1INTRO(2) System Calls Manual INTRO(2)
2
6 intro - introduction to system calls and error numbers
7
9 #include <sys/errno.h>
10
12 This section describes all of the system calls. Most of these calls
13 have one or more error returns. An error condition is indicated by an
14 otherwise impossible return value. This is almost always -1; the indi‐
15 vidual descriptions specify the details. Note that a number of system
16 calls overload the meanings of these error numbers, and that the mean‐
17 ings must be interpreted according to the type and circumstances of the
18 call.
19 As with normal arguments, all return codes and values from functions
21 are of type integer unless otherwise noted. An error number is also
22 made available in the external variable errno, which is not cleared on
23 successful calls. Thus errno should be tested only after an error has
24 occurred.
25 The following is a complete list of the errors and their names as given
27 in <sys/errno.h>.
28 0 Error 0
30 Unused.
31 1 EPERM Not owner
33 Typically this error indicates an attempt to modify a file in
34 some way forbidden except to its owner or super-user. It is
35 also returned for attempts by ordinary users to do things
36 allowed only to the super-user.
37 2 ENOENT No such file or directory
39 This error occurs when a file name is specified and the file
40 should exist but doesn't, or when one of the directories in a
41 path name does not exist.
42 3 ESRCH No such process
44 The process or process group whose number was given does not
45 exist, or any such process is already dead.
46 4 EINTR Interrupted system call
48 An asynchronous signal (such as interrupt or quit) that the user
49 has elected to catch occurred during a system call. If execu‐
50 tion is resumed after processing the signal and the system call
51 is not restarted, it will appear as if the interrupted system
52 call returned this error condition.
53 5 EIO I/O error
55 Some physical I/O error occurred during a read or write. This
56 error may in some cases occur on a call following the one to
57 which it actually applies.
58 6 ENXIO No such device or address
60 I/O on a special file refers to a subdevice that does not exist,
61 or beyond the limits of the device. It may also occur when, for
62 example, an illegal tape drive unit number is selected or a disk
63 pack is not loaded on a drive.
64 7 E2BIG Arg list too long
66 An argument list longer than 20480 bytes (or the current limit,
67 NCARGS in <sys/param.h>) is presented to execve.
68 8 ENOEXEC Exec format error
70 A request is made to execute a file that, although it has the
71 appropriate permissions, does not start with a valid magic num‐
72 ber, (see a.out(5)).
73 9 EBADF Bad file number
75 Either a file descriptor refers to no open file, or a read
76 (resp. write) request is made to a file that is open only for
77 writing (resp. reading).
78 10 ECHILD No children
80 Wait and the process has no living or unwaited-for children.
81 11 EAGAIN No more processes
83 In a fork, the system's process table is full or the user is not
84 allowed to create any more processes.
85 12 ENOMEM Not enough memory
87 During an execve or break, a program asks for more core or swap
88 space than the system is able to supply, or a process size limit
89 would be exceeded. A lack of swap space is normally a temporary
90 condition; however, a lack of core is not a temporary condition;
91 the maximum size of the text, data, and stack segments is a sys‐
92 tem parameter. Soft limits may be increased to their corre‐
93 sponding hard limits.
94 13 EACCES Permission denied
96 An attempt was made to access a file in a way forbidden by the
97 protection system.
98 14 EFAULT Bad address
100 The system encountered a hardware fault in attempting to access
101 the arguments of a system call.
102 15 ENOTBLK Block device required
104 A plain file was mentioned where a block device was required,
105 e.g., in mount.
106 16 EBUSY Device busy
108 An attempt to mount a device that was already mounted or an
109 attempt was made to dismount a device on which there is an
110 active file (open file, current directory, mounted-on file, or
111 active text segment). A request was made to an exclusive access
112 device that was already in use.
113 17 EEXIST File exists
115 An existing file was mentioned in an inappropriate context,
116 e.g., link.
117 18 EXDEV Cross-device link
119 A hard link to a file on another device was attempted.
120 19 ENODEV No such device
122 An attempt was made to apply an inappropriate system call to a
123 device, e.g., to read a write-only device, or the device is not
124 configured by the system.
