NAME

intro, errno − introduction to system calls and error numbers

SYNOPSIS

#include <errno.h>

DESCRIPTION

Section 2 of this manual describes all the entries into the system.  Distinctions as to the status of the entries are made in the headings:

(2)System call entries which are standard in Version 7 UNIX systems. 

(2J)System call entries added in support of the job control mechanisms of csh(1). These system calls are not available in standard Version 7 UNIX systems, and should be used only when necessary; to prevent inexplicit use they are contained in the jobs library which must be specifically requested with the −ljobs loader option.  The use of conditional compilation is recommented when possible so that programs which use these features will gracefully degrade on systems which lack job control. 

(2V)System calls added for the Virtual Memory version of UNIX distributed by Berkeley.  Some of these calls are likely to be replaced by new facilities in future versions; in cases where this is imminent, this is indicated in the individual manual pages. 

(2X)System calls added in an intermediate 4.1x release; these provide access to facilities planned for 4.2bsd, but the 4.2bsd interface to the facilities may differ. 

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 perror.

0    Error 0
Unused.

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 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 dialed in or no disk pack is loaded on a drive.

7  E2BIG  Arg list too long
An argument list longer than 10240 bytes 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 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 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 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 system call.

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 directory. (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 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: dismounting a non-mounted device, mentioning an unknown signal 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 accepted. 

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 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
The size of a file exceeded the maximum (about 109 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-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 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 function.

34  ERANGE  Result too large
The value of a function in the math package (3M) is unrepresentable within machine precision.

35  EWOULDBLOCK  Operation would block
An operation which would cause a process to block was attempted on a object in non-blocking mode (see ioctlnew (2X)). 

36  EINPROGRESS  Operation now in progress
An operation which takes a long time to complete (such as a connect (2X)) was attempted on a non-blocking object (see ioctlnew (2X)). 

37  EALREADY  Operation already in progress
An operation was attempted on a non-blocking object which already had an operation in progress.

38  ENOTSOCK  Socket operation on non-socket
Self-explanatory.

39  EDESTADDRREQ  Destination address required
A required address was omitted from an operation on a socket.

40  EMSGSIZZE  Message too long
A message sent on a socket was larger than the internal message buffer.

41  EPROTOTYPE  Protocol wrong type for socket
A protocol was specified which does not support the semantics of the socket type requested. For example you cannot use the internet UDP protocol with type SOCK_STREAM.

42  ENOPROTOOPT  Protocol not available
In this incarnation of the system.

43  EPROTONOSUPPORT  Protocol not supported
In this incarnation of the system.

44  ESOCKTNOSUPPORT  Socket type not supported
In this incarnation of the system.

45  EOPNOTSUPP  Operation not supported on socket
For example, trying to accept a connection on a datagram socket. 

46  EPFNOSUPPORT  Protocol family not supported
In this incarnation of the system.

47  EAFNOSUPPORT  Address family not supported by protocol family
An address incompatible with the requested protocol was used. For example, you shouldn’t necessarily expect to be able to use PUP Internet addresses with ARPA Internet protocols.

48  EADDRINUSE  Address already in use
Only one usage of each address is normally permitted.

49  EADDRNOTAVAIL  Can’t assign requested address
Normally results from an attempt to create a socket with an address not on this machine.

50  ENETDOWN  Network is down
A socket operation encounted a dead network.

51  ENETUNREACH  Network is unreachable
A socket operation was attempted to an unreachable network.

52  ENETRESET  Network dropped connection on reset
The host you were connected to crashed and rebooted.

53  ECONNABORTED  Software caused connection abort
A connection abort was caused internal to your host machine.

54  ECONNRESET  Connection reset by peer

55  ENOBUFS  No buffer space available
For a socket or a pipe in the buffer pool.

56  EISCONN  Socket is already connected

57  ENOTCONN  Socket is not connected

58  ESHUTDOWN  Can’t send after socket shutdown

59  unused

60  ETIMEDOUT  Connection timed out
Due to failure to initiate properly or because keep-alives failed.

61  ECONNREFUSED  Connection refused
No connection could be made because the target machine actively refused it.

62  ELOOP  Too many levels of symbolic links
A path name lookup involved more than 8 symbolic links.

63  ENAMETOOLONG  File name too long
A component of a path name exceeded 14 characters, or an entire path name exceeded 1023 characters.

SEE ALSO

intro(3)

ASSEMBLER (PDP-11)

as /usr/include/sys.s file ... 

The PDP11 assembly language interface is given for each system call.  The assembler symbols 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 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 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:

la%2,errno
l%0,&callno
svc0,args

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 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 pipe) and is −1 in case of error. The overflow bit in the program status word is also set when errors occur.

On the VAX-11 a system call follows exactly the same conventions as a C procedure.  Namely, register ap points to a long word containing the number of arguments, and the arguments follow in successive long words.  Values are returned in registers r0 and r1.  An error is indicated by setting the C (carry) bit in the processor status word; the error number is placed in r0. 

BUGS

The message “Mount device busy” is reported when a terminal is inaccessible because the “exclusive use” bit is set; this is confusing. 

7th Edition  —  3/17/82