intro(2)
NAME
intro, errno − introduction to system calls and error numbers
SYNTAX
#include <errno.h>
DESCRIPTION
Section 2 describes the ULTRIX system calls, which are the entries into the ULTRIX kernel. Certain distinctions of purpose are made in the headings. Pages ending in (2yp) are specific to the Yellow Pages (YP) service. Pages ending in (2nfs) are specific to the Network File System (NFS) service.
SYSTEM V COMPATIBILITY
Some system calls contain System V compatibility features that are available to general ULTRIX-32 programs. This compatibility sometimes conflict with features already present in ULTRIX; the function performed may be slightly different in the System V environment. These features are provided for applications that are being ported from System V.
The descriptions in this section include an ENVIRONMENT section that describes any differences in function between System V and the standard C runtime library.
The System V compatibility features are not contained in the standard C runtime library. To get System V specific behavior, you must specify that the System V environment is to be used in compiling and linking programs. There are two ways to do this:
.3iUse the −Y option for the cc(1) command.
.3iGlobally set the environment variable PROG_ENV to SYSTEM_FIVE. If you are using the C shell, execute the following line or include it in your .login file:
setenv PROG_ENV SYSTEM_FIVE
If you are using the Bourne shell, execute the following line or include it in your .profile file:
PROG_ENV=SYSTEM_FIVE ; export PROG_ENV
In both cases, the cc command defines the preprocessor symbol SYSTEM_FIVE, so that the C preprocessor, /lib/cpp, will select the System V version of various data structures and symbol definitions.
In addition, if cc invokes ld, the library libcV.a (the System V version of the Standard C library) is searched before libc.a to resolve references to the System-V-specific routines. Also, if the −lm option is specified on either the cc(1) or the ld(1) command line, then the System V version of the math library will be used instead of the ULTRIX math library.
RETURN VALUE
Most of the system calls have one or more return values. An error condition is indicated by an otherwise impossible return value. This is almost always −1; the individual descriptions specify the details.
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.
Here is a 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 superuser. It is also returned for attempts by ordinary users to do things allowed only to the superuser.
2 ENOENT No such file or directory
This error occurs when a file name is specified and the file should exist but does not, or when one of the directories in a pathname does not exist.
3 ESRCH No such process
The process whose number was given to kill(2) and ptrace(2) 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 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 limits of the device. It may also occur when 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(2).
8 ENOEXEC Exec format error
A request is made to execute a file that does not start with a valid magic number, although it has the appropriate permissions. For further information, see a.out(5).
9 EBADF Bad file number
Either a file descriptor refers to no open file or a read (respectively, write) request is made to a file which is open only for writing (respectively, reading).
10 ECHILD No children
A 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(2) or brk, 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 some 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, for example, in mount(2).
16 EBUSY Mount device busy
An attempt was made to mount a device that was already mounted or to unmount a device on which there was an active file directory, an open file, current directory, mounted-on file, or active text segment. Or, an attempt was made to modify a partition table incorrectly. See the restrictions in chpt(8).
17 EEXIST File exists
An existing file was mentioned in an inappropriate context.
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; for example, 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(2).
21 EISDIR Is a directory
An attempt to write on a directory.
22 EINVAL Invalid argument
An invalid argument: dismounting a non-mounted device, mentioning an unknown signal in signal(3), 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 accepted.
24 EMFILE Too many open files
Customary configuration limit is 20 files per process.
25 ENOTTY Not a typewriter
The file mentioned in an ioctl(2) 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 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 109 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(2) was issued to a pipe. This error may also be issued for other non-seekable devices.
30 EROFS Restricted operation on a file system
An attempt to access a file or directory on a mounted file system when that permission has been revoked. For example, trying to write a file on a file system 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 Argument too large
The argument of a function in the math package Section (3m) is out of the domain of the function.
34 ERANGE Result too large
The value of a function in the math package Section (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. For further information, see ioctl(2).
36 EINPROGRESS Operation now in progress
An operation which takes a long time to complete, such as connect(2), was attempted on a non-blocking object. For further information, 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 Protocol not available
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 Error--operation not supported
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 should not expect to be able to use PUP Internet addresses with ARPA Internet protocols.
48 EADDRINUSE Address already in use
Each address can be used only once.
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 usually 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.) A connect request or remote file operation failed because the connected party did not properly respond after a period of time which is dependent on the communication protocol. For example, this occurs when an NFS file system is mounted with the “soft” option and the server is not responding to file operation requests.
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 8 symbolic links.
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 failed because the destination host was down.
