4.3BSD-UWisc/man/cat2/intro.2
INTRO(2) UNIX Programmer's Manual INTRO(2)
NAME
intro - introduction to system calls and error numbers
SYNOPSIS
#include <sys/errno.h>
DESCRIPTION
This section describes all of the system calls. Most of
these calls have one or more error returns. An error condi-
tion is indicated by an otherwise impossible return value.
This is almost always -1; the individual descriptions
specify the details. 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 cir-
cumstances of the call.
As with normal arguments, all return codes and values from
functions are of type integer unless otherwise noted. An
error number is also made available in the external variable
_e_r_r_n_o, which is not cleared on successful calls. Thus _e_r_r_n_o
should be tested only after an error has occurred.
The following is a complete list of the errors and their
names as given in <_s_y_s/_e_r_r_n_o._h>.
0 Error 0
Unused.
1 EPERM Not owner
Typically this error indicates an attempt to modify a
file in some way forbidden except to its owner or
super-user. It is also returned for attempts by ordi-
nary users to do things allowed only to the super-user.
2 ENOENT No such file or directory
This error occurs when a file name is specified and the
file should exist but doesn't, or when one of the
directories in a path name does not exist.
3 ESRCH No such process
The process or process group whose number was given
does not exist, or any such process is already dead.
4 EINTR Interrupted system call
An asynchronous signal (such as interrupt or quit) that
the user has elected to catch occurred during a system
call. If execution is resumed after processing the
signal and the system call is not restarted, it will
appear as if the interrupted system call returned this
error condition.
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5 EIO I/O error
Some physical I/O error occurred during a _r_e_a_d or
_w_r_i_t_e. This error may in some cases occur on a call
following the one to which it actually applies.
6 ENXIO No such device or address
I/O on a special file refers to a subdevice that does
not exist, or beyond the limits of the device. It may
also occur when, for example, an illegal tape drive
unit number is selected or a disk pack is not loaded on
a drive.
7 E2BIG Arg list too long
An argument list longer than 20480 bytes (or the
current limit, NCARGS in <_s_y_s/_p_a_r_a_m._h>) is presented to
_e_x_e_c_v_e.
8 ENOEXEC Exec format error
A request is made to execute a file that, although it
has the appropriate permissions, does not start with a
valid magic number, (see _a._o_u_t(5)).
9 EBADF Bad file number
Either a file descriptor refers to no open file, or a
read (resp. write) request is made to a file that is
open only for writing (resp. reading).
10 ECHILD No children
_W_a_i_t and the process has no living or unwaited-for
children.
11 EAGAIN No more processes
In a _f_o_r_k, the system's process table is full or the
user is not allowed to create any more processes.
12 ENOMEM Not enough memory
During an _e_x_e_c_v_e or _b_r_e_a_k, a program asks for more core
or swap space than the system is able to supply, or a
process size limit would be exceeded. A lack of swap
space is normally a temporary condition; however, a
lack of core is not a temporary condition; the maximum
size of the text, data, and stack segments is a system
parameter. 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 the protection system.
14 EFAULT Bad address
The system encountered a hardware fault in attempting
to access the arguments of a system call.
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15 ENOTBLK Block device required
A plain file was mentioned where a block device was
required, e.g., in _m_o_u_n_t.
16 EBUSY Device busy
An attempt to mount a device that was already mounted
or an attempt was made to dismount a device on which
there is an active file (open file, current directory,
mounted-on file, or active text segment). A request
was made to an exclusive access device that was already
in use.
17 EEXIST File exists
An existing file was mentioned in an inappropriate con-
text, e.g., _l_i_n_k.
18 EXDEV Cross-device link
A hard link to a file on another device was attempted.
19 ENODEV No such device
An attempt was made to apply an inappropriate system
call to a device, e.g., to read a write-only device, or
the device is not configured by the system.
20 ENOTDIR Not a directory
A non-directory was specified where a directory is
required, for example, in a path name or as an argument
to _c_h_d_i_r.
21 EISDIR Is a directory
An attempt to write on a directory.
22 EINVAL Invalid argument
Some invalid argument: dismounting a non-mounted dev-
ice, mentioning an unknown signal in _s_i_g_n_a_l, or some
other argument inappropriate for the call. Also set by
math functions, (see _m_a_t_h(3)).
23 ENFILE File table overflow
The system's table of open files is full, and tem-
porarily no more _o_p_e_n_s can be accepted.
24 EMFILE Too many open files
As released, the limit on the number of open files per
process is 64. _G_e_t_d_t_a_b_l_e_s_i_z_e(2) will obtain the
current limit. Customary configuration limit on most
other UNIX systems is 20 per process.
25 ENOTTY Inappropriate ioctl for device
The file mentioned in an _i_o_c_t_l is not a terminal or one
of the devices to which this call applies.
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26 ETXTBSY Text file busy
An attempt to execute a pure-procedure program that is
currently open for writing. Also an attempt to open
for writing a pure-procedure program that is being exe-
cuted.
27 EFBIG File too large
The size of a file exceeded the maximum (about 2.1E9
bytes).
28 ENOSPC No space left on device
A _w_r_i_t_e to an ordinary file, the creation of a direc-
tory or symbolic link, or the creation of a directory
entry failed because no more disk blocks are available
on the file system, or the allocation of an inode for a
newly created file failed because no more inodes are
available on the file system.
29 ESPIPE Illegal seek
An _l_s_e_e_k was issued to a socket or pipe. This error
may also be issued for other non-seekable devices.
30 EROFS Read-only file system
An attempt to modify a file or directory was made on a
device mounted read-only.
