NAME
intro, errno - introduction to system calls and error
numbers
SYNOPSIS
#include <errno.h>
DESCRIPTION
This section describes all of the system calls. Most of
these calls have one or more error returns. An error condi-
tion is indicated by an otherwise impossible return value.
This is almost always -1; the individual descriptions
specify the details. Note that a number of system calls
overload the meanings of these error numbers, and that the
meanings must be interpreted according to the type and cir-
cumstances of the call.
As with normal arguments, all return codes and values from
functions are of type integer unless otherwise noted. An
error number is also made available in the external variable
errno, which is not cleared on successful calls. Thus errno
should be tested only after an error has occurred.
The following is a complete list of the errors and their
names as given in <sys/errno.h>:
0 OK Error 0
Unused. (The symbol "OK" is only used inside the ker-
nel source.)
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 ordi-
nary 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 or process group whose number was given
does not exist, or any such process is already dead.
4 EINTR Interrupted system call
An asynchronous signal (such as interrupt or quit) that
the user has elected to catch occurred during a system
call. If execution is resumed after processing the
signal and the system call is not restarted, 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 an I/O opera-
tion, usually read or write. Operations on file
descriptors that refer to devices that are forcefully
taken away or in a bad state will also provoke this
error.
6 ENXIO No such device or address
I/O on a special file refers to a subdevice that does
not exist, or beyond the limits of the device. It may
also occur when, for example, an illegal tape drive
unit number is selected or a disk pack is not loaded on
a drive.
7 E2BIG Arg list too long
An argument list longer than ARG_MAX bytes is presented
to execve. ARG_MAX is set to 4096 bytes for 16-bit
MINIX 3, 16384 bytes for 32-bit MINIX 3, and unlimited
for Minix-vmd as these systems are released.
8 ENOEXEC Exec format error
A request is made to execute a file that, although it
has the appropriate permissions, 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 Resource temporarily unavailable
In a fork, the system's process table is full or the
user is not allowed to create any more processes, oth-
erwise an operation that would cause a process to block
was attempted on an object in non-blocking mode (see
fcntl(2)).
12 ENOMEM Not enough core
During an execve or brk, a program asks for more (vir-
tual) memory than the system is able to supply, or a
process size limit would be exceeded. The maximum size
of the data+stack segment is set by the chmem(1) pro-
gram. For Minix-vmd a small data+stack size is
increased to 3 megabytes when a program is executed.
13 EACCES Permission denied
An attempt was made to access a file in a way forbidden
by the protection system. Also an attempt to open a
device for writing that is physically write protected.
14 EFAULT Bad address
An argument of a system call is outside the address
space allocated to a process.
15 ENOTBLK Block device required
A plain file was mentioned where a block device was
required, e.g., in mount.
16 EBUSY Resource 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, or active text segment). A request
was made to an exclusive access device that was already
in use.
17 EEXIST File exists
An existing file was mentioned in an inappropriate con-
text, e.g., link.
18 EXDEV Cross-device link
A hard link to a file on another device was attempted.
19 ENODEV No such device
An attempt was made to access a device that is not con-
figured by the system, i.e., there is no driver for the
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: dismounting a non-mounted dev-
ice, mentioning an unknown signal in signal, or some
other argument inappropriate for the call. Also set by
math functions, (see math(3)).
23 ENFILE File table overflow
The system's table of open files is full, and tem-
porarily no more opens can be accepted.
24 EMFILE Too many open files
The limit on the number of open files per process,
OPEN_MAX, is reached. As released, this limit is 20
for MINIX 3, and 30 for Minix-vmd.
25 ENOTTY Not a typewriter
The file mentioned in an ioctl is not a terminal or one
of the devices to which this call applies. (Often seen
error from programs with bugs in their error reporting
code.)
26 ETXTBSY Text file busy
Attempt to execute a program that is open for writing.
Obsolete under MINIX 3.
27 EFBIG File too large
The size of a file exceeded the maximum (little over 64
megabytes for the V2 file system).
28 ENOSPC No space left on device
A write to an ordinary file, the creation of a direc-
tory or symbolic link, or the creation of a directory
entry failed because no more disk blocks are available
on the file system, or the allocation of an inode for a
newly created file failed because no more inodes are
available on the file system.
29 ESPIPE Illegal seek
An lseek was issued to a pipe or TCP/IP channel. This
error may also be issued for other non-seekable dev-
ices.
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 a certain number of hard
links to a file. The advertized maximum, LINK_MAX, is
127, but Minix-vmd uses a much larger maximum of 32767
for the V2 file system.
32 EPIPE Broken pipe
A write on a pipe or TCP/IP channel for which there is
no process to read the data. This condition normally
generates the signal SIGPIPE; the error is returned if
the signal is caught or ignored.
33 EDOM Math argument
The argument of a function in the math package is out
of the domain of the function.
34 ERANGE Result too large
The value of a function in the math package is
unrepresentable within machine precision.
35 EDEADLK Resource deadlock avoided
A process attempts to place a blocking lock on a file
that is already locked by another process and that pro-
cess is waiting for the first process to unlock a file
that first process already has a lock on. (The classic
"lock A, lock B" by process 1, and "lock B, lock A" by
process 2.)
36 ENAMETOOLONG File name too long
The path name exceeds PATH_MAX characters. PATH_MAX
equals 255 as distributed.
37 ENOLCK No locks available
The system's table of active locks is full.
38 ENOSYS Function not implemented
The system call is not supported. Either an old pro-
gram uses an obsolete call, or a program for a more
capable system is run on a less capable system.
39 ENOTEMPTY Directory not empty
A directory with entries other than "." and ".." was
supplied to a remove directory or rename call.
