INTRO(2) BSD Programmer's Manual INTRO(2)
intro - introduction to system calls and error numbers
This section provides an overview of the system calls, their error re-
turns, and other common definitions and concepts.
Nearly all of the system calls provide an error number in the external
variable errno, which is defined as:
extern int errno
When a system call detects an error, it returns an integer value indicat-
ing 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 un-
til 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 call.
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 opera-
tion 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 (such as SIGINT
or SIGQUIT) was caught by the process 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.
5 EIO Input/output error. 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 (over written) by any
6 ENXIO No such device or address. Input or output on a special file re-
ferred 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
7 E2BIG Arg list too long. The number of bytes used for the argument and
environment list of the new process exceeded the current limit of
20480 bytes (NCARGS 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 for-
mat required for an executable file.
9 EBADF 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).
10 ECHILD No child processes. A wait or waitpid function was executed by
a process that had no existing or unwaited-for child processes.
11 EDEADLK Resource deadlock avoided. An attempt was made to lock a sys-
tem resource that would have resulted in a deadlock situation.
12 ENOMEM Cannot allocate memory. The new process image required more
memory than was allowed by the hardware or by system-imposed mem-
ory 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.
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 attempt-
ing to use an argument of a call.
15 ENOTBLK Not a block device. A block device operation was attempted on
a non-block device or file.
16 EBUSY Resource 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 inappropriate
context, for instance, as the new link name in a link function.
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 pathname exist-
ed, but it was not a directory, when a directory was expected.
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 ex-
ample, specifying an undefined signal to a signal or kill func-
23 ENFILE Too many open files in system. Maximum number of file descrip-
tors allowable on the system has been reached and a requests for
an open cannot be satisfied until at least one has been closed.
24 EMFILE Too many open files. <As released, the limit on the number of
open files per process is 64.> Getdtablesize(2) will obtain the
25 ENOTTY Inappropriate ioctl for device. A control function (see
ioctl(2)) was attempted for a file or special device for which
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 process, or
while the pure procedure file was being executed an open call re-
quested write access.
27 EFBIG File too large. The size of a file exceeded the maximum (about
28 ENOSPC Device out of space. A write 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
29 ESPIPE Illegal seek. An lseek function was issued on a socket, pipe or
30 EROFS Read-only file system. An attempt was made to modify a file or
directory was made on a file system that was read-only at the
31 EMLINK Too many links. Maximum allowable hard links to a single file
has been exceeded (limit of 32767 hard links per file).
32 EPIPE Broken pipe. A write on a pipe, socket or FIFO for which there
is no process to read the data.
33 EDOM Numerical argument out of domain. A numerical input argument was
outside the defined domain of the mathematical function.
34 ERANGE Numerical result out of range. A numerical result of the func-
tion was too large to fit in the available space (perhaps exceed-
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 non-
blocking object (see fcntl(2)).
37 EALREADY Operation already in progress. An operation was attempted on
a non-blocking object that already had an operation in progress.
38 ENOTSOCK Socket operation on non-socket. Self-explanatory.
39 EDESTADDRREQ Destination address required. A required address was
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 limit.
41 EPROTOTYPE 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 ARPA Internet UDP protocol with
42 ENOPROTOOPT Protocol not available. A bad option or level was speci-
fied in a getsockopt(2) or setsockopt(2) call.
43 EPROTONOSUPPORT Protocol not supported. The protocol has not been con-
figured into the system or no implementation for it exists.
44 ESOCKTNOSUPPORT Socket type not supported. The support for the socket
type has not been configured into the system or no implementation
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 that cannot
support this operation, for example, trying to accept a connec-
tion on a datagram socket.
46 EPFNOSUPPORT Protocol family not supported. The protocol family has
not been configured into the system or no implementation for it
47 EAFNOSUPPORT Address family not supported by protocol family. An ad-
dress incompatible with the requested protocol was used. For ex-
ample, you shouldn't necessarily expect to be able to use NS ad-
dresses with ARPA Internet protocols.
