intro, errno - introduction to system calls and error numbers
Section 2 of this manual lists all the entries into the system. Most of these calls have
an error return. An error condition is indicated by an otherwise impossible returned
value. Almost always this is -1; the individual sections specify the details. An error
number is also made available in the external variable errno. 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(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 per-
0 Error 0
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.
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 and ptrace does not exist, or is
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 condi-
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 that does not exist, or beyond the lim-
its 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 5120 bytes is presented to exec.
8 ENOEXEC Exec format error
A request is made to execute a file which, although it has the appropriate permis-
sions, does not start with a valid magic number, see a.out(5).
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 that 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 or the user is not allowed to create
any more processes.
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 requires too many segmentation registers.
13 EACCES Permission denied
An attempt was made to access a file in a way forbidden by the protection system.
14 EFAULT Bad address
The system encountered a hardware fault in attempting to access the arguments of a
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 active file (open file, current directory,
mounted-on file, active text segment).
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
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: dismounting a non-mounted device, mentioning an unknown sig-
nal in signal, reading or writing a file for which seek has generated a negative
pointer. Also set by math functions, see intro(3).
23 ENFILE File table overflow
The system's table of open files is full, and temporarily no more opens can be
24 EMFILE Too many open files
Customary configuration limit is 20 per process.
25 ENOTTY Not a typewriter
The file mentioned in stty or gtty is not a terminal or one of the other devices to
which these calls apply.
26 ETXTBSY Text file busy
An attempt to execute a pure-procedure program that 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
The size of a file exceeded the maximum (about 1.0E9 bytes).
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 Illegal seek
An lseek was issued to a pipe. This error should also be issued for other non-
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 32767 links to a file.
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.
33 EDOM Math argument
The argument of a function in the math package (3M) is out of the domain of the
34 ERANGE Result too large
The value of a function in the math package (3M) is unrepresentable within machine
as /usr/include/sys.s file ...
The PDP11 assembly language interface is given for each system call. The assembler sym-
bols are defined in `/usr/include/sys.s'.
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 turn-
ing on the c-bit of the condition codes. The error number is returned in r0. The pres-
ence 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.
On the Interdata 8/32, the system call arguments correspond well to the arguments of the C
routines. The sequence is:
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 nomencla-
ture is identical to that on the PDP11. The argument of the svc is the address of the
arguments, laid out in storage as in the C calling sequence. The return value is in reg-
ister 2 (possibly 3 also, as in pipe) and is -1 in case of error. The overflow bit in the
program status word is also set when errors occur.