INTRO(2)

NAME
intro - introduction to system calls and error numbers

SYNOPSIS
#include <errno.h>

DESCRIPTION
This section describes all of the system calls. Most of these calls have one or more error returns. An error condition is indicated by an otherwise impossible return value. This is almost always -1; the individual descriptions specify the details.

As with normal arguments, all return codes and values from functions are of type integer unless otherwise noted. An error number is also made available in the external variable errno, which is not cleared on successful calls. Thus errno should be tested only after an error has occurred.

The following is a complete list of the errors and their names as given in <errno.h>. 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 kill 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, an illegal tape drive unit number is selected or a disk pack is not loaded on a drive. An argument list longer than 10240 bytes is presented to execve. 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 execve or break, a program asks for more core or swap space than the system is able to supply. A lack of swap space is normally a temporary condition, however a lack of core is not a temporary condition; the maximum size of the text, data, and stack segments is a system parameter. 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). Also generated on an attempt to open a device already opened for exclusive access. An existing file was mentioned in an inappropriate context, e.g. link. A hard 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). One of the System's inode, file, or mfile tables has overflowed. The first two temporarily prevent further opens or creates. The latter temporarily prevents further execs or mmaps. Customary configuration limit is 20 per process, more can be binary configured to suit individual site needs. The file mentioned in an ioctl 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. 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). A write to an ordinary file, the creation of a directory or symbolic link, or the creation of a directory entry failed because no more disk blocks are available on the file system, or the allocation of an inode for a newly created file failed because no more inodes are available on the file system. An lseek was issued to a pipe. This error may 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 hard links to a file. A write on a pipe or socket 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. An operation which would cause a process to block was attempted on a object in non-blocking mode (see ioctl (2)). An operation which takes a long time to complete (such as a connect (2)) was attempted on a non-blocking object (see ioctl (2)). An operation was attempted on a non-blocking object which already had an operation in progress. Self-explanatory. A required address was omitted from an operation on a socket. A message sent on a socket was larger than the internal message buffer. A protocol was specified which does not support the semantics of the socket type requested. For example you cannot use the ARPA Internet UDP protocol with type SOCK_STREAM. A bad option was specified in a getsockopt(2) or setsockopt(2) call. The protocol has not been configured into the system or no implementation for it exists. The support for the socket type has not been configured into the system or no implementation for it exists. For example, trying to accept a connection on a datagram socket. The protocol family has not been configured into the system or no implementation for it exists. 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. Only one usage of each address is normally permitted. Normally results from an attempt to create a socket with an address not on this machine. A socket operation encountered a dead network. A socket operation was attempted to an unreachable network. The host you were connected to crashed and rebooted. A connection abort was caused internal to your host machine. A connection was forcibly closed by a peer. This normally results from the peer executing a shutdown (2) call. An operation on a socket or pipe was not performed because the system lacked sufficient buffer space. A connect request was made on an already connected socket; or, a sendto or sendmsg request on a connected socket specified a destination other than the connected party. An request to send or receive data was disallowed because the socket is not connected. A request to send data was disallowed because the socket had already been shut down with a previous shutdown(2) call. A connect request failed because the connected party did not properly respond after a period of time. (The timeout period is dependent on the communication protocol.) No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service which is inactive on the foreign host. A path name lookup involved more than N symbolic links, where N is binary configurable (default is 8). A component of a path name exceeded 255 characters, or an entire path name exceeded 1023 characters. A socket operation encountered a dead host. A socket operation was attempted to an unreachable host. A directory with entries other than “.” and “..” was supplied to a remove directory or rename call. A client referenced a an open file, when the file has been deleted. An attempt was made to remotely mount a file system into a path which already has a remotely mounted component. A potential deadlock among a set of record locks was avoided.

DEFINITIONS

Process ID
    Each active process in the system is uniquely identified by a positive integer called a process ID. The range of this ID is from 1 to 30000.

Parent process ID
    A new process is created by a currently active process; see fork(2). The parent process ID of a process is the process ID of its creator.

Process Group ID
    Each active process is a member of a process group that is identified by a positive integer called the process group ID. This is the process ID of the group leader. This grouping permits the signaling of related processes (see killpg(2)) and the job control mechanisms of csh(1).

Tty Group ID
    Each active process can be a member of a terminal group that is identified by a positive integer called the tty group ID. This grouping is used to arbitrate between multiple jobs contending for the same terminal; see csh(1), and tty(4).

Real User ID and Real Group ID
    Each user on the system is identified by a positive integer termed the real user ID.

    Each user is also a member of one or more groups. One of these groups is distinguished from others and used in implementing accounting facilities. The positive integer corresponding to this distinguished group is termed the real group ID.

    All processes have a real user ID and real group ID. These are initialized from the equivalent attributes of the process which created it.

