— introduction to system calls and error
The manual pages in section 2 provide an overview of the system
calls, their error returns, and other common definitions and concepts.
Programs may be restricted to a subset of system calls with
Nearly all of the system calls provide an error number via the
identifier errno, which expands to an addressable
location of type int. The address of
errno in each thread is guaranteed to be unique for
the lifetime of the thread. Applications must use
errno as defined in
<errno.h> and not attempt to
use a custom 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
The following is a complete list of the errors and their names as
- Not used.
1 EPERM Operation not
- 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
- A component of a specified pathname did not exist, or the pathname was an
3 ESRCH No such process.
- No process could be found which corresponds to the given process ID.
- An asynchronous signal (such as
SIGQUIT) was caught by the thread 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.
- 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 (overwritten) by any subsequent errors.
6 ENXIO Device not
- 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.
Argument list too
- The number of bytes used for the argument and environment list of the new
process exceeded the limit
- 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
- 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
10 ECHILD No child
- 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
- An attempt was made to lock a system resource that would have resulted in
a deadlock situation.
- 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.
- An attempt was made to access a file in a way forbidden by its file access
14 EFAULT Bad address.
- The system detected an invalid address in attempting to use an argument of
- 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)
- A hard link to a file on another file system was attempted.
19 ENODEV Operation not supported by
- 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.
Is a directory.
- An attempt was made to open a directory with write mode specified.
- Some invalid argument was supplied. (For example, specifying an undefined
signal to a signal(3) or
23 ENFILE Too many open files in
- Maximum number of file descriptors allowable on the system has been
reached and a request for an open cannot be satisfied until at least one
has been closed. The sysctl(2) variable
kern.maxfiles contains the current limit.
24 EMFILE Too many open
- The maximum number of file descriptors allowable for this process has been
reached and a request for an open cannot be satisfied until at least one
has been closed.
getdtablesize(3) will obtain the
25 ENOTTY Inappropriate ioctl for
- A control function (see ioctl(2)) was
attempted for a file or special device for which the operation was
Text file busy.
- An attempt was made either to execute a pure procedure (shared text) file
which was open for writing by another process, or to open with write
access a pure procedure file that is currently being executed.
27 EFBIG File too large.
- The size of a file exceeded the maximum. (The system-wide maximum file
size is 2**63 bytes. Each file system may impose a lower limit for files
contained within it.)
28 ENOSPC No space left on
- 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.
- An attempt was made to modify a file or create a directory on a file
system that was read-only at the time.
31 EMLINK Too many links.
- The maximum allowable number of hard links to a single file has been
exceeded (see pathconf(2) for how to
obtain this value).
- A write on a pipe, socket or FIFO for which there is no process to read
Numerical argument out
- A numerical input argument was outside the defined domain of the
- A result of the function was too large to fit in the available space
(perhaps exceeded precision).
35 EAGAIN Resource temporarily
- This is a temporary condition and later calls to the same routine may
36 EINPROGRESS Operation now in
- 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
- An operation was attempted on a non-blocking object that already had an
operation in progress.
38 ENOTSOCK Socket operation on
- A required address was omitted from an operation on a socket.
- A message sent on a socket was larger than the internal message buffer or
some other network limit.
41 EPROTOTYPE Protocol wrong type
- A protocol was specified that does not support the semantics of the socket
type requested. For example, you cannot use the Internet UDP protocol with
42 ENOPROTOOPT Protocol not
- A bad option or level was specified in a
43 EPROTONOSUPPORT Protocol not
- The protocol has not been configured into the system or no implementation
for it exists.
44 ESOCKTNOSUPPORT Socket type not
- The support for the socket type has not been configured into the system or
no implementation for it exists.
45 EOPNOTSUPP Operation
- 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
connection on a datagram socket.
46 EPFNOSUPPORT Protocol family not
- The protocol family has not been configured into the system or no
implementation for it exists.
47 EAFNOSUPPORT Address family not
supported by protocol family.
- An address incompatible with the requested protocol was used. For example,
you shouldn't necessarily expect to be able to use NS addresses with
48 EADDRINUSE Address already in
- Only one usage of each address is normally permitted.
49 EADDRNOTAVAIL Can't assign
- Normally results from an attempt to create a socket with an address not on
50 ENETDOWN Network is
- A socket operation encountered a dead network.
51 ENETUNREACH Network is
- 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.
- A connection abort was caused internal to your host machine.
54 ECONNRESET Connection reset by
- 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
55 ENOBUFS No buffer space
- 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
- A connect(2) request was made on an
already connected socket; or, a
sendmsg(2) request on a connected
socket specified a destination when already connected.
57 ENOTCONN Socket is not
- A request to send or receive data was disallowed because the socket was
not connected and (when sending on a datagram socket) no address was
58 ESHUTDOWN Can't send after socket
- A request to send data was disallowed because the socket had already been
shut down with a previous shutdown(2)
59 ETOOMANYREFS Too many references:
- Not used in OpenBSD.
