INTRO(II) 11/5/73 INTRO(II) 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) 1 EPERM Not 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. 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 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 dialled in or no disk pack is loaded on a drive. 7 E2BIG Arg list too long An argument list longer than 512 bytes (counting the null at the end of each argument) 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 one of the magic numbers 407 or 410. 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 and no more processes can for the moment be created. 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 is such as to require more than the existing 8 segmentation registers. 13 EACCES Permission denied An attempt was made to access a file in a way forbidden by the protection system. 14 - (unused) 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 open file or some process's current directory. 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: currently, dismounting a non-mounted device, mentioning an unknown signal in signal, and giving an unknown request in stty to the TIU special file. 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 Only 15 files can be open per process. 25 ENOTTY Not a typewriter The file mentioned in stty or gtty is not a typewriter 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 An attempt to make a file larger than the maximum of 32768 blocks. 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 Seek on pipe A seek 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 127 links to a file. 32 EPIPE Write 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.