path: root/static/unix-v10/man2/intro.2

.TH INTRO 2
.de en
.HP
\\$1  \\fL\\$2\\fP  \\$3
.br
..
.SH NAME
intro, errno \(mi introduction to system calls and error numbers
.SH SYNOPSIS
.B #include <errno.h>
.SH DESCRIPTION
Section 2
describes the entries into the kernel.
.PP
Calls that deal with file systems refer to directory entries by name.
In order to
.I read
or
.IR write (2)
data on a file, the file must be opened
.RI ( open,
.I pipe,
.IR creat (2));
an open file is known by a integer called a
.IR "file descriptor" .
Certain open files admit control operations that alter their behavior;
see in particular
.IR ioctl (2),
.IR stream (4),
and
.IR ttyld (4).
.PP
Besides ordinary file systems, which are passive repositories of data,
organized in a hierarchical directory structure,
there are
special file systems \(em virtual constructs with an interface
that mimics an ordinary file system.
In particular, access to files on remote computers is
mediated by the special file system
.IR netfs (8).
Other special file systems support
.IR proc (4)
and
.IR faced (9.5).
.SH SEE ALSO
.IR intro (3)
.SH DIAGNOSTICS
A `Diagnostics' paragraph appears below for each system call
that has an error return.
Unless otherwise stated, the error value is the integer \-1,
and the success value is 0.
Upon an error return,
an error number is assigned to
the external variable
.IR errno .
.I Errno
is not cleared on successful calls, so it should be tested only
after an error has occurred.
.PP
There is a table of messages that describe the errors
and a routine for printing them;
see
.IR perror (3).
The list below gives
the number, the name (as defined in
.BR <errno.h> ),
and the
.I perror
message for each error type.
The reasons for error returns are explained in general terms;
further explanations for less obvious error returns
appear in the writeups of individual system calls.
.en 0 \h'\w'EIO'u' "Error 0
No error has occurred.
.en 1 EPERM "Not owner
An attempt was made to modify a file in some way forbidden
except to its owner or the super-user,
or an ordinary user attempted to do something
allowed only to the super-user.
.en 2 ENOENT "No such file or directory
A file name was specified
and the file should exist but doesn't, or one
of the directories in a path name does not exist.
.en 3 ESRCH "No such process
The process whose number was given to
.I kill
does not exist, or is already dead.
.en 4 EINTR "Interrupted system call
An asynchronous signal (such as interrupt or quit),
which the user has elected to catch,
occurred during a system call.
If execution is resumed
after processing the signal,
it will appear as if the interrupted system call
returned this error condition.
.en 5 EIO "I/O error
A physical I/O error or timeout occurred during 
.IR read ,
.I write,
or
.I ioctl.
This error may in some cases be returned
on a call following the one to which it actually applies.
.en 6 ENXIO "No such device or address
I/O on a special file refers to a subdevice which does not
exist,
or beyond the limits of the device.
It may also occur when, for example, a tape drive
is off line or no disk pack is loaded on a drive.
.en 7 E2BIG "Arg list too long
An argument list longer than 16384 bytes
was presented to
.IR exec .
.en 8 ENOEXEC "Exec format error
A request was made to execute a file
which, although it has the appropriate permissions,
does not start with a valid magic number, see
.IR a.out (5).
.en 9 EBADF "Bad file number
A file descriptor referred to no
open file,
or a read (resp. write) request was made to
a file which is open only for writing (resp. reading).
.en 10 ECHILD "No children
In a
.IR wait ,
the process had no
living or unwaited-for children.
.en 11 EAGAIN "No more processes
In a
.IR fork ,
the system's process table was full
or the user is not allowed to create any more
processes.
.en 12 ENOMEM "Not enough memory
During an
.I exec
or
.I brk,
a program asked for more memory or swap space
than the system is able to supply.
.en 13 EACCES "Permission denied
An attempt was made to access a file in a way forbidden
by the protection system.
