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)

1EPERMNot 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.

2ENOENTNo 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.

3ESRCHNo such process
The process whose number was given to signal does not exist, or is already dead. 

4EINTRInterrupted 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.

5EIOI/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. 

6ENXIONo 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.

7E2BIGArg list too long
An argument list longer than 512 bytes (counting the null at the end of each argument) is presented to exec. 

8ENOEXECExec 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.

9EBADFBad 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).

10ECHILDNo children
Wait and the process has no living or unwaited-for children. 

11EAGAINNo more processes
In a fork, the system’s process table is full and no more processes can for the moment be created. 

12ENOMEMNot 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. 

13EACCESPermission denied
An attempt was made to access a file in a way forbidden by the protection system.

14−(unused)

15ENOTBLKBlock device required
A plain file was mentioned where a block device was required, e.g. in mount. 

16EBUSYMount 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.

17EEXISTFile exists
An existing file was mentioned in an inappropriate context, e.g. link. 

18EXDEVCross-device link
A link to a file on another device was attempted.

19ENODEVNo such device
An attempt was made to apply an inappropriate system call to a device; e.g. read a write-only device.

20ENOTDIRNot a directory
A non-directory was specified where a directory is required, for example in a path name or as an argument to chdir. 

21EISDIRIs a directory
An attempt to write on a directory.

22EINVALInvalid 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. 

23ENFILEFile table overflow
The system’s table of open files is full, and temporarily no more opens can be accepted. 

24EMFILEToo many open files
Only 15 files can be open per process.

25ENOTTYNot a typewriter
The file mentioned in stty or gtty is not a typewriter or one of the other devices to which these calls apply. 

26ETXTBSYText 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.

27EFBIGFile too large
An attempt to make a file larger than the maximum of 32768 blocks.

28ENOSPCNo space left on device
During a write to an ordinary file, there is no free space left on the device. 

29ESPIPESeek on pipe
A seek was issued to a pipe.  This error should also be issued for other non-seekable devices. 

30EROFSRead-only file system
An attempt to modify a file or directory was made on a device mounted read-only.

31EMLINKToo many links
An attempt to make more than 127 links to a file.

32EPIPEWrite 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.