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
, 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
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
Undefined error: 0.
- 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
No such file or directory.
- A component of a specified pathname did not exist, or the
pathname was an empty string.
No such process.
- No process could be found which corresponds to the given
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.
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.
Argument list too long.
- The number of bytes used for the argument and environment
list of the new process exceeded the limit
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
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).
No child processes.
function was executed by a process that had no existing or unwaited-for
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
- An attempt was made to access a file in a way forbidden by
its file access permissions.
- The system detected an invalid address in attempting to
use an argument of a call.
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.
- 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
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
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
- Some invalid argument was supplied. (For example,
specifying an undefined signal to a
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
variable kern.maxfiles contains the
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.
will obtain the current limit.
Inappropriate ioctl for device.
- A control function (see
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
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.)
No space left on device.
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
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.
Too many links.
- The maximum allowable number of hard links to a single
file has been exceeded (see
for how to obtain this value).
- A write on a pipe, socket or FIFO for which there is no
process to read the data.
Numerical argument out of domain.
- A numerical input argument was outside the defined domain
of the mathematical function.
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 complete normally.
Operation now in progress.
- An operation that takes a long time to complete (such as a
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
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 type
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 Internet protocols.
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 this machine.
Network is down.
- A socket operation encountered a dead network.
Network is unreachable.
- A socket operation was attempted to an unreachable
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
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.
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
Socket is already connected.
request was made on an already connected socket; or, a
request on a connected socket specified a destination when already
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 supplied.
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.
request failed because the connected party did not properly respond after
a period of time. (The timeout period is dependent on the communication
- 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.
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
No route to host.
- A socket operation was attempted to an unreachable
Directory not empty.
- A directory with entries other than
..’ was supplied to a remove
directory or rename call.
Too many processes.
Too many users.
- The quota system ran out of table entries.
Disk quota exceeded.
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 occurred.
RPC struct is bad.
- Exchange of
information was unsuccessful.
RPC version wrong.
- The version of
on the remote peer is not compatible with the local version.
RPC program not available.
- The requested
program is not registered on the remote host.
Program version wrong.
- The requested version of the
program is not available on the remote host.
Bad procedure for program.
call was attempted for a procedure which doesn't exist in the remote
No locks available.
- A system-imposed limit on the number of simultaneous file
locks was reached.
Function not implemented.
- Attempted a system call that is not available on this
Inappropriate file type or format.
- The file contains invalid data or set to invalid
- Attempted to use an invalid authentication ticket to mount
a NFS filesystem.
- An authentication ticket must be obtained before the given
NFS filesystem may be mounted.
IPsec processing failure.
- IPsec subsystem error. Not used in
Attribute not found.
- A UFS Extended Attribute is not found for the specified
Illegal byte sequence.
- An illegal sequence of bytes was used when using wide
No medium found.
- Attempted to use a removable media device with no medium
Wrong medium type.
- Attempted to use a removable media device with incorrect
or incompatible medium.
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 it.
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
- The operation has requested an unsupported value.
- 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
- Parent Process ID
- A new process is created by a currently active process;
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
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
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
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 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
- 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
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 process.
- 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
- 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
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
(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 0.
- Special Processes
- The processes with process IDs of 0 and 1 are special.
Process 0 is the scheduler. Process 1 is the initialization process
and is the ancestor of every other process in the system. It is used to
control the process structure.
- An integer assigned by the system when a file is referenced
or when a socket is created by
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
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 slash ‘
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
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 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
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;
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 page appeared in
Version 6 AT&T UNIX