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 returned value.  This is almost always −1; the individual descriptions 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 been indicated. 

There is a table of messages associated with each error, and a routine for printing the message; see perror(3). All of the possible error numbers are not listed in each system call description because many errors are possible for most of the calls. The following is a complete list of the error numbers and their names as defined in <errno.h>. 

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
No process can be found corresponding to that specified by pid in kill or ptrace.

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. 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 on-line or no disk pack is loaded on a drive.

7  E2BIG  Arg list too long
An argument list longer than 5,120 bytes is presented to a member of the exec family. 

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(4)).

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 child processes
A wait was executed by a process that had no existing or unwaited-for child processes. 

11  EAGAIN  No more processes
A fork failed because the system’s process table is full or the user is not allowed to create any more processes. 

12  ENOMEM  Not enough space
During an exec, brk, or sbrk, a program asks for more space than the system is able to supply. This is not a temporary condition; the maximum space size is a system parameter. The error may also occur if the arrangement of text, data, and stack segments requires too many segmentation registers, or if there is not enough swap space during a fork.

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 use an argument of a system call.

15  ENOTBLK  Block device required
A non-block 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 (open file, current directory, mounted-on file, active text segment). It will also occur if an attempt is made to enable accounting when it is already enabled.

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 prefix or as an argument to chdir(2).

21  EISDIR  Is a directory
An attempt to write on a directory.

22  EINVAL  Invalid argument
Some invalid argument (e.g., dismounting a non-mounted device; mentioning an undefined signal in signal, or kill; reading or writing a file for which lseek has generated a negative pointer).  Also set by the math functions described in the (3M) entries of this manual. 

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
No process may have more than 40 file descriptors open at a time.

25  ENOTTY  Not a typewriter
The file mentioned in stty or gtty 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 file size (1,082,201,088 bytes) or ULIMIT; see ulimit(2).

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 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 the maximum number of links (1000) to a file.

32  EPIPE  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.

35  EWOULDBLOCK  Operation would block
An operation which would cause a process to block was attempted on an 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

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 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 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 a peer executing a shutdown(2) call.

55  ENOBUFS  No buffer space available
An operation on a socket or pipe 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
A request to send or receive data was disallowed because the socket was 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.

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

65  EHOSTUNREACH  No route to host

66  ENOTEMPTY  Directory not empty

67  ENOMSG  No message of desired type

68  EIDRM  Identifier removed

69 

70  ESTALE  Stale NFS file handle

71  ETEXTABLEFULL  Text table is full

72  EINODETABLEFULL  Inode table is full

73  ENOFPA  No fpa to run program

74  EGRBUSY  Graphics already in use

75  ENOGR  No graphics hardware present

76-79  rfu

80  EDEADLK  Deadlock condition

81  ENOLCK  No record locks available

82  ENOSTR  Device not a stream

83  ENODATA  No data (for no delay io)

84  ETIME  Timer expired

85  ENOSR  Out of streams resources

86  EBADMSG  Trying to read unreadable message

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 29,999. 

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 ID is the process ID of the group leader.  This grouping permits the signaling of related processes; see kill(2).

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 terminate a group of related process upon termination of one of the processes in the group; see exit(2) and signal(2).

Real User ID and Real Group ID

Each user allowed on the system is identified by a positive integer called a real user ID. 

Each user is also a member of a group.  The group is identified by a positive integer called the real group ID. 

An active process has a real user ID and real group ID that are set to the real user ID and real group ID, respectively, of the user responsible for the creation of the process. 

Effective User ID and Effective Group ID

An active process has an effective user ID and an effective group ID that are used to determine file access permissions (see below).  The effective user ID and effective group ID are equal to the process’s real user ID and real group ID respectively, unless the process or one of its ancestors evolved from a file that had the set-user-ID bit or set-group ID bit set; see exec(2).

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 of 0 and a process ID of 1 are special processes and are referred to as proc0 and proc1.

proc0 is the scheduler.  proc1 is the initialization process (init). proc1 is the ancestor of every other process in the system and is used to control the process structure. 

File Name

Names consisting of 1 to 14 characters may be used to name an ordinary file, special file or directory. 

These characters may be selected from the set of all character values excluding \0 (null) and the ASCII code for / (slash). 

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.  See sh(1). Although permitted, it is advisable to avoid the use of unprintable characters in file names.

Path Name and Path Prefix

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. 

More precisely, a path name is a null-terminated character string constructed as follows:

<path-name>::=<file-name>|<path-prefix><file-name>|/
<path-prefix>::=<rtprefix>|/<rtprefix>
<rtprefix>::=<dirname>/|<rtprefix><dirname>/

where <file-name> is a string of 1 to 14 characters other than the ASCII slash and null, and <dirname> is a string of 1 to 14 characters (other than the ASCII slash and null) that names a directory. 

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. 

Unless specifically stated otherwise, the null path name is treated as if it named a non-existent file. 

Directory.

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.

Read, write, and execute/search permissions on a file are granted to a process if one or more of the following is true:

The process’s effective user ID is super-user. 

The process’s effective user ID matches the user ID of the owner of the file and the appropriate access bit of the “owner” portion (0700) of the file mode is set. 

The process’s effective user ID does not match the user ID of the owner of the file, and the process’s effective group ID matches the group of the file and the appropriate access bit of the “group” portion (070) of the file mode is set. 

The process’s effective user ID does not match the user ID of the owner of the file, and the process’s effective group ID does not match the group ID of the file, and the appropriate access bit of the “other” portion (07) of the file mode is set. 

Otherwise, the corresponding permissions are denied. 

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 a 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 is true:

The process’s effective user ID is super-user. 

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

The process’s effective user ID does not match msg_perm.[c]uid and the process’s effective group ID 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 a 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 is true:

The process’s effective user ID is super-user. 

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

The process’s effective user ID does not match sem_perm.[c]uid and the process’s effective group ID 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:

structipc_perm shm_perm;/∗ operation permission struct ∗/
intshm_segsz;/∗ size of segment ∗/
ushortshm_cpid;/∗ creator pid ∗/
ushortshm_lpid;/∗ pid of last operation ∗/
shortshm_nattch;/∗ number of current attaches ∗/
time_tshm_atime;/∗ last attach time ∗/
time_tshm_dtime;/∗ last detach time ∗/
time_tshm_ctime;/∗ last change time ∗/
/∗ Times measured in secs since ∗/
/∗ 00:00:00 GMT, Jan. 1, 1970 ∗/

shm_perm is a ipc_perm structure that specifies the shared memory 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 ∗/

shm_segsz 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 is true:

The process’s effective user ID is super-user. 

The process’s effective user ID 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. 

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

The process’s effective user ID does not match shm_perm.[c]uid and the process’s effective group ID 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. 

SEE ALSO

intro(3). 

Version 3.6  —  December 20, 1987