INTRO(II) 11/5/73 INTRO(II)
INTRODUCTION TO SYSTEM CALLS
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 bes and bec (``branch on error set (or
clear)''). These are synonyms for the bcs and bcc instructions.
From C, an error condition is indicated by an otherwise
impossible returned value. Almost always this is -1; the
individual sections specify the details.
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 errno is set to the error number. 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 perror (III).
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 perror. A short explanation is also provided.
0 - (unused)
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.
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.
3 ESRCH No such process
The process whose number was given to signal does not exist,
or is already dead.
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.
5 EIO I/O error
Some physical I/O error occurred during a read or write. This
error may in some cases occur on a call following the one to
which it actually applies.
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.
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 exec.
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.
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).
10 ECHILD No children
Wait and the process has no living or unwaited-for children.
11 EAGAIN No more processes
In a fork, the system's process table is full and no more
processes can for the moment be created.
12 ENOMEM Not enough core
During an exec or 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.
13 EACCES Permission denied
An attempt was made to access a file in a way forbidden by the
protection system.
14 - (unused)
15 ENOTBLK Block device required
A plain file was mentioned where a block device was required,
e.g. in mount.
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.
17 EEXIST File exists
An existing file was mentioned in an inappropriate context,
e.g. link.
18 EXDEV Cross-device link
A link to a file on another device was attempted.
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.
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 chdir.
21 EISDIR Is a directory
An attempt to write on a directory.
22 EINVAL Invalid argument
Some invalid argument: currently, dismounting a non-mounted
device, mentioning an unknown signal in signal, and giving an
unknown request in stty to the TIU special file.
23 ENFILE File table overflow
The system's table of open files is full, and temporarily no
more opens can be accepted.
24 EMFILE Too many open files
Only 15 files can be open per process.
25 ENOTTY Not a typewriter
The file mentioned in stty or gtty is not a typewriter or one
of the other devices to which these calls apply.
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.
27 EFBIG File too large
An attempt to make a file larger than the maximum of 32768
blocks.
28 ENOSPC No space left on device
During a write to an ordinary file, there is no free space
left on the device.
29 ESPIPE Seek on pipe
A seek was issued to a pipe. This error should also be issued
for other non-seekable devices.
30 EROFS Read-only file system
An attempt to modify a file or directory was made on a device
mounted read-only.
31 EMLINK Too many links
An attempt to make more than 127 links to a file.
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.