INTRO(2) System Calls Manual INTRO(2)
intro - introduction to system calls and error numbers
This section describes all of the system calls. Most of these calls have one or more error returns. An error condition is indicated by an
otherwise impossible return value. This is almost always -1; the individual descriptions specify the details. Note that a number of sys-
tem calls overload the meanings of these error numbers, and that the meanings must be interpreted according to the type and circumstances
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 errno, which is not cleared on successful calls. Thus errno should be tested only after an error
The following is a complete list of the errors and their names as given in <sys/errno.h>.
0 Error 0
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 ordinary 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.
5 EIO I/O error
Some physical I/O error occurred during a read or write. This error may in some cases occur on a call following the one to which it
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 <sys/param.h>) is presented to execve.
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,
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
10 ECHILD No children
Wait and the process has no living or unwaited-for children.
11 EAGAIN No more processes
In a fork, 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 execve or break, 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
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.
15 ENOTBLK Block device required
A plain file was mentioned where a block device was required, e.g., in mount.
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 context, e.g., link.
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 con-
figured 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 chdir.
21 EISDIR Is a directory
An attempt to write on a directory.
22 EINVAL Invalid argument
Some invalid argument: dismounting a non-mounted device, mentioning an unknown signal in signal, or some other argument inappropri-
ate for the call. Also set by math functions, (see math(3)).
23 ENFILE File table overflow
The system's table of open files is full, and temporarily no more opens can be accepted.
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 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 ioctl is not a terminal or one of the devices to which this call applies.
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-pro-
cedure program that is being executed.
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 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 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 lseek 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 process to read the data. This condition normally generates 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 fcntl(2)).
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
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 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 on socket
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 shouldn't 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 time-
out 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 desti-
nation 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 shutdown(2) call.
60 ETIMEDOUT Connection timed out
A connect or send 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 ser-
vice 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 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 exhausted, or the allocation of an inode for a newly created file failed because the user's quota of
inodes was exhausted.
Each active process in the system is uniquely identified by a positive integer called a process ID. The range of this ID is from 0 to
Parent process ID
A new process is created by a currently active process; (see fork(2)). The parent process ID of a process is the process ID of its
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 killpg(2)) and the job control mecha-
nisms of csh(1).
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 group-
ing is used to arbitrate between multiple jobs contending for the same terminal; (see csh(1) and tty(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 account-
ing 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
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 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)).
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''.
A process is recognized as a super-user process and is granted special privileges if its effective user ID is 0.
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
init, and is the ancestor of every other process in the system. It is used to control the process structure. Process 2 is the paging
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.
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.
A path name is a null-terminated character string starting with an optional slash (/), followed by zero or more directory names sepa-
rated by slashes, optionally followed by a file name. The total length of a path name must be less than 1024 (MAXPATHLEN) characters.
If a path name begins with a slash, the path search begins at the root directory. Otherwise, the search begins from the current work-
ing directory. A slash by itself names the root directory. A null 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 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 per-
form 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.
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
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 cer-
tain 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.
4th Berkeley Distribution May 23, 1986 INTRO(2)