48 EADDRINUSE Address already in use. Only one usage of each address is
49 EADDRNOTAVAIL Cannot assign requested address. Normally results from
an attempt to create a socket with an address not on this ma-
50 ENETDOWN Network is down. A socket operation encountered a dead net-
51 ENETUNREACH Network is unreachable. A socket operation was attempted
to an unreachable network.
52 ENETRESET Network dropped connection on reset. The host you were con-
nected 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 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.
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 request was made on an
already connected socket; or, a sendto or sendmsg request on a
connected socket specified a destination when already connected.
57 ENOTCONN Socket is not connected. An 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.
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.
60 ETIMEDOUT Operation timed out. A connect or send request failed be-
cause the connected party did not properly respond after a period
of time. (The timeout period is dependent on the communication
61 ECONNREFUSED Connection refused. 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 for-
62 ELOOP Too many levels of symbolic links. A path name lookup involved
more than 8 symbolic links.
63 ENAMETOOLONG File name too long. A component of a path name exceeded
255 (MAXNAMELEN) characters, or an entire path name exceeded 1023
64 EHOSTDOWN Host is down. A socket operation failed because the destina-
tion 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 other than `.'
and `..' was supplied to a remove directory or rename call.
67 EPROCLIM Too many processes.
68 EUSERS Too many users. The quota system ran out of table entries.
69 EDQUOT Disc quota exceeded. A write 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 exhaust-
ed, or the allocation of an inode for a newly created file 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 as refer-
enced by the file descriptor. This may indicate the file was
deleted on the NFS server or some other catastrophic event oc-
72 EBADRPC RPC struct is bad. Exchange of RPC information was unsuccess-
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 regis-
tered on the remote host.
75 EPROGMISMATCH Program version wrong. The requested version of the pro-
gram is not available on the remote host (RPC).
76 EPROCUNAVAIL Bad procedure for program. An RPC call was attempted for
a procedure which doesn't exist in the remote program.
77 ENOLCK No locks available. A system-imposed limit on the number of si-
multaneous file locks was reached.
78 ENOSYS Function not implemented. Attempted a system call that is not
available on this system.
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 30000.
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 pro-
Each active process is a member of a process group that is iden-
tified by a non-negative integer called the process group ID.
This is the process ID of the group leader. This grouping per-
mits 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 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
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
A session leader with a controlling terminal is a controlling
A terminal that is associated with a session is known as the con-
trolling 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 csh(1) and
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 pro-
cess for its children is changed to be init, which is in a sepa-
rate 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
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 ac-
counting 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
Effective User Id, Effective Group Id, and Group Access List
Access to system resources is governed by two values: the effec-
tive 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 supplemen-
tary 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 its ancestors) (see execve(2)). By con-
vention, 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)
The group access list is a set of group IDs used only in deter-
mining 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 ef-
fective 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 effec-
tive 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 set-
gid, but this does not work as desired for the super-user.)
A process is recognized as a super-user process and is granted
special privileges if its effective user ID is 0.
The processes with process IDs of 0, 1, and 2 are special. Pro-
cess 0 is the scheduler. Process 1 is the initialization process
init, and is the ancestor of every other process in the system.
It is used to control the process structure. Process 2 is the
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),
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 ASCII char-
acter excluding 0 (NUL) and the ASCII code for `/' (slash). (The
parity bit, bit 7, must be 0.)
Note that it is generally unwise to use `*', `?', `[' or `]' as
part of file names because of the special meaning attached to
these characters by the shell.
A path name is a NUL-terminated character string starting with an
optional slash `/', followed by zero or more directory names sep-
arated by slashes, optionally followed by a file name. The total
length of a path name must be less than 1024 (MAXPATHLEN) charac-
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 refers to the current directory.
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,
`.' and `..', referred to as dot and dot-dot respectively. Dot
refers to the directory itself and dot-dot refers to its parent
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 chmod(2) call.
File access is broken down according to whether a file may be:
read, written, or executed. Directory files use the execute per-
mission to control if the directory may be searched.
File access permissions are interpreted by the system as they ap-
ply 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 super-user. (Note:
even the super-user 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 al-
low 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'' al-
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 com-
munications 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.
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