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>.

0    Error 0
Unused.

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 kill 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 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, an illegal tape drive unit number is selected or a disk pack is not loaded on a drive.

7  E2BIG  Arg list too long
An argument list longer than 10240 bytes is presented to execve.

8  ENOEXEC  Exec format error
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).

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 or the user is not allowed to create any more processes. 

12  ENOMEM  Not enough core
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. 

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 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.

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

18  EXDEV  Cross-device link
A hard 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 signal 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
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. 

24  EMFILE  Too many open files
Customary configuration limit is 20 per process, more can be binary configured to suit individual site needs.

25  ENOTTY  Not a typewriter
The file mentioned in an ioctl 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 which is currently open for writing. 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 109 bytes). 

28  ENOSPC  No space left on device
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. 

29  ESPIPE  Illegal seek
An lseek was issued to a pipe.  This error may 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 hard links to a file.

32  EPIPE  Broken pipe
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.

33  EDOM  Math argument
The argument of a function in the math package (3M) is out of the domain of the function.

34  ERANGE  Result too large
The value of a function in the math package (3M) is unrepresentable within machine precision.

35  EWOULDBLOCK  Operation would block
An operation which would cause a process to block was attempted on a object in non-blocking mode (see ioctl (2)). 

36  EINPROGRESS  Operation now in progress
An operation which takes a long time to complete (such as a connect (2)) was attempted on a non-blocking object (see ioctl (2)). 

37  EALREADY  Operation already in progress
An operation was attempted on a non-blocking object which already had an operation in progress.

38  ENOTSOCK  Socket operation on non-socket
Self-explanatory.

39  EDESTADDRREQ  Destination address required
A required address was omitted from an operation on a socket.

40  EMSGSIZE  Message too long
A message sent on a socket was larger than the internal message buffer.

41  EPROTOTYPE  Protocol wrong type for socket
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.

42  ENOPROTOOPT  Bad protocol option
A bad option was specified in a getsockopt(2) or setsockopt(2) call.

43  EPROTONOSUPPORT  Protocol not supported
The protocol has not been configured into the system or no implementation for it exists.

44  ESOCKTNOSUPPORT  Socket type not supported
The support for the socket type has not been configured into the system or no implementation for it exists.

45  EOPNOTSUPP  Operation not supported on socket
For example, trying to accept a connection on a datagram socket. 

46  EPFNOSUPPORT  Protocol family not supported
The protocol family has not been configured into the system or no implementation for it exists.

47  EAFNOSUPPORT  Address family not supported by protocol family
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.

48  EADDRINUSE  Address already in use
Only one usage of each address is normally permitted.

49  EADDRNOTAVAIL  Can’t assign requested address
Normally results from an attempt to create a socket with an address not on this machine.

50  ENETDOWN  Network is down
A socket operation encountered a dead network.

51  ENETUNREACH  Network is unreachable
A socket operation was attempted to an unreachable network.

52  ENETRESET  Network dropped connection on reset
The host you were connected to crashed and rebooted.

53  ECONNABORTED  Software caused connection abort
A connection abort was caused internal to your host machine.

54  ECONNRESET  Connection reset by peer
A connection was forcibly closed by a peer.  This normally results from the peer executing a shutdown (2) call. 

55  ENOBUFS  No buffer space available
An operation on a socket or pipe was not performed because the system lacked sufficient buffer space.

56  EISCONN  Socket is already connected
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. 

57  ENOTCONN  Socket is not connected
An request to send or receive data was disallowed because the socket is not connected.

58  ESHUTDOWN  Can’t send after socket shutdown
A request to send data was disallowed because the socket had already been shut down with a previous shutdown(2) call.

59  ETOOMANYREFS  Too many references: can’t splice

60  ETIMEDOUT  Connection timed out
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.) 

61  ECONNREFUSED  Connection refused
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.

62  ELOOP  Too many levels of symbolic links
A path name lookup involved more than N symbolic links, where N is binary configurable (default is 8).

63  ENAMETOOLONG  File name too long
A component of a path name exceeded 255 characters, or an entire path name exceeded 1023 characters.

64  EHOSTDOWN  Host is down
A socket operation encountered a dead host.

65  EHOSTUNREACH  No route to host
A socket operation was attempted to an unreachable host.

66  ENOTEMPTY  Directory not empty
A directory with entries other than “.” and “..” was supplied to a remove directory or rename call.

67  EPROCLIM  Too many processes

68  EUSERS  Too many users

70  ESTALE  Stale NFS file handle
A client referenced a an open file, when the file has been deleted.

71  EREMOTE  Too many levels of remote in path
An attempt was made to remotely mount a file system into a path which already has a remotely mounted component.

72  EDEADLK  Deadlock condition if locked
A potential deadlock among a set of record locks was avoided.

73  ENOLCK  No locks available

74  ENOMSG  No message of desired type

75  EIDRM  Identifier removed

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 signalling 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:

structipc_perm msg_perm;/∗ operation permission struct ∗/
ushortmsg_qnum;/∗ number of msgs on q ∗/
ushortmsg_qbytes;/∗ max number of bytes on q ∗/
ushortmsg_lspid;/∗ pid of last msgsnd operation ∗/
ushortmsg_lrpid;/∗ pid of last msgrcv operation ∗/
time_tmsg_stime;/∗ last msgsnd time ∗/
time_tmsg_rtime;/∗ last msgrcv time ∗/
time_tmsg_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:

ushortcuid;/∗ creator user id ∗/
ushortcgid;/∗ creator group id ∗/
ushortuid;/∗ user id ∗/
ushortgid;/∗ group id ∗/
ushortmode;/∗ 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:

structipc_perm sem_perm;/∗ operation permission struct ∗/
ushortsem_nsems;/∗ number of sems in set ∗/
time_tsem_otime;/∗ last operation time ∗/
time_tsem_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:

ushortcuid;/∗ creator user id ∗/
ushortcgid;/∗ creator group id ∗/
ushortuid;/∗ user id ∗/
ushortgid;/∗ group id ∗/
ushortmode;/∗ 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:

ushortsemval;/∗ semaphore value ∗/
shortsempid;/∗ pid of last operation  ∗/
ushortsemncnt;/∗ # awaiting semval > cval ∗/
ushortsemzcnt;/∗ # 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)

4BSD