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INTRODUCTION TO SYSTEM CALLS
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Section II of this manual
lists all the entries into the system.
In most cases two calling sequences are specified,
one of which is usable from assembly language, and the other from C.
Most of these calls have an error return.
From assembly language an erroneous call is always
indicated by turning on the c-bit of the condition codes.
The presence of an error is most easily tested
by the instructions
.it bes
and
.it bec
(``branch on error set (or clear)'').
These are synonyms for
the
.it bcs
and
.it bcc
instructions.
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From C, an error condition is indicated by an otherwise
impossible returned value.
Almost always this is \(mi1;
the individual sections specify the details.
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In both cases an error number is also available.
In assembly language,
this number is returned in r0 on erroneous calls.
From C,
the external variable
.it errno
is set to the error number.
.it Errno
is not cleared on successful calls, so it should be tested only
after an error has occurred.
There is a table of messages
associated with each error, and a routine for printing the
message.
See
.it "perror (III)."
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The possible error numbers
are not recited with each writeup in section II, since many
errors are possible for most of the calls.
Here is a list of the error numbers,
their names inside the system (for the benefit of
system-readers),
and the messages available using
.it perror.
A short explanation is also provided.
.en 0 \(mi (unused)
.en 1 EPERM "Not owner and not super-user"
Typically this error indicates
an attempt to modify a file in some way forbidden
except to its owner.
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
.it signal
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
.it read
or
.it 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 512 bytes
(counting the null at the end of each argument)
is presented to
.it 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 one of the magic numbers
407 or 410.
.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"
.it Wait
and the process has no
living or unwaited-for children.
.en 11 EAGAIN "No more processes"
In a
.it fork,
the system's process table is full and no
more processes can for the moment be created.
.en 12 ENOMEM "Not enough core"
During an
.it exec
or
.it 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 is such as to
require more than the existing 8 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 \(mi (unused)
.en 15 ENOTBLK "Block device required"
A plain file was mentioned where a block device was required,
e.g. in
.it 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 open file or some process's current
directory.
.en 17 EEXIST "File exists"
An existing file was mentioned in an inappropriate context,
e.g.
.it 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
.it chdir.
.en 21 EISDIR "Is a directory"
An attempt to write on a directory.
.en 22 EINVAL "Invalid argument"
Some invalid argument:
currently, dismounting a non-mounted
device,
mentioning an unknown signal in
.it signal,
and giving an unknown request in
.it stty
to the TIU special file.
.en 23 ENFILE "File table overflow"
The system's table of open files is full,
and temporarily no more
.it opens
can be accepted.
.en 24 EMFILE "Too many open files"
Only 15 files can be open per process.
.en 25 ENOTTY "Not a typewriter"
The file mentioned in
.it stty
or
.it gtty
is not a typewriter 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"
An attempt to make a file larger than the maximum of 32768 blocks.
.en 28 ENOSPC "No space left on device"
During a
.it write
to an ordinary file,
there is no free space left on the device.
.en 29 ESPIPE "Seek on pipe"
A
.it seek
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 127 links to a file.
.en 32 EPIPE "Write on 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.