Effective User Id, Effective Group Id, and Access Groups
    Access to system resources is governed by three values: the effective user ID, the effective group ID, and the group access list.

    The effective user ID and effective group ID are initially the process's real user ID and real group ID respectively. Either may be modified through execution of a set-user-ID or set-group-ID file (possibly by one its ancestors); see execve(2).

    The group access list is an additional set of group ID's used only in determining resource accessibility. Access checks are performed as described below in ``File Access Permissions''.

Super-user
    A process is recognized as a super-user process and is granted special privileges if its effective user ID is 0.

Special Processes
    The processes with a process ID's of 0, 1, and 2 are special. Process 0 is the scheduler. Process 1 is the initialization process init, and is the ancestor of every other process in the system. It is used to control the process structure. Process 2 is the paging daemon.

Descriptor
    An integer assigned by the system when a file is referenced by open(2), dup(2), or pipe(2) or a socket is referenced by socket(2) or socketpair(2) which uniquely identifies an access path to that file or socket from a given process or any of its children.

File Name
    Names consisting of up to 1024 characters may be used to name an ordinary file, special file, or directory.

    These characters may be selected from the set of all ASCII character excluding 0 (null) and the ASCII code for / (slash). (The parity bit, bit 8, must be 0.)

    Note that it is generally unwise to use *, ?, [ or ] as part of file names because of the special meaning attached to these characters by the shell.

Path Name
    A path name is a null-terminated character string starting with an optional slash (/), followed by zero or more directory names separated by slashes, optionally followed by a file name. The total length of a path name must be less than {PATHNAME_MAX} characters.

    If a path name begins with a slash, the path search begins at the root directory. Otherwise, the search begins from the current working directory. A slash by itself names the root directory. A null pathname refers to the current directory.

Directory
    A directory is a special type of file which contains entries which are references to other files. Directory entries are called links. By convention, a directory contains at least two links, . and .., referred to as dot and dot-dot respectively. Dot refers to the directory itself and dot-dot refers to its parent directory.

Root Directory and Current Working Directory
    Each process has associated with it a concept of a root directory and a current working directory for the purpose of resolving path name searches. A process's root directory need not be the root directory of the root file system.

File Access Permissions
    Every file in the file system has a set of access permissions. These permissions are used in determining whether a process may perform a requested operation on the file (such as opening a file for writing). Access permissions are established at the time a file is created. They may be changed at some later time through the chmod(2) call.

    File access is broken down according to whether a file may be: read, written, or executed. Directory files use the execute permission to control if the directory may be searched.

    File access permissions are interpreted by the system as they apply to three different classes of users: the owner of the file, those users in the file's group, anyone else. Every file has an independent set of access permissions for each of these classes. When an access check is made, the system decides if permission should be granted by checking the access information applicable to the caller.

    Read, write, and execute/search permissions on a file are granted to a process if:

    The process's effective user ID is that of the super-user.

    The process's effective user ID matches the user ID of the owner of the file and the owner permissions allow the access.

    The process's effective user ID does not match the user ID of the owner of the file, and either the process's effective group ID matches the group ID of the file, or the group ID of the file is in the process's group access list, and the group permissions allow the access.

    Neither the effective user ID nor effective group ID and group access list of the process match the corresponding user ID and group ID of the file, but the permissions for ``other users'' allow access.

    Otherwise, permission is denied.

Sockets and Address Families

    A socket is an endpoint for communication between processes. Each socket has queues for sending and receiving data.

    Sockets are typed according to their communications properties. These properties include whether messages sent and received at a socket require the name of the partner, whether communication is reliable, the format used in naming message recipients, etc.

    Each instance of the system supports some collection of socket types; consult socket(2) for more information about the types available and their properties.

    Each instance of the system supports some number of sets of communications protocols. Each protocol set supports addresses of a certain format. An Address Family is the set of addresses for a specific group of protocols. Each socket has an address chosen from the address family in which the socket was created.

SYSTEM V IPC SUPPORT

  Message Queue Identifier
A message queue identifier (msqid) is a unique positive integer created by a msgget(2) system call. Each msqid has a message queue and a data structure associated with it. The data structure is referred to as msqid_ds and contains the following members:

    struct	ipc_perm msg_perm;	/* operation permission struct */
    ushort	msg_qnum;	/* number of msgs on q */
    ushort	msg_qbytes;	/* max number of bytes on q */
    ushort	msg_lspid;	/* pid of last msgsnd operation */
    ushort	msg_lrpid;	/* pid of last msgrcv operation */
    time_t	msg_stime;	/* last msgsnd time */
    time_t	msg_rtime;	/* last msgrcv time */
    time_t	msg_ctime;	/* last change time */
    		/* Times measured in secs since */
    		/* 00:00:00 GMT, Jan. 1, 1970 */

