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 resources.
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 process ID.
Interrupted system call.
- An asynchronous signal (such
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
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 executable file.
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.
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 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 link(2) function.
- 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.
Not a directory.
- A component of the specified
pathname existed, but it was not a directory, when a directory was
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
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
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.
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.
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.
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
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 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
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
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
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.
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.
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 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 full.
Socket is already connected.
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 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
Operation timed out.
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.
Too many levels of symbolic links.
- A path name lookup involved
more than 32 (
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.
Too many users.
- The quota system ran out of
Disk quota exceeded.
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 occurred.
RPC struct is bad.
- Exchange of
rpc(3) information was
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.
rpc(3) call was attempted for a
procedure which doesn't exist in the remote program.
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 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 filesystem.
- 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 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
- 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 message.
- 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;
(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 ksh(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 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 Terminal
- A terminal that is associated with a session is known as
the controlling terminal for that session and its members.
- Terminal Process Group
- 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 ksh(1),
- 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 process.
- Real User ID and Real Group
- 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 the
- 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
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 open(2) or
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 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
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
- 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
- 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 page appeared in
Version 6 AT&T UNIX