31 EMLINK Too many links
An attempt to make more than 32767 hard links to a
file.
32 EPIPE Broken pipe
A write on a pipe or socket for which there is no pro-
cess to read the data. This condition normally gen-
erates a signal; the error is returned if the signal is
caught or ignored.
33 EDOM Argument too large
The argument of a function in the math package (3M) is
out of the domain of the function.
34 ERANGE Result too large
The value of a function in the math package (3M) is
unrepresentable within machine precision.
35 EWOULDBLOCK Operation would block
An operation that would cause a process to block was
attempted on an object in non-blocking mode (see
_f_c_n_t_l(2)).
36 EINPROGRESS Operation now in progress
An operation that takes a long time to complete (such
as a _c_o_n_n_e_c_t(2)) was attempted on a non-blocking object
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(see _f_c_n_t_l(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 type
SOCK_STREAM.
42 ENOPROTOOPT Option not supported by protocol
A bad option or level was specified in a _g_e_t_s_o_c_k_o_p_t(2)
or _s_e_t_s_o_c_k_o_p_t(2) call.
43 EPROTONOSUPPORT Protocol not supported
The protocol has not been configured 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 on socket
For example, trying to _a_c_c_e_p_t a connection 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 exists.
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 proto-
cols.
48 EADDRINUSE Address already in use
Only one usage of each address is normally permitted.
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49 EADDRNOTAVAIL Can't assign requested address
Normally results from an attempt to create a socket
with an address not on this machine.
50 ENETDOWN Network is down
A socket operation encountered a dead network.
51 ENETUNREACH Network is unreachable
A socket operation was attempted to an unreachable net-
work.
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 forcibly closed by a peer. This nor-
mally 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 _c_o_n_n_e_c_t request was made on an already connected
socket; or, a _s_e_n_d_t_o or _s_e_n_d_m_s_g 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 is not connected and (when sending
on a datagram socket) no address was supplied.
58 ESHUTDOWN Can't send after socket shutdown
A request to send data was disallowed because the
socket had already been shut down with a previous _s_h_u_t_-
_d_o_w_n(2) call.
59 _u_n_u_s_e_d
60 ETIMEDOUT Connection timed out
A _c_o_n_n_e_c_t or _s_e_n_d request failed because the connected
party did not properly respond after a period of time.
(The timeout period is dependent on the communication
protocol.)
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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 foreign
host.
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
(MAXPATHLEN-1) characters.
64 EHOSTDOWN Host is down
A socket operation failed because the destination host
was down.
65 EHOSTUNREACH Host is unreachable
A socket operation was attempted to an unreachable
host.
66 ENOTEMPTY Directory not empty
A directory with entries other than "." and ".." was
supplied to a remove directory or rename call.
69 EDQUOT Disc quota exceeded
A _w_r_i_t_e to an ordinary file, the creation of a direc-
tory 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.
DEFINITIONS
Process ID
Each active process in the system is uniquely identi-
fied by a positive 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 _f_o_r_k(2)). The parent process ID of a process is
the process ID of its creator.
Process Group ID
Each active process is a member of a process group that
is identified by a positive integer called the process
group ID. This is the process ID of the group leader.
This grouping permits the signaling of related
processes (see _k_i_l_l_p_g(2)) and the job control mechan-
isms of _c_s_h(1).
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Tty Group ID
Each active process can be a member of a terminal group
that is identified by a positive integer called the tty
group ID. This grouping is used to arbitrate between
multiple jobs contending for the same terminal; (see
_c_s_h(1) and _t_t_y(4)).
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 Access Groups
Access to system resources is governed by three values:
the effective user ID, the effective group ID, and the
group access list.
The effective user ID and effective group ID are ini-
tially 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 _e_x_e_c_v_e(2)).
The group access list is an additional set of group
ID's used only in determining resource accessibility.
Access checks are performed as described below in
``File Access Permissions''.
Super-user
A process is recognized as a _s_u_p_e_r-_u_s_e_r process and is
granted special privileges if its effective user ID is
0.
Special Processes
The processes with a process ID's of 0, 1, and 2 are
special. Process 0 is the scheduler. Process 1 is the
initialization process _i_n_i_t, and is the ancestor of
every other process in the system. It is used to con-
trol the process structure. Process 2 is the paging
daemon.
Descriptor
An integer assigned by the system when a file is
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referenced by _o_p_e_n(2) or _d_u_p(2), or when a socket is
created by _p_i_p_e(2), _s_o_c_k_e_t(2) or _s_o_c_k_e_t_p_a_i_r(2), 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 (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 character excluding 0 (null) and the ASCII code
for / (slash). (The parity bit, bit 8, 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.
Path Name
A path name is a null-terminated character string
starting with an optional slash (/), followed by zero
or more directory names separated by slashes, option-
ally followed by a file name. The total length of a
path name must be less than 1024 (MAXPATHLEN) charac-
ters.
If a path name begins with a slash, the path search
begins at the _r_o_o_t directory. Otherwise, the search
begins from the current working directory. A slash by
itself names the root directory. A null pathname
refers to the current directory.
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
_d_o_t and _d_o_t-_d_o_t respectively. Dot refers to the direc-
tory itself and dot-dot refers to its parent directory.
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 pur-
pose 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 per-
missions. 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
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permissions are established at the time a file is
created. They may be changed at some later time
through the _c_h_m_o_d(2) call.
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 process if:
The process's effective user ID is that of the super-
user.
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
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used in naming message recipients, etc.
Each instance of the system supports some collection of
socket types; consult _s_o_c_k_e_t(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.
SEE ALSO
intro(3), perror(3)
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