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INTRO(2)										 INTRO(2)

       intro, errno - introduction to system calls and error numbers

       #include <errno.h>

       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
	      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 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
	      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 (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 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-
	      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

       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:

	      la   %2,errno
	      l    %0,&callno
	      svc  0,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 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.

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