introduction to system calls and error numbers
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
, which expands to an addressable
location of type int
. The address of
in each thread is guaranteed to be
unique for the lifetime of the thread. Applications must use
as defined in
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
accordingly. (This allows
interpretation of the failure on receiving a -1 and to take action
accordingly.) Successful calls never set
; 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 given in
- Not used.
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
3 ESRCH No
- No process could be found which corresponds to the given process ID.
Interrupted system call.
- 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
- 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 long.
- The number of bytes used for the argument and environment list of the new
process exceeded the limit
8 ENOEXEC Exec
- 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
- 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
- A wait(2) or
waitpid(2) function was
executed by a process that had no existing or unwaited-for child
Resource deadlock avoided.
- An attempt was made to lock a system resource that would have resulted in
a deadlock situation.
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.
- An attempt was made to access a file in a way forbidden by its file access
14 EFAULT Bad
- The system detected an invalid address in attempting to use an argument of
Block device required.
- A block device operation was attempted on a non-block device or file.
- 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
- An existing file was mentioned in an inappropriate context, for instance,
as the new link name in a
- A hard link to a file on another file system was attempted.
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 component of the specified pathname existed, but it was not a directory,
when a directory was expected.
21 EISDIR Is a
- 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 system.
- 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
kern.maxfiles contains the current
24 EMFILE Too
many open files.
- 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.
obtain the current limit.
Inappropriate ioctl for device.
- A control function (see
ioctl(2)) was attempted for a
file or special device for which the operation was inappropriate.
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
- 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 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.
- An lseek(2) function was
issued on a socket, pipe or FIFO.
Read-only file system.
- 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
- 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 of domain.
- A numerical input argument was outside the defined domain of the
Result too large.
- A result of the function was too large to fit in the available space
(perhaps exceeded precision).
Resource temporarily unavailable.
- This is a temporary condition and later calls to the same routine may
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
Operation already in progress.
- An operation was attempted on a non-blocking object that already had an
operation in progress.
Socket operation on non-socket.
Destination address required.
- A required address was omitted from an operation on a socket.
Message too long.
- A message sent on a socket was larger than the internal message buffer or
some other network limit.
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 Internet UDP protocol with
Protocol not available.
- A bad option or level was specified in a
Protocol not supported.
- The protocol has not been configured into the system or no implementation
for it exists.
Socket type not supported.
- The support for the socket type has not been configured into the system or
no implementation for it exists.
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
Protocol family not supported.
- The protocol family has not been configured into the system or no
implementation for it exists.
Address family not supported by protocol
- An address incompatible with the requested protocol was used. For example,
you shouldn't necessarily expect to be able to use NS addresses with
Address already in use.
- Only one usage of each address is normally permitted.
Can't assign requested address.
- Normally results from an attempt to create a socket with an address not on
Network is down.
- A socket operation encountered a dead network.
Network is unreachable.
- A socket operation was attempted to an unreachable network.
Network dropped connection on reset.
- The host you were connected to crashed and rebooted.
Software caused connection abort.
- A connection abort was caused internal to your host machine.
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
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.
Socket is already connected.
- 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.
Socket is not connected.
- 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
Can't send after socket shutdown.
- A request to send data was disallowed because the socket had already been
shut down with a previous
Too many references: can't splice.
- Not used in OpenBSD.
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.)
- 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
SYMLOOP_MAX) symbolic links.
File name too long.
- A component of a pathname exceeded 255
NAME_MAX) characters, or an entire
pathname (including the terminating NUL) exceeded 1024
Host is down.
- A socket operation failed because the destination host was down.
No route to host.
- A socket operation was attempted to an unreachable host.
Directory not empty.
- A directory with entries other than
..’ was supplied to a remove
directory or rename call.
Too many processes.
68 EUSERS Too
- The quota system ran out of table entries.
69 EDQUOT Disk
- 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 handle.
- 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 bad.
- Exchange of rpc(3) information
RPC version wrong.
- The version of rpc(3) on the
remote peer is not compatible with the local version.
RPC program not available.
- The requested rpc(3) program is
not registered on the remote host.
Program version wrong.
- The requested version of the
rpc(3) program is not available
on the remote host.
Bad procedure for program.
- An rpc(3) call was attempted
for a procedure which doesn't exist in the remote program.
77 ENOLCK No
- A system-imposed limit on the number of simultaneous file locks was
Function not implemented.
- Attempted a system call that is not available on this system.
Inappropriate file type or format.
- 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 processing failure.
- IPsec subsystem error. Not used in OpenBSD.
Attribute not found.
- A UFS Extended Attribute is not found for the specified pathname.
Illegal byte sequence.
- An illegal sequence of bytes was used when using wide characters.
No medium found.
- Attempted to use a removable media device with no medium present.
Wrong medium type.
- 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.
- The requested operation was canceled.
- An IPC identifier was removed while the current thread was waiting on
90 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.
91 ENOTSUP Not
- The operation has requested an unsupported value.
92 EBADMSG Bad
- A corrupted message was detected.
State not recoverable.
- The state protected by a robust mutex is not recoverable.
Previous owner died.
- The owner of a robust mutex terminated while holding the mutex lock.
- 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,
- 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
- 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 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.
- 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
- 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 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
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
- A process is recognized as a superuser
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 pipe(2),
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
NAME_MAX) characters may be used to
name an ordinary file, special file, or directory.
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
- Path Name
- A path name 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 path name 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 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 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
dot and dot-dot
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
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 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.
manual for section 2 first appeared
in Version 6 AT&T UNIX