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 open files allowable on the system has been reached and
requests for an open cannot be satisfied until at least one has been
closed.
24 EMFILE
Too
many open files.
- Maximum number of file descriptors allowable in the process has been
reached and requests for an open cannot be satisfied until at least one
has been closed. 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
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 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
Can't 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.
89 EBADMSG
Bad
message.
- A corrupted message was detected.
90 EMULTIHOP
Multihop attempted.
- This error code is unused, but present for compatibility with other
systems.
91 ENOLINK
Link has been severed.
- This error code is unused, but present for compatibility with other
systems.
92 EPROTO
Protocol error.
- A device or socket encountered an unrecoverable protocol error.
93 ENOTCAPABLE
Capabilities insufficient.
- An operation on a capability file descriptor requires greater privilege
than the capability allows.
94 ECAPMODE
Not permitted in capability mode.
- The system call or operation is not permitted for capability mode
processes.
95 ENOTRECOVERABLE
State not recoverable.
- The state protected by a robust mutex is not recoverable.
96 EOWNERDEAD
Previous owner died.
- The owner of a robust mutex terminated while holding the mutex lock.
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)