60 ETIMEDOUT Operation timed
- 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
- 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
- A pathname lookup involved more than 32
SYMLOOP_MAX) symbolic links.
63 ENAMETOOLONG File name too
- A component of a pathname exceeded 255 (
characters, or an entire pathname (including the terminating NUL) exceeded
Host is down.
- A socket operation failed because the destination host was down.
65 EHOSTUNREACH No route to
- A socket operation was attempted to an unreachable host.
- A directory with entries other than
..’ was supplied to a remove
directory or rename call.
67 EPROCLIM Too many
68 EUSERS Too many users.
- The quota system ran out of table entries.
- 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.
Stale NFS file
- An attempt was made to access an open file on an NFS filesystem which is
now unavailable as referenced by the file descriptor. This may indicate
the file was deleted on the NFS server or some other catastrophic event
72 EBADRPC RPC struct is
- Exchange of rpc(3) information was
73 ERPCMISMATCH RPC version
- The version of rpc(3) on the remote peer
is not compatible with the local version.
74 EPROGUNAVAIL RPC program not
- The requested rpc(3) program is not
registered on the remote host.
- The requested version of the rpc(3)
program is not available on the remote host.
76 EPROCUNAVAIL Bad procedure for
- An rpc(3) call was attempted for a
procedure which doesn't exist in the remote program.
77 ENOLCK No locks
- A system-imposed limit on the number of simultaneous file locks was
- Attempted a system call that is not available on this system.
79 EFTYPE Inappropriate file type or
- The file contains invalid data or set to invalid modes.
- Attempted to use an invalid authentication ticket to mount a NFS
- An authentication ticket must be obtained before the given NFS filesystem
may be mounted.
- IPsec subsystem error. Not used in OpenBSD.
- A UFS Extended Attribute is not found for the specified pathname.
84 EILSEQ Illegal byte
- An illegal sequence of bytes was used when using wide characters.
85 ENOMEDIUM No medium
- Attempted to use a removable media device with no medium present.
- Attempted to use a removable media device with incorrect or incompatible
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.
88 ECANCELED Operation
- The requested operation was canceled.
- An IPC identifier was removed while the current thread was waiting on
90 ENOMSG No message of desired
- An IPC message queue does not contain a message of the desired type, or a
message catalog does not contain the requested message.
91 ENOTSUP Not supported.
- The operation has requested an unsupported value.
92 EBADMSG Bad message.
- A corrupted message was detected.
- The state protected by a robust mutex is not recoverable.
- The owner of a robust mutex terminated while holding the mutex lock.
95 EPROTO Protocol error.
- A device-specific protocol error occurred.
- A process is a collection of one or more threads, plus the resources
shared by those threads such as process ID, address space, user IDs and
group IDs, and root directory and current working directory.
- 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 ksh(1) and
- 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
- 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;
- 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.
- A thread is a preemptively scheduled flow of control within a process,
with its own set of register values, floating point environment, thread
ID, signal mask, pending signal set, alternate signal stack, thread
control block address, resource utilization, errno variable location, and
values for thread-specific keys. A process initially has just one thread,
a duplicate of the thread in the parent process that created this
- 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
- 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 of 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
- A process is recognized as a
process and is granted special privileges if its effective user ID is
- Special Processes
- The processes with process IDs of 0 and 1 are special. Process 0 is the
scheduler. Process 1 is the initialization process
init(8), and is the ancestor of every
other process in the system. It is used to control the process
- An integer assigned by the system when a file is referenced by
dup(2), or when a socket is created by
socketpair(2), which uniquely
identifies an access path to that file or socket from a given process or
any of its children.
- File Name
- Names consisting of up to 255 (
characters may be used to name an ordinary file, special file, or
These characters may be arbitrary eight-bit values, excluding
0 (NUL) and the ASCII code for ‘
Note that it is generally unwise to use
]’ as part of file names because
of the special meaning attached to these characters by the shell.
Note also that
NAME_MAX is an upper
limit fixed by the kernel, meant to be used for sizing buffers. Some
filesystems may have additional restrictions. These can be queried using
- A pathname is a NUL-terminated character string starting with an optional
/’, followed by zero or more
directory names separated by slashes, optionally followed by a file name.
The total length of a pathname must be less than 1024
PATH_MAX) characters. Additional restrictions may
apply, depending upon the filesystem, to be queried with
fpathconf(2) if needed.
If a pathname begins with a slash, the path search
begins at the
Otherwise, the search begins from the current working directory. A slash
by itself names the root directory. An empty pathname is invalid.
- 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,
..’, referred to as
respectively. Dot refers to the directory itself and dot-dot refers to its
- 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
- 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
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 superuser.
(Note: even the superuser 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
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
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 manual for section 2 first
appeared in Version 5 AT&T UNIX.