.en 14 EFAULT "Bad address
The system encountered a hardware fault in attempting to
access the arguments of a system call.
.en 15 EHASF "Directory not empty
An attempt was made to remove a nonempty directory.
.en 16 EBUSY "In use
An attempt was made to mount a device that was already mounted
(or crashed or was copied in mounted state),
to dismount a device
on which there is an active file
(open file, current directory, mounted-on file, active text segment),
or to remove the current directory of some process.
.en 17 EEXIST "File exists
An existing file was mentioned in an inappropriate context,
e.g.
.IR link .
.en 18 EXDEV "Cross-device link
A link to a file on another device
was attempted.
.en 19 ENODEV "No such device
An attempt was made to apply an inappropriate
system call to a device;
e.g. read a write-only device.
.en 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
.IR chdir .
.en 21 EISDIR "Is a directory
An attempt to write on a directory.
.en 22 EINVAL "Invalid argument
Some invalid argument:
dismounting a non-mounted
device,
mentioning an unknown signal in
.IR signal ,
reading or writing a file for which
.I lseek
has generated a negative pointer.
Also set by math functions, see
.IR intro (3).
.en 23 ENFILE "File table overflow
The system's table of open files is full,
and temporarily no more
.I opens
can be accepted.
.en 24 EMFILE "Too many open files
The limit is 128 per process.
.en 25 ENOTTY "Illegal ioctl
The function code mentioned in
.I ioctl
does not apply to the file or device.
.en 26 ETXTBSY "Text file busy
An attempt to execute a pure-procedure
program which is currently open.
Also an attempt to open for writing a pure-procedure
program that is being executed.
.en 27 EFBIG "File too large
The size of a file exceeded the maximum (about
.if t 10\u\s-29\s+2\d
.if n 1.0E9
bytes).
.en 28 ENOSPC "No space left on device
During a
.I write
to an ordinary file,
there was no free space left on the device.
.en 29 ESPIPE "Illegal seek
An
.I lseek
was issued to a pipe or
other non-seekable device.
.en 30 EROFS "Read-only file system
An attempt to modify a file or directory
was made
on a device mounted read-only.
.en 31 EMLINK "Too many links
An attempt to make more than 32767 links to a file.
.en 32 EPIPE "Broken pipe
A write on a pipe for which there is no process
to read the data, or on a file with insufficient
security clearance.
This condition normally generates a signal;
the error is returned if the signal is ignored.
.en 33 EDOM "Math argument
The argument of a function in the math package (3M)
is out of the domain of the function.
.en 34 ERANGE "Result too large
The value of a function in the math package (3M)
is unrepresentable within machine precision.
.en 35 ELOOP "Link loop
An endless cycle of symbolic links was encountered.
.en 36 ECONC "Concurrency violation
The open was in violation of the concurrent access specified
for the file,
or an attempted IO operation violated a process exclusive (pex) lock.
.en 37 EGREG "It's all Greg's fault
Something went wrong.
.en 38 ELAB "Security label violation
An action which would, if completed, break security rules; see
.IR getplab (2).
.en 39 ENOSYS "No such system call
An attempt to execute a nonexistent or unsupported system call.
.en 40 ENLAB "Out of security labels
A system table for security labels is full:
a trouble similar to 
.BR ENFILE .
.en 41 EPRIV "Insufficient privilege
An attempt was made to execute a priviledged system call, or
exercise a privileged feature of a regular system call.
.SH SEE ALSO
.IR perror (3)
.SH BUGS
Device drivers and special file systems may use error codes in
unexpected or unconventional ways;
it is infeasible to list them all.
For example, the crash of a remote computer or the
failure of a communication protocol can produce
.L ENOENT
(no such file or directory) in response to
an attempt to open a file through the network file system.
.br
To avoid publishing duplicates of trivially changed manual pages,
.L ELAB
is not recorded among diagnostics when that would be the only
difference from v9.