INTRO(2)

NAME
intro, errno - introduction to system calls and error numbers

SYNOPSIS
#include <errno.h>

DESCRIPTION
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 perror. Unused. 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. 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. The process whose number was given to signal and ptrace does not exist, or is already dead. 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. 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. 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. An argument list longer than 5120 bytes is presented to exec. 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). 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). Wait and the process has no living or unwaited-for children. In a fork, the system's process table is full or the user is not allowed to create any more processes. 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. An attempt was made to access a file in a way forbidden by the protection system. The system encountered a hardware fault in attempting to access the arguments of a system call. A plain file was mentioned where a block device was required, e.g. in mount. 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). An existing file was mentioned in an inappropriate context, e.g. link. A link to a file on another device was attempted. An attempt was made to apply an inappropriate system call to a device; e.g. read a write-only device. A non-directory was specified where a directory is required, for example in a path name or as an argument to chdir. An attempt to write on a directory. 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). The system's table of open files is full, and temporarily no more opens can be accepted. Customary configuration limit is 20 per process. The file mentioned in stty or gtty is not a terminal or one of the other devices to which these calls apply. 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. The size of a file exceeded the maximum (about 1.0E9 bytes). During a write to an ordinary file, there is no free space left on the device. An lseek was issued to a pipe. This error should also be issued for other non-seekable devices. An attempt to modify a file or directory was made on a device mounted read-only. An attempt to make more than 32767 links to a file. 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. The argument of a function in the math package (3M) is out of the domain of the function. The value of a function in the math package (3M) is unrepresentable within machine precision.

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
    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 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.