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.TH INTRO 2 
.de en
.HP
\\$1  \\$2  \\$3
.br
..
.SH NAME
intro, errno \- introduction to system calls and error numbers
.SH SYNOPSIS
.B #include <errno.h>
.SH DESCRIPTION
Section 2 of this manual
lists all the entries into the system.
Most of these calls have an error return.
.pg
An error condition is indicated by an otherwise
impossible returned value.
Almost always this is \(mi1;
the individual sections specify the details.
An error number is also made available
in 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
associated with each error, and a routine for printing the
message;
See
.IR perror (3).
The possible error numbers
are not recited with each writeup in section 2, since many
errors are possible for most of the calls.
Here is a list of the error numbers,
their names as defined in <errno.h>,
and the messages available using
.IR perror .
.en 0 \h'\w'EIO'u' "Error 0
Unused.
.en 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.
.en 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.
.en 3 ESRCH "No such process
The process whose number was given to
.I signal
and
.I ptrace
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
Some physical I/O error occurred during a
.I read
or
.IR write .
This error may in some cases occur
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 not dialled in or no disk pack is loaded on a drive.
.en 7 E2BIG "Arg list too long
An argument list longer than 5120 bytes
is presented to
.IR exec .
.en 8 ENOEXEC "Exec format error
A request is 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
Either a file descriptor refers to no
open file,
or a read (resp. write) request is made to
a file which is open only for writing (resp. reading).
.en 10 ECHILD "No children
.I Wait
and the process has no
living or unwaited-for children.
.en 11 EAGAIN "No more processes
In a
.I fork,
the system's process table is full
or the user is not allowed to create any more
processes.
.en 12 ENOMEM "Not enough core
During an
.I exec
or
.I break,
a program asks for more core than the system is able to supply.
This is not a temporary condition; the maximum core size
is a system parameter.
The error may also occur if the arrangement
of text, data, and stack segments
requires too many segmentation registers.
.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 ENOTBLK "Block device required
A plain file was mentioned where a block device was required,
e.g. in
.IR mount .
.en 16 EBUSY "Mount 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
directory.
(open file, current directory, mounted-on file, active text segment).
.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
.I signal,
reading or writing a file for which
.I seek
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
Customary configuration limit is 20 per process.
.en 25 ENOTTY "Not a typewriter
The file mentioned in
.I stty
or
.I gtty
is not a terminal or one of the other
devices to which these calls apply.
.en 26 ETXTBSY "Text file busy
An attempt to execute a pure-procedure
program which is currently open for writing
(or reading!).
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 is no free space left on the device.
.en 29 ESPIPE "Illegal seek
An
.I lseek
was issued to a pipe.
This error should also be issued for
other non-seekable devices.
.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.
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.
.SH SEE ALSO
intro(3)
.SH "ASSEMBLER (PDP-11)"
.B as /usr/include/sys.s file ...
.PP
The PDP11 assembly language interface is given for each
system call.
The assembler symbols are defined in `/usr/include/sys.s'.
.PP
Return values appear in registers r0 and r1;
it is unwise to count on these registers being preserved
when no value is expected.
An erroneous call is always
indicated by turning on the c-bit of the condition codes.
The error number is returned in r0.
The presence of an error is most easily tested
by the instructions
.I bes
and
.I bec
(`branch on error set (or clear)').
These are synonyms for
the
.I bcs
and
.I bcc
instructions.
.PP
On the Interdata 8/32, the system call arguments
correspond well to the arguments of the C routines.
The sequence is:
.IP
.nf
la	%2,errno
l	%0,&callno
svc	0,args
.fi
.PP
Thus register 2 points to a word into which the error number will be
stored as needed; it is cleared if no error occurs.
Register 0 contains the system call number; the nomenclature
is identical to that on the PDP11.
The argument of the
.I svc
is the address of the arguments, laid out in storage
as in the C calling sequence.
The return value is in register 2 (possibly 3 also, as in
.IR pipe )
and is \-1 in case of error.
The overflow bit in the program status word is also
set when errors occur.
.PP
On the VAX-11 a system call follows exactly the same conventions as a
C procedure.  Namely, register
.B ap
points to a long word containing the number of arguments, and the
arguments follow in successive long words.  Values are returned in registers
.B r0
and
.BR r1 .
An error is indicated by setting the C (carry) bit in the processor status
word; the error number is placed in
.BR r0 .