INTRO(2) BSD System Calls Manual INTRO(2)
intro, errno -- introduction to system calls and error numbers
This section provides an overview of the system calls, their error returns, and other common definitions and concepts.
Nearly all of the system calls provide an error number in the external variable errno. errno is implemented as a macro which expands to a
modifiable lvalue of type int.
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 has been reported, because otherwise a leftover value from
some previous error may be found instead. (Many library functions that are not system calls also set errno on return, in the same fashion.
In these cases a nonzero value may be left in errno even upon successful return if some internal action failed.)
The manual page for each system call will list some of the common errno codes that system call can return, but that should not be considered
an exhaustive list, i.e. a properly written program should be able to gracefully recover from any error that a system call might return.
Documenting all the error codes that a system call can return in a more specification-like manner would take more resources than this project
Note also that a number of system calls overload the meanings of these error numbers, and that in these cases the meanings must be inter-
preted according to the type and circumstances of the call.
The following is a complete list of the errors and their names as given in <errno.h>.
0 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 function 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 function call will seem to have returned the
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 lim-
its 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 Arg list too long. The number of bytes used for the argument and environment list of the new process exceeded the current limit of
2**18 bytes (ARG_MAX in <sys/syslimits.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, had been revoked by revoke(2), or a
read(2) (or write(2)) request was made to a file that was only open for writing (or 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 pro-
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. The reliable detection of this
error cannot be guaranteed and when not detected may result in the generation of a signal, indicating an address violation, which is
sent to the process.
15 ENOTBLK Block device required. A block device operation was attempted on a non-block device or file.
16 EBUSY Resource busy. An attempt to use a system resource which was in use at the time in a manner which would have conflicted with the
17 EEXIST File exists. An existing file was mentioned in an inappropriate context, for instance, as the new link name in a link(2) function.
18 EXDEV Improper 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) or kill(2)
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 getrlimit(2) call with the
RLIMIT_NOFILE resource 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 opera-
tion 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. (The system-wide maximum file size is 2**63 bytes. Each file system may
impose a lower limit for files contained within it).
28 ENOSPC Device out of space. 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) function was issued on a socket, pipe or FIFO.
30 EROFS Read-only file system. An attempt was made to modify a file or directory was made on a file system that was read-only at the time.
31 EMLINK Too many links. The number of hard links to a single file has exceeded the maximum. (The system-wide maximum number of hard links
is 32767. Each file system may impose a lower limit for files contained within it).
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 or too small. The result of the function is too large or too small to be represented in the available space.
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 option 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
46 EPFNOSUPPORT Protocol family not supported. The protocol family has not been configured into the system or no implementation for it
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 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 send-
ing 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.
59 ETOOMANYREFS Too many references: can't splice. The resource is used up to capacity.
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 255 (MAXNAMELEN) characters, or an entire path name exceeded 1023
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 filesystem) 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.
71 EREMOTE Too many levels of remote in path. NFS version 3 RPC return code 71.
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 doesn't 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. Attempted a file operation on a file of a type for which it was invalid.
80 EAUTH Authentication error. Attempted to use an invalid authentication ticket to mount an NFS filesystem.
81 ENEEDAUTH Need authenticator. An authentication ticket must be obtained before the given NFS filesystem 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 the 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
85 EILSEQ Illegal byte sequence. A wide character/multibyte character encoding error occurred.
86 ENOTSUP Not supported. An attempt was made to set or change a parameter to an unsupported value.
87 ECANCELED Operation canceled. The requested operation was canceled.
88 EBADMSG Bad or corrupt message. A message in the specified message catalog did not satisfy implementation defined criteria, or a STREAMS
operation encountered an invalid message or a file descriptor at the STREAM head.
89 ENODATA No message available. No message is available on the STREAM head read queue
90 ENOSR No STREAM resources. Buffers could not be allocated due to insufficient STREAMs memory resources.
91 ENOSTR Not a STREAM. A STREAM is not associated with the specified file descriptor.
92 ETIME STREAM ioctl timeout. The timer set for a STREAMS ioctl(2) operation has expired.
93 ENOATTR Attribute not found. The specified extended attribute does not exist.
94 EMULTIHOP Multihop attempted. Components of path require hopping to multiple remote machines and the file system does not allow it. It
occurs when users try to access remote resources which are not directly accessible.
95 ENOLINK Link has been severed. Occurs when the link (virtual circuit) connecting to a remote machine is gone.
96 EPROTO Protocol error. Some protocol error occurred. This error is device-specific, but is generally not related to a hardware failure.
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 30000.
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).
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.
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)).
A session leader with a controlling terminal is a controlling process.
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) for more
information on job control.)
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 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 cre-
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 of 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 effec-
tive 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.)
A process is recognized as a super-user process and is granted special privileges if its effective user ID is 0.
The processes with process IDs of 0 and 1 are special. Process 0 is the scheduler. Process 1 is the initialization process init(8),
and is the ancestor (parent) of every other process in the system. It is used to control the process structure. The kernel will
allocate other kernel threads to handle certain periodic tasks or device related tasks, such as:
acctwatch System accounting disk watcher, see acct(2), acct(5).
aiodoned Asynchronous I/O done handler, see uvm(9).
atabusX ATA bus handler, see ata(4).
cardslotX CardBus slot watcher thread, see cardslot(4).
cryptoret The software crypto daemon.
fssbsX File system snapshot thread, see fss(4).
ioflush The in-kernel periodic flush the buffer cache to disk task, which replaces the old update program.
NFS handing daemons.
lfs_writer Log filesystem writer.
pagedaemon The page daemon.
raidX, raidioX, raid_parity, raid_recon, raid_reconip, raid_copyback
Raid framework related threads, see raid(4).
scsibusX SCSI bus handler, see scsi(4).
SMBFS handling daemon, see netsmb(4).
swdmover The software data mover I/O thread, see dmoverio(4).
sysmon The systems monitoring framework daemon.
USB bus handler, see usb(4).
There are more machine-dependent kernel threads allocated by different drivers. See the specific driver manual pages for more infor-
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 (NUL) and the ASCII code for '/' (slash). (The par-
ity bit, bit 7, 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 NUL-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) charac-
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 string is not a valid pathname.
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 permis-
sion 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
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 avail-
able 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.
An intro manual page appeared in Version 6 AT&T UNIX.
July 23, 2009 BSD