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     intro — introduction to system calls and error numbers


     Standard C Library (libc, -lc)


     #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 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

     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

     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

     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

     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

     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

     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

     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

     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

     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

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


     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 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).

             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 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

     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

             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

     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.)

             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 {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.

             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)