NAME¶
intro —
introduction to system calls
and error numbers
LIBRARY¶
Standard C Library (libc, -lc)
SYNOPSIS¶
#include <errno.h>
DESCRIPTION¶
This section provides an overview of the system calls, their error returns, and
other common definitions and concepts.
RETURN VALUES¶
Nearly all of the system calls provide an error number referenced via the
external identifier errno. This identifier is defined in
<sys/errno.h> as
extern int * __error();
#define errno (* __error())
The
__error() function returns a pointer to a field in the
thread specific structure for threads other than the initial thread. For the
initial thread and non-threaded processes,
__error()
returns a pointer to a global
errno variable that is
compatible with the previous definition.
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 the failure 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 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 in
<sys/errno.h>.
0
Undefined 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
system 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 system
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 subsequent errors.
6 ENXIO
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.
7 E2BIG
Argument
list too long.
- The number of bytes used for
the argument and environment list of the new process exceeded the current
limit (
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
format 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(2)
or waitpid(2) 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
system 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
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.
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
Block
device required.
- A block device operation was
attempted on 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 request.
17 EEXIST
File
exists.
- An existing file was
mentioned in an inappropriate context, for instance, as the new link name
in a link(2) system call.
18 EXDEV
Cross-device link.
- A hard link to a file on
another file system was attempted.
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 existed, 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 example, specifying an undefined signal to a
signal(3) function or a kill(2) system
call).
23 ENFILE
Too many
open files in system.
- Maximum number of file
descriptors 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.) The
getdtablesize(2) system call will obtain the current
limit.
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(2) call requested write access.
27 EFBIG
File too
large.
- The size of a file exceeded
the maximum.
28 ENOSPC
No space
left on device.
- 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 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.
29 ESPIPE
Illegal
seek.
- An lseek(2)
system call was issued on a socket, pipe or FIFO.
30 EROFS
Read-only
file system.
- An attempt was made to modify
a file or directory on a file system that was read-only at the time.
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
Result too
large.
- A numerical 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 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 type
SOCK_STREAM
.
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 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.
- 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.
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 should not 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 normally permitted.
49 EADDRNOTAVAIL
Cannot 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 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 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(2) request was made on an already connected
socket; or, a sendto(2) or sendmsg(2)
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(2) or 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.)
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 32 (
MAXSYMLINKS
) symbolic links.
63 ENAMETOOLONG
File
name too long.
- A component of a path name
exceeded {
NAME_MAX
} characters, or an entire path
name exceeded {PATH_MAX
} characters. (See also the
description of _PC_NO_TRUNC
in
pathconf(2).)
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 unreachable host.
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(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.
70 ESTALE
Stale NFS
file handle.
- An attempt was made to access
an open file (on an NFS file system) 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.
72 EBADRPC
RPC
struct is bad.
- Exchange of RPC information
was unsuccessful.
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 attempted for
a procedure which does not exist in the remote program.
77 ENOLCK
No locks
available.
- A system-imposed limit on the
number of 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 was the wrong type
for the operation, or a data file had the wrong format.
80 EAUTH
Authentication error.
- Attempted to use an invalid
authentication ticket to mount a NFS file system.
81 ENEEDAUTH
Need
authenticator.
- An authentication ticket must
be obtained before the given NFS file system may be mounted.
82 EIDRM
Identifier
removed.
- An IPC identifier was removed
while the current process was waiting on it.
83 ENOMSG
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.
84 EOVERFLOW
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.
85 ECANCELED
Operation canceled.
- The scheduled operation was
canceled.
86 EILSEQ
Illegal
byte sequence.
- While decoding a multibyte
character the function came along an invalid or an incomplete sequence of
bytes or the given wide character is invalid.
87 ENOATTR
Attribute
not found.
- The specified extended
attribute does not exist.
88 EDOOFUS
Programming error.
- A function or API is being
abused in a way which could only be detected at run-time.
93 ENOTCAPABLE
Capabilities insufficient.
- An operation on a capability
file descriptor requires greater privilege than the capability
allows.
DEFINITIONS¶
- 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
to 99999.
- 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,
init(8).
- 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 csh(1).
- Session
- 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.
- 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
termios(4)).
- Controlling process
- A session leader with a controlling terminal is a
controlling process.
- 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 csh(1) and
tty(4)).
- 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.
- 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 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
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
super-user.)
- Super-user
- A process is recognized as a super-user
process and is granted special privileges if its effective user ID is
0.
- Descriptor
- 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 children.
- File Name
- Names consisting of up to
{
NAME_MAX
} characters may be used to name an
ordinary file, special file, or directory.
These characters may be arbitrary eight-bit values, excluding
NUL
(ASCII 0) and the
‘/
’ character (slash, ASCII 47).
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
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
{PATH_MAX
} characters. (On some systems, this
limit may be infinite.)
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.
- 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
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 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 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. (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 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;
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.
SEE ALSO¶
intro(3),
perror(3)