intro - introduction to system calls and error numbers
The manual pages in section 2 provide an overview of the
their error returns, and other common definitions and concepts.
Nearly all of the system calls provide an error number in
variable errno, which is currently defined as:
extern int errno;
Portable applications must not depend on this definition,
and should only
use errno as defined in <errno.h>. 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
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
be interpreted according to the type and circumstances of
The following is a complete list of the errors and their
names as given
0 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 function call. An asynchronous signal
SIGINT or SIGQUIT) was caught by the process during
of an interruptible function. If the signal handler
normal return, the interrupted function 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
the same file descriptor and may be lost (over written) by any
6 ENXIO No such device or address. 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
example, a tape drive is not online or no disk pack
is loaded on
7 E2BIG Arg list too long. The number of bytes used for the
environment list of the new process exceeded the
(specified 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
range, referred to no open file, or a read (write)
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 unwaitedfor child
11 EDEADLK Resource deadlock avoided. An attempt was made
to lock a system
resource that would have resulted in a deadlock
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
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 Not a block device. A block device operation was
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
17 EEXIST File exists. An existing file was mentioned in an
context, for instance, as the new link name in a
18 EXDEV Improper link. A hard link to a file on another
file system was
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 printer.
20 ENOTDIR Not a directory. A component of the specified
but it was not a directory, when a directory was
21 EISDIR Is a directory. An attempt was made to open a directory with
write mode specified.
22 EINVAL Invalid argument. Some invalid argument was supplied. (For
example, specifying an undefined signal to a signal(3) or kill(2)
23 ENFILE Too many open files in system. Maximum number of
allowable on the system has been reached and a
an open cannot be satisfied until at least one has
24 EMFILE Too many open files. (As released, the limit on
the number of
open files per process is 64.) getdtablesize(3)
will obtain the
25 ENOTTY Inappropriate ioctl for device. A control function (see
ioctl(2)) was attempted for a file or special device
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
while the pure procedure file was being executed an
requested write access.
27 EFBIG File too large. The size of a file exceeded the
system-wide maximum file size is 2**63 bytes. Each
may impose a lower limit for files contained within
28 ENOSPC Device out of space. 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
on the file system, or the allocation of an inode
for a newly
created file failed because no more inodes were
available on the
29 ESPIPE Illegal seek. An lseek(2) function was issued on
pipe or FIFO.
30 EROFS Read-only file system. An attempt was made to modify a file or
create a directory on a file system that was readonly at the
31 EMLINK Too many links. Maximum allowable hard links to a
has been exceeded (limit of 32767 hard links per
32 EPIPE Broken pipe. A write on a pipe, socket or FIFO for
is no process to read the data.
33 EDOM Numerical argument out of domain. A numerical input
outside the defined domain of the mathematical function.
34 ERANGE Result out of range. A 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 nonblocking
object (see fcntl(2)).
37 EALREADY Operation already in progress. An operation was
a non-blocking object that already had an operation
38 ENOTSOCK Socket operation on non-socket. Self-explanatory.
39 EDESTADDRREQ Destination address required. A required
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
41 EPROTOTYPE Protocol wrong type for socket. A protocol
that does not support the semantics of the socket
For example, you cannot use the ARPA Internet UDP
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
configured into the system or no implementation for
44 ESOCKTNOSUPPORT Socket type not supported. The support
for the socket
type has not been configured into the system or no
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
support this operation, for example, trying to
accept a connection
on a datagram socket.
46 EPFNOSUPPORT Protocol family not supported. The protocol
not been configured into the system or no implementation for it
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 ARPA Internet protocols.
48 EADDRINUSE Address already in use. Only one usage of
each address is
49 EADDRNOTAVAIL Cannot assign requested address. Normally
an attempt to create a socket with an address not on
50 ENETDOWN Network is down. A socket operation encountered
a dead network.
51 ENETUNREACH Network is unreachable. A socket operation
to an unreachable network.
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
by a peer. This normally results from a loss of the
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
on a connected socket specified a destination
57 ENOTCONN Socket is not connected. A request to send or
was disallowed because the socket was not connected
sending on a datagram socket) no address was supplied.
58 ESHUTDOWN Cannot 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.
59 ETOOMANYREFS Too many references: can't splice. Not used
60 ETIMEDOUT Operation timed out. A connect(2) or send(2)
because the connected party did not properly respond
after a period
of time. (The timeout period is dependent on
61 ECONNREFUSED Connection refused. No connection could be
the target machine actively refused it. This usually results
from trying to connect to a service that is inactive
on the foreign
62 ELOOP Too many levels of symbolic links. A path name
more than 32 symbolic links.