Msg_perm is an ipc_perm structure that specifies the message operation permission (see below). This structure includes the following members:

    ushort	cuid;	/* creator user id */
    ushort	cgid;	/* creator group id */
    ushort	uid;	/* user id */
    ushort	gid;	/* group id */
    ushort	mode;	/* r/w permission */

Msg_qnum is the number of messages currently on the queue. Msg_qbytes is the maximum number of bytes allowed on the queue. Msg_lspid is the process id of the last process that performed a msgsnd operation. Msg_lrpid is the process id of the last process that performed a msgrcv operation. Msg_stime is the time of the last msgsnd operation, msg_rtime is the time of the last msgrcv operation, and msg_ctime is the time of the last msgctl(2) operation that changed a member of the above structure.

  Message Operation Permissions
In the msgop(2) and msgctl(2) system call descriptions, the permission required for an operation is given as "{token}", where "token" is the type of permission needed interpreted as follows:

    00400
        Read by user
    00200
        Write by user
    00060
        Read, Write by group
    00006
        Read, Write by others

Read and Write permissions on a msqid are granted to a process if one or more of the following are true:

    The effective user ID of the process is super-user.

    The effective user ID of the process matches msg_perm.[c]uid in the data structure associated with msqid and the appropriate bit of the “user” portion (0600) of msg_perm.mode is set.

    The effective user ID of the process does not match msg_perm.[c]uid and the effective group ID of the process matches msg_perm.[c]gid and the appropriate bit of the “group” portion (060) of msg_perm.mode is set.

    The effective user ID of the process does not match msg_perm.[c]uid and the effective group ID of the process does not match msg_perm.[c]gid and the appropriate bit of the “other” portion (06) of msg_perm.mode is set.

Otherwise, the corresponding permissions are denied.

  Semaphore Identifier
A semaphore identifier (semid) is a unique positive integer created by a semget(2) system call. Each semid has a set of semaphores and a data structure associated with it. The data structure is referred to as semid_ds and contains the following members:

    struct	ipc_perm sem_perm;	/* operation permission struct */
    ushort	sem_nsems;	/* number of sems in set */
    time_t	sem_otime;	/* last operation time */
    time_t	sem_ctime;	/* last change time */
    		/* Times measured in secs since */
    		/* 00:00:00 GMT, Jan. 1, 1970 */

Sem_perm is an ipc_perm structure that specifies the semaphore operation permission (see below). This structure includes the following members:

    ushort	cuid;	/* creator user id */
    ushort	cgid;	/* creator group id */
    ushort	uid;	/* user id */
    ushort	gid;	/* group id */
    ushort	mode;	/* r/a permission */

The value of sem_nsems is equal to the number of semaphores in the set. Each semaphore in the set is referenced by a positive integer referred to as a sem_num. Sem_num values run sequentially from 0 to the value of sem_nsems minus 1. Sem_otime is the time of the last semop(2) operation, and sem_ctime is the time of the last semctl(2) operation that changed a member of the above structure.

A semaphore is a data structure that contains the following members:

    ushort	semval;	/* semaphore value */
    short	sempid;	/* pid of last operation  */
    ushort	semncnt;	/* # awaiting semval > cval */
    ushort	semzcnt;	/* # awaiting semval = 0 */

Semval is a non-negative integer. Sempid is equal to the process ID of the last process that performed a semaphore operation on this semaphore. Semncnt is a count of the number of processes that are currently suspended awaiting this semaphore's semval to become greater than its current value. Semzcnt is a count of the number of processes that are currently suspended awaiting this semaphore's semval to become zero.

  Semaphore Operation Permissions
In the semop(2) and semctl(2) system call descriptions, the permission required for an operation is given as "{token}", where "token" is the type of permission needed interpreted as follows:

    00400
        Read by user
    00200
        Alter by user
    00060
        Read, Alter by group
    00006
        Read, Alter by others

Read and Alter permissions on a semid are granted to a process if one or more of the following are true:

    The effective user ID of the process is super-user.

    The effective user ID of the process matches sem_perm.[c]uid in the data structure associated with semid and the appropriate bit of the “user” portion (0600) of sem_perm.mode is set.

    The effective user ID of the process does not match sem_perm.[c]uid and the effective group ID of the process matches sem_perm.[c]gid and the appropriate bit of the “group” portion (060) of sem_perm.mode is set.

    The effective user ID of the process does not match sem_perm.[c]uid and the effective group ID of the process does not match sem_perm.[c]gid and the appropriate bit of the “other” portion (06) of sem_perm.mode is set.

Otherwise, the corresponding permissions are denied.

SEE ALSO
intro(3), perror(3)