intro - introduction to system calls and error numbers


Standard C Library (libc, -lc)


#include <errno.h>


This section provides an overview of the system calls, their
returns, and other common definitions and concepts.


Nearly all of the system calls provide an error number ref
erenced via the
external identifier errno. This identifier is defined in

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
When a system call detects an error, it returns an integer
value indicating failure (usually -1) and sets the variable errno accord
ingly. <This
allows interpretation of the failure on receiving a -1 and
to take action
accordingly.> Successful calls never set errno; once set, it
until another error occurs. It should only be examined af
ter 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 Undefined error: 0. Not used.
1 EPERM Operation not permitted. An attempt was made to
perform an oper
ation limited to processes with appropriate privi
leges or to the
owner of a file or other resources.
2 ENOENT No such file or directory. A component of a speci
fied pathname
did not exist, or the pathname was an empty string.
3 ESRCH No such process. No process could be found corre
sponding 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 ex
ecution of an
interruptible function. If the signal handler per
forms 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 er
ror occurred.
This error will not be reported until a subsequent
operation on
the same file descriptor and may be lost (over writ
ten) by any
subsequent errors.
6 ENXIO Device not configured. Input or output on a special
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 argu
ment and environment list of the new process exceed
ed the current
limit (NCARGS in
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 (read
10 ECHILD No child processes. A wait(2) or waitpid(2) func
tion was exe
cuted by a process that had no existing or unwaited
for child
11 EDEADLK Resource deadlock avoided. An attempt was made
to lock a sys
tem resource that would have resulted in a deadlock
12 ENOMEM Cannot allocate memory. The new process image re
quired more
memory than was allowed by the hardware or by sys
tem-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 ad
dress 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 con
flicted with the
17 EEXIST File exists. An existing file was mentioned in an
ate 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 anoth
er file system
was attempted.
19 ENODEV Operation not supported by device. An attempt was
made to
apply an inappropriate function to a device, for ex
ample, trying
to read a write-only device such as a printer.
20 ENOTDIR Not a directory. A component of the specified
existed, but it was not a directory, when a directo
ry was
21 EISDIR Is a directory. An attempt was made to open a di
rectory with
write mode specified.
22 EINVAL Invalid argument. Some invalid argument was sup
plied. (For
example, specifying an undefined signal to a sig
nal(3) function
or a kill(2) system call).
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.> The getdtablesize(2)
system call
will obtain the current limit.
25 ENOTTY Inappropriate ioctl for device. A control func
tion (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 pro
cedure (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
28 ENOSPC No space left on device. A write(2) to an ordi
nary 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 mod
ify a file or
directory 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
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 func
34 ERANGE Result too large. A numerical result of the func
tion was too
large to fit in the available space (perhaps exceed
ed precision).
35 EAGAIN Resource temporarily unavailable. This is a tem
porary condi
tion and later calls to the same routine may com
plete normally.
36 EINPROGRESS Operation now in progress. An operation that
takes a long
time to complete (such as a connect(2)) was attempt
ed on a nonblocking 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-explanato
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
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 lev
el was speci
fied 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
for it exists.
45 EOPNOTSUPP Operation not supported. The attempted opera
tion is not
supported for the type of object referenced. Usual
ly 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 implemen
tation for it
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
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 connec
tion 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
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 suffi
cient buffer
space or because a queue was full.
56 EISCONN Socket is already connected. A connect(2) re
quest was made on
an already connected socket; or, a sendto(2) or
request on a connected socket specified a destina
tion 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 sup
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 usual
ly 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 ex
ceeded {PATH_MAX}
characters. (See also the description of
64 EHOSTDOWN Host is down. A socket operation failed be
cause the desti
nation 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
69 EDQUOT Disc quota exceeded. A write(2) to an ordinary
file, the cre
ation of a directory or symbolic link, or the cre
ation 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
70 ESTALE Stale NFS file handle. An attempt was made to ac
cess an open
file (on an NFS file system) which is now unavail
able as referenced by the file descriptor. This may indicate the
file was
deleted on the NFS server or some other catastrophic
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 ver
sion 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 pro
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 for
80 EAUTH Authentication error. Attempted to use an invalid
tion 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
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 con
tain 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
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
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 at
tribute 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.


Process ID.
Each active process in the system is uniquely iden
tified 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 pro
cess (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).
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 suc
cessful call to
setsid(2), is known as a session leader. Only a
session leader
may acquire a terminal as its controlling terminal
Controlling process
A session leader with a controlling terminal is a
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 ses
sion, 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 arbi
trate 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 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 cre
ating 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
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 im
accounting facilities. The positive integer corre
sponding to
this distinguished group is termed the real group
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 val
ues: 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 respective
ly. Either
may be modified through execution of a set-user-ID
or set-groupID file (possibly by one its ancestors) (see ex
ecve(2)). By convention, the effective group ID (the first member of
the group
access list) is duplicated, so that the execution of
a set-groupID program does not result in the loss of the origi
nal (real)
group ID.
The group access list is a set of group IDs used on
ly 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 ac
cess 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 pro
cess 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
A process is recognized as a super-user process and
is granted
special privileges if its effective user ID is 0.
An integer assigned by the system when a file is
referenced by
open(2) or dup(2), or when a socket is created by
socket(2) or socketpair(2), which uniquely identi
fies an access
path to that file or socket from a given process or
any of its
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 direc
tory names separated by slashes, optionally followed by a file
name. The total
length of a path name must be less than {PATH_MAX}
(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
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 respec
tively. 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
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 sys
tem 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
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 su
(Note: even the super-user cannot execute a non-exe
cutable file.)
The process's effective user ID matches the user ID
of the owner
of the file and the owner permissions allow the ac
The process's effective user ID does not match the
user ID of the
owner of the file, and either the process's effec
tive 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
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 ``oth
er users''
allow access.
Otherwise, permission is denied.
Sockets and Address Families
A socket is an endpoint for communication between
Each socket has queues for sending and receiving da
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 com
munication is
reliable, the format used in naming message recipi
ents, 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 ad
dresses for a
specific group of protocols. Each socket has an ad
dress chosen
from the address family in which the socket was cre


intro(3), perror(3)
BSD February 27, 1995
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