125 20 ENOTDIR Not a directory
127 A non-directory was specified where a directory is required, for
128 example, in a path name or as an argument to chdir.
129 21 EISDIR Is a directory
131 An attempt to write on a directory.
132 22 EINVAL Invalid argument
134 Some invalid argument: dismounting a non-mounted device, men‐
135 tioning an unknown signal in signal, or some other argument
136 inappropriate for the call. Also set by math functions, (see
137 math(3)).
138 23 ENFILE File table overflow
140 The system's table of open files is full, and temporarily no
141 more opens can be accepted.
142 24 EMFILE Too many open files
144 As released, the limit on the number of open files per process
145 is 64. Getdtablesize(2) will obtain the current limit. Custom‐
146 ary configuration limit on most other UNIX systems is 20 per
147 process.
148 25 ENOTTY Inappropriate ioctl for device
150 The file mentioned in an ioctl is not a terminal or one of the
151 devices to which this call applies.
152 26 ETXTBSY Text file busy
154 An attempt to execute a pure-procedure program that is currently
155 open for writing. Also an attempt to open for writing a pure-
156 procedure program that is being executed.
157 27 EFBIG File too large
159 The size of a file exceeded the maximum (about 2.1E9 bytes).
160 28 ENOSPC No space left on device
162 A write to an ordinary file, the creation of a directory or sym‐
163 bolic link, or the creation of a directory entry failed because
164 no more disk blocks are available on the file system, or the
165 allocation of an inode for a newly created file failed because
166 no more inodes are available on the file system.
167 29 ESPIPE Illegal seek
169 An lseek was issued to a socket or pipe. This error may also be
170 issued for other non-seekable devices.
171 30 EROFS Read-only file system
173 An attempt to modify a file or directory was made on a device
174 mounted read-only.
175 31 EMLINK Too many links
177 An attempt to make more than 32767 hard links to a file.
178 32 EPIPE Broken pipe
180 A write on a pipe or socket for which there is no process to
181 read the data. This condition normally generates a signal; the
182 error is returned if the signal is caught or ignored.
183 33 EDOM Argument too large
185 The argument of a function in the math package (3M) is out of
186 the domain of the function.
187 34 ERANGE Result too large
189 The value of a function in the math package (3M) is unrepre‐
190 sentable within machine precision.
191 35 EWOULDBLOCK Operation would block
193 An operation that would cause a process to block was attempted
194 on an object in non-blocking mode (see fcntl(2)).
195 36 EINPROGRESS Operation now in progress
197 An operation that takes a long time to complete (such as a con‐
198 nect(2)) was attempted on a non-blocking object (see fcntl(2)).
199 37 EALREADY Operation already in progress
201 An operation was attempted on a non-blocking object that already
202 had an operation in progress.
203 38 ENOTSOCK Socket operation on non-socket
205 Self-explanatory.
206 39 EDESTADDRREQ Destination address required
208 A required address was omitted from an operation on a socket.
209 40 EMSGSIZE Message too long
211 A message sent on a socket was larger than the internal message
212 buffer or some other network limit.
213 41 EPROTOTYPE Protocol wrong type for socket
215 A protocol was specified that does not support the semantics of
216 the socket type requested. For example, you cannot use the ARPA
217 Internet UDP protocol with type SOCK_STREAM.
218 42 ENOPROTOOPT Option not supported by protocol
220 A bad option or level was specified in a getsockopt(2) or set‐
221 sockopt(2) call.
222 43 EPROTONOSUPPORT Protocol not supported
224 The protocol has not been configured into the system or no
225 implementation for it exists.
226 44 ESOCKTNOSUPPORT Socket type not supported
228 The support for the socket type has not been configured into the
229 system or no implementation for it exists.
230 45 EOPNOTSUPP Operation not supported on socket
232 For example, trying to accept a connection on a datagram socket.
233 46 EPFNOSUPPORT Protocol family not supported
235 The protocol family has not been configured into the system or
236 no implementation for it exists.
237 47 EAFNOSUPPORT Address family not supported