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
The user would exceed the number of user processes that are available. This limit can be changed by changing the maxuprc option in the configuration file when the kernel is built.
68 EUSERS Too many users
This error is returned when a login process would exceed the maximum allowable login processes that the system is licensed for.
69 EDQUOT Disk quota exceeded
A write to an ordinary file, the creation of a directory or symbolic link, or the creation of a directory entry failed because the user’s quota of disk blocks was exhausted, or the allocation of an inode for a newly created file failed because the user’s quota of inodes was exhausted.
70 ESTALE
An NFS stale file handle. Information used by the operating system to identify a file in an NFS file system is no longer valid. This error code results from operating on a remote file that no longer exists on the server or resides in a file system that has been moved to a different device on the server.
71 EREMOTE
Remote NFS client has requested an operation on a file that is remote to the server as well. An attempt has been made to mount an NFS remote file system that is not local to the specified server. This error code cannot occur except in response to a failed mount(2).
72 ENOMSG No message of desired type
An attempt was made to receive a message of a type that does not exist on the specified message queue; see msgop(2).
73 EIDRM Identifier removed
In semaphores, shared memory, or message queues, the caller tried to access the identifier after it had been removed from the system.
74 EALIGN alignment error
Alignment error of some type, for example, cluster, page, or block.
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 0 to {PROC_MAX}.
Parent process ID
A new process is created by a currently active process. For further information, 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. For further information, 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. For further information, 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. For further information, 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”.
Superuser
A process is recognized as a superuser 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 {FILENAME_MAX} 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 superuser.
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 the 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.
If the process is trying to “exec” an image and the file system is mounted “no exec”, execute permission will be denied.
If the process’s effective UID is not root, the process is attempting to access a character or block special device, and the file system is mounted with “nodev”, access will be denied.
If the process’s effective UID is not root, the process is trying to execute an image with setuid or setgid, bit set in the file’s permissions, and the file system is mounted “nosuid”, execute permission will be denied.
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, and whether the format is used in naming message recipients.
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.
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 message 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 msgrcv(2) operation. The msg_lrpid is the process id of the last process that performed a msgop operation. Th3 msg_stime is the time of the last msgop(2) 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 are granted to a process if one or more of the following are true:
1.The effective user ID of the process is superuser.
2.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.
3.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.
4.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. 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. The sem_num values run sequentially from 0 to the value of sem_nsems minus 1. The 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:
1.The effective user ID of the process is superuser.
2.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.
3.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.
4.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.
Shared Memory Identifier
A shared memory identifier (shmid) is a unique positive integer created by a shmget(2) system call. Each shmid has a segment of memory (referred to as a shared memory segment) and a data structure associated with it. The data structure is referred to as shmid_ds and contains the following members:
struct ipc_perm shm_perm; /*operation permission struct*/
int shm_segsz; /*size of segment*/
ushort shm_cpid; /*creator pid*/
ushort shm_lpid; /*pid of last operation*/
short shm_nattch; /*number of current attaches*/
time_t shm_atime; /*last attach time*/
time_t shm_dtime; /*last detach time*/
time_t shm_ctime; /*last change time*/
/*Times measured in secs since*/
/*00:00:00 GMT, Jan. 1, 1970*/
Shm_perm is an ipc_perm structure that specifies the shared memory operation permission. 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*/
Shm_segz specifies the size of the shared memory segment. Shm_cpid is the process id of the process that created the shared memory identifier. Shm_lpid is the process id of the last process that performed a shmop(2) operation. Shm_nattch is the number of processes that currently have this segment attached. Shm_atime is the time of the last shmat operation, shm_dtime is the time of the last shmdt operation, and shm_ctime is the time of the last shmctl(2) operation that changed one of the members of the above structure.
Shared Memory Operation Permissions
In the shmop(2) and shmctl(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 shmid are granted to a process if one or more of the following are true:
1.The effective user ID of the process is superuser.
2.The effective user ID of the process matches shm_perm.[c]uid in the data structure associated with shmid and the appropriate bit of the user portion (0600) of shm_perm.mode is set.
3.The effective user ID of the process does not match shm_perm.[c]uid and the effective group ID of the process matches shm_perm.[c]gid and the appropriate bit of the group portion (060) of shm_perm.mode is set.
4.The effective user ID of the process does not match shm_perm.[c]uid and the effective group ID of the process does not match shm_perm.[c]gid and the appropriate bit of the other portion (06) of shm_perm.mode is set.
Otherwise, the corresponding permissions are denied.