOSTDOWN Host is down.  The destination host has been determined to
             be down or disconnected.

     Process ID
             Each active process in the system is uniquely identified by a
             positive integer called a process ID.  The range of this ID is
             from 1 to 29999.

     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 init,

     Process Group
             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 termios(4) ).

             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

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

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

     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.  (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

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

             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

     File Name
             Names consisting of up to 60 (NAME_MAX) characters may be used to
             name an ordinary file, special file, or directory.

             These characters may be selected from the set of all ASCII
             character excluding 0 (NUL) and the ASCII code for '/' (slash).
             (The parity bit, bit 7, 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.

             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 255 (PATH_MAX)

             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 string is not a valid pathname.

             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

     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.

     intro(3), perror(3)

BSD                              July 14, 2010                             BSD