intro —
introduction to system calls and error numbers
SYNOPSIS
#include <errno.h>
DESCRIPTION
The manual pages in section 2 provide an overview of the system calls,
their error returns, and other common definitions and concepts.
DIAGNOSTICS
Nearly all of the system calls provide an error number in the external
variable
errno,
which is currently defined as:
extern int errno;
Portable applications must not depend on this definition, and should only
use
errno
as defined in
‹errno.h›.
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›.
Not used.
An attempt was made to perform an operation limited to processes
with appropriate privileges or to the owner of a file or other
resources.
A component of a specified pathname did not exist, or the
pathname was an empty string.
No process could be found which corresponds to the given process ID.
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 error condition.
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.
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.
The number of bytes used for the argument and environment
list of the new process exceeded the limit
NCARGS
(specified in
‹sys/param.h)›.
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.
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).
A
wait(2)
or
waitpid(2)
function was executed by a process that had no existing or unwaited-for
child processes.
An attempt was made to lock a system resource that
would have resulted in a deadlock situation.
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.
An attempt was made to access a file in a way forbidden
by its file access permissions.
The system detected an invalid address in attempting to
use an argument of a call.
A block device operation was attempted on a non-block device or file.
An attempt to use a system resource which was in use at the time
in a manner which would have conflicted with the request.
An existing file was mentioned in an inappropriate context,
for instance, as the new link name in a
link(2)
function.
A hard link to a file on another file system was attempted.
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.
A component of the specified pathname existed, but it was
not a directory, when a directory was expected.
An attempt was made to open a directory with write mode specified.
Some invalid argument was supplied.
(For example, specifying an undefined signal to a
signal(3)
or
kill(2)
function).
Maximum number of file descriptors allowable on the system
has been reached and a request for an open cannot be satisfied
until at least one has been closed.
The
sysctl(3)
variable
kern.maxfiles
contains the current limit.
The maximum number of file descriptors allowable for this process
has been reached and a request for an open cannot be satisfied
until at least one has been closed.
getdtablesize(3)
will obtain the current limit.
A control function (see
ioctl(2))
was attempted for a file or
special device for which the operation was inappropriate.
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.
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.)
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.
An
lseek(2)
function was issued on a socket, pipe or
FIFO.
An attempt was made to modify a file or create a directory
on a file system that was read-only at the time.
The maximum allowable number of hard links to a single file has been
exceeded (see
pathconf(2)
for how to obtain this value).
A write on a pipe, socket or
FIFO
for which there is no process to read the data.
A numerical input argument was outside the defined domain of
the mathematical function.
A result of the function was too large to fit in the
available space (perhaps exceeded precision).
This is a temporary condition and later calls to the
same routine may complete normally.
An operation that takes a long time to complete (such as a
connect(2))
was attempted on a non-blocking object (see
fcntl(2/)).
An operation was attempted on a non-blocking object that already
had an operation in progress.
Self-explanatory.
A required address was omitted from an operation on a socket.
A message sent on a socket was larger than the internal message buffer
or some other network limit.
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.
A bad option or level was specified in a
getsockopt(2)
or
setsockopt(2)
call.
The protocol has not been configured into the
system or no implementation for it exists.
The support for the socket type has not been configured into the
system or no implementation for it exists.
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.
The protocol family has not been configured into the
system or no implementation for it exists.
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.
Only one usage of each address is normally permitted.
Normally results from an attempt to create a socket with an
address not on this machine.
A socket operation encountered a dead network.
A socket operation was attempted to an unreachable network.
The host you were connected to crashed and rebooted.
A connection abort was caused internal to your host machine.
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.
An operation on a socket or pipe was not performed because
the system lacked sufficient buffer space or because a queue was full.
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.
A 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.
A request to send data was disallowed because the socket
had already been shut down with a previous
shutdown(2)
call.
Not used in
OpenBSD.
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.)
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.
A path name lookup involved more than 32
(SYMLOOP_MAX)
symbolic links.
A component of a path name exceeded 255
(MAXNAMLEN)
characters, or an entire path name exceeded 1023
(MAXPATHLEN)
characters.
A socket operation failed because the destination host was down.
A socket operation was attempted to an unreachable host.
A directory with entries other than
"\&."
and
"\&.."
was supplied to a remove directory or rename call.
The quota system ran out of table entries.
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.
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.
Exchange of
RPC
information was unsuccessful.
The version of
RPC
on the remote peer is not compatible with the local version.
The requested program is not registered on the remote host.
The requested version of the program is not available on the remote host
(RPC).
An
RPC
call was attempted for a procedure which doesn't exist
in the remote program.
A system-imposed limit on the number of simultaneous file
locks was reached.
Attempted a system call that is not available on this
system.
The file contains invalid data or set to invalid modes.
Attempted to use an invalid authentication ticket to mount a
NFS
filesystem.
An authentication ticket must be obtained before the given
NFS
filesystem may be mounted.
IPsec subsystem error.
Not used in
OpenBSD.
A UFS Extended Attribute is not found for the specified pathname.
An illegal sequence of bytes was used when using wide characters.
Attempted to use a removable media device with no medium present.
Attempted to use a removable media device with incorrect or incompatible
medium.
A numerical result of the function was too large to be stored in the
caller provided space.
The requested operation was canceled.
An IPC identifier was removed while the current process was waiting on it.
An IPC message queue does not contain a message of the desired type,
or a message catalog does not contain the requested message.
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 1 to 32766.
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 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 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 superuser.)
Superuser
A process is recognized as a
superuser
process and is granted special privileges if its effective user ID is 0.
Special Processes
The processes with process IDs of 0, 1, and 2 are special.
Process 0 is the scheduler.
Process 1 is the initialization process
init(8),
and is the ancestor of every other process in the system.
It is used to control the process structure.
Process 2 is the paging daemon.
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 255
(MAXNAMLEN)
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).
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.
Note also that
(MAXNAMLEN)
is an upper limit fixed by the kernel, meant to be used for sizing buffers.
Some filesystems may have additional restrictions.
These can be queried using
pathconf(2)
and
fpathconf(2).
Path Name
A path name is a
NUL
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 1024
(MAXPATHLEN)
characters.
Additional restrictions may apply, depending upon the filesystem, to be
queried with
pathconf(2)
or
fpathconf(2)
if needed.
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 is invalid.
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 superuser.
(Note: even the superuser 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.