63 ENAMETOOLONG File name too long. A component of a path
255 (MAXNAMELEN) characters, or an entire path name
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
66 ENOTEMPTY Directory not empty. A directory with entries
`.' and `..' was supplied to a remove directory or
67 EPROCLIM Too many processes.
68 EUSERS Too many users. The quota system ran out of table
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
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 filesystem) which is now unavailable
by the file descriptor. This may indicate the
deleted on the NFS server or some other catastrophic
72 EBADRPC RPC struct is bad. Exchange of RPC information
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 host.
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
a procedure which doesn't exist in the remote program.
77 ENOLCK No locks available. A system-imposed limit on the
simultaneous file locks was reached.
78 ENOSYS Function not implemented. Attempted a system call
that is not
available on this system.
79 EFTYPE Inappropriate file type or format. The file contains invalid
data or set to invalid modes.
80 EAUTH Authentication error. Attempted to use an invalid
ticket to mount a NFS filesystem.
81 ENEEDAUTH Need authenticator. An authentication ticket
must be obtained
before the given NFS filesystem may be mounted.
82 EIPSEC IPsec processing failure. IPsec subsystem error.
Not used in
83 ENOATTR Attribute not found. A UFS Extended Attribute is
for the specified pathname.
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 1 to 32766.
Parent Process ID
A new process is created by a currently active process; (see
fork(2)). The parent process ID of a process is
process ID of its creator. If the creating process
parent process ID of each child is set to the ID of
a system process,
Each active process is a member of a process group
that is identified
by a non-negative integer called the process
This is the process ID of the group leader. This
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
created by a successful call to setsid(2), which
caller to become the only member of the only process
group in the
A process that has created a new session by a successful call to
setsid(2), is known as a session leader. Only a
may acquire a terminal as its controlling terminal
A session leader with a controlling terminal is a
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
terminal. Once a terminal is associated with a session, any of
the process groups within the session may be placed
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,
group is orphaned when none of its members has a
that is in the same session as the group, but is in
process group. Note that when a process exits, the
for its children is changed to be init(8),
which is in a
separate session. Not all members of an orphaned
are necessarily orphaned processes (those whose creating process
has exited). The process group of a session leader
Real User ID and Real Group ID
Each user on the system is identified by a positive
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.
initialized from the equivalent attributes of the
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
group IDs, and it is unspecified whether the
ID is a member of the list.)
The effective user ID and effective group ID are
process's real user ID and real group ID respectively. Either
may be modified through execution of a set-user-ID
file (possibly by one its ancestors) (see execve(2)). By convention,
the effective group ID (the first member of
access list) is duplicated, so that the execution of
program does not result in the loss of the original (real)
The group access list is a set of group IDs used only in determining
resource accessibility. Access checks are
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-groupID. The effective
user ID of the process is then recorded as the
and the effective group ID of the process is
the saved set-group-ID. These values may be used to
values as the effective user or group ID after reverting to the
real ID (see setuid(2)). (In POSIX.1, the saved
saved set-group-ID are optional, and are used in setuid and setgid,
but this does not work as desired for the superuser.)
A process is recognized as a superuser process and
special privileges if its effective user ID is 0.
The processes with process IDs of 0, 1, and 2 are
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 structure.
Process 2 is
the paging daemon.
An integer assigned by the system when a file is
open(2) or dup(2), or when a socket is created by
socket(2) or socketpair(2), which uniquely identifies an access
path to that file or socket from a given process or
any of its
Names consisting of up to 255 (MAXNAMELEN) characters may be used
to name an ordinary file, special file, or directory.
These characters may be selected from the set of all
excluding 0 (NUL) and the ASCII code for `/'
Note that it is generally unwise to use `*', `?',
`[' or `]' as
part of file names because of the special meaning
these characters by the shell.
Note also that (MAXNAMELEN) is an upper limit fixed
by the kernel,
meant to be used for sizing buffers. Some
have additional restrictions. These can be queried
pathconf(2) and fpathconf(2).
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 1024 (MAXPATHLEN) characters.
Additional restrictions may apply, depending
filesystem, to be queried with pathconf(2) or fpathconf(2) if
If a path name begins with a slash, the path search
begins at the
root directory. Otherwise, the search begins from
working directory. A slash by itself names the root
An empty pathname is invalid.
A directory is a special type of file that contains
are references to other files. Directory entries
links. By convention, a directory contains at least
`.' and `..', referred to as dot and dot-dot respectively. Dot
refers to the directory itself and dot-dot refers to
Root Directory and Current Working Directory
Each process has associated with it a concept of a
and a current working directory for the purpose of
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
These permissions are used in determining whether a
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
the chmod(2) call.
File access is broken down according to whether a
file may be:
read, written, or executed. Directory files use the
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
When an access check is made, the system decides if
should be granted by checking the access information
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
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
Neither the effective user ID nor effective group ID
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
Each socket has queues for sending and receiving data.
Sockets are typed according to their communications
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
types; consult socket(2) for more information about
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.
An intro manual page appeared in Version 6 AT&T UNIX.
OpenBSD 3.6 December 11, 1993
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