40 ELOOP Too many symbolic links
A path name lookup involved more than SYMLOOP symbolic
links. SYMLOOP equals 8 as distributed. (Minix-vmd)
50 EPACKSIZE Invalid packet size
51 EOUTOFBUFS Not enough buffers left
52 EBADIOCTL Illegal ioctl for device
53 EBADMODE Bad mode in ioctl
54 EWOULDBLOCK Would block
55 EBADDEST Bad destination address
56 EDSTNOTRCH Destination not reachable
57 EISCONN Already connected
58 EADDRINUSE Address in use
59 ECONNREFUSED Connection refused
60 ECONNRESET Connection reset
61 ETIMEDOUT Connection timed out
62 EURG Urgent data present
63 ENOURG No urgent data present
64 ENOTCONN No connection
65 ESHUTDOWN Already shutdown
66 ENOCONN No such connection
67 EINPROGRESS Operation now in progress
68 EALREADY Operation already in progress
DEFINITIONS
Process ID
Each active process in the system is uniquely identi-
fied by a positive integer called a process ID. The
range of this ID is from 1 to 29999. The special pro-
cess with process ID 1 is init, the ancestor of all
processes.
Parent process ID
A new process is created by a currently active process;
(see fork(2)). The parent process ID of a process is
the process ID of its creator, unless the creator dies,
then init becomes the parent of the orphaned process.
Process Group ID
Each active process is a member of a process group that
is identified by a positive integer called the process
group ID. This is the process ID of the group leader.
This grouping permits the signaling of related
processes (see kill(2)).
Real User ID and Real Group ID
Each user on the system is identified by a positive
integer termed the real user ID.
Each user is also a member of one or more groups. One
of these groups is distinguished from others and used
in implementing accounting facilities. The positive
integer corresponding to this distinguished group is
termed the real group ID. (Under standard MINIX 3 this
is the only group a process can be a member of.)
All processes have a real user ID and real group ID.
These are initialized from the equivalent attributes of
the process that created it.
Effective User Id, Effective Group Id, and Access Groups
Access to system resources is governed by three values:
the effective user ID, the effective group ID, and the
group access list.
The effective user ID and effective group ID are ini-
tially the process's real user ID and real group ID
respectively. Either may be modified through execution
of a set-user-ID or set-group-ID file (possibly by one
its ancestors) (see execve(2)).
The group access list is an additional set of group
ID's used only in determining resource accessibility.
Access checks are performed as described below in
``File Access Permissions''. The maximum number of
additional group ID's is NGROUPS_MAX. For MINIX 3 this
is 0, but Minix-vmd supports a list of up to 16 addi-
tional group ID's. (Also known as ``supplemental''
group ID's.)
Super-user
A process is recognized as a super-user process and is
granted special privileges if its effective user ID is
0.
Descriptor
An integer assigned by the system when a file or device
is referenced by open(2), dup(2) or fcntl(2) which
uniquely identifies an access path to that file or dev-
ice from a given process or any of its children.
File Descriptor
Older, and often used name for a descriptor.
File Name
Names consisting of up to NAME_MAX characters may be
used to name an ordinary file, special file, or direc-
tory. NAME_MAX is the maximum of the maximum file name
lengths of the supported file systems. Excess charac-
ters are ignored when too long file names are used for
files in a given file system. The maximum file name
length of the V1 and V2 file systems is 14 characters.
The Minix-vmd "flex" variants of V1 and V2 have a 60
character maximum.
The characters in a file name may assume any value
representable in eight bits excluding 0 (null) and the
ASCII code for / (slash).
Note that it is generally unwise to use one of
\'"<>();~$^&*|{}[]? as part of file names because of
the special meaning attached to these characters by the
shell.
Path Name
A path name is a null-terminated character string
starting with an optional slash (/), followed by zero
or more directory names separated by slashes, option-
ally followed by a file name. The total length of a
path name must be less than PATH_MAX characters (255 as
distributed.)
If a path name begins with a slash, the path search
begins at the root directory. Otherwise, the search
begins from the current working directory. A slash by
itself names the root directory. A null pathname is
illegal, use "." to refer to the current working direc-
tory.
Directory
A directory is a special type of file that contains
entries that are references to other files. Directory
entries are called links. By convention, a directory
contains at least two links, . and .., referred to as
dot and dot-dot respectively. Dot refers to the direc-
tory itself and dot-dot refers to its parent directory.
Root Directory and Current Working Directory
Each process has associated with it a concept of a root
directory and a current working directory for the pur-
pose of resolving path name searches. A process's root
directory need not be the root directory of the root
file system.
File Access Permissions
Every file in the file system has a set of access per-
missions. These permissions are used in determining
whether a process may perform a requested operation on
the file (such as opening a file for writing). Access
permissions are established at the time a file is
created. They may be changed at some later time
through the chmod(2) call.
File access is broken down according to whether a file
may be: read, written, or executed. Directory files
use the execute permission to control if the directory
may be searched.
File access permissions are interpreted by the system
as they apply to three different classes of users: the
owner of the file, those users in the file's group,
anyone else. Every file has an independent set of
access permissions for each of these classes. When an
access check is made, the system decides if permission
should be granted by checking the access information
applicable to the caller.
Read, write, and execute/search permissions on a file
are granted to a process if:
The process's effective user ID is that of the super-
user.
The process's effective user ID matches the user ID of
the owner of the file and the owner permissions allow
the access.
The process's effective user ID does not match the user
ID of the owner of the file, and either the process's
effective group ID matches the group ID of the file, or
the group ID of the file is in the process's group
access list, and the group permissions allow the
access.
Neither the effective user ID nor effective group ID
and group access list of the process match the
corresponding user ID and group ID of the file, but the
permissions for ``other users'' allow access.
Otherwise, permission is denied.
SEE ALSO
intro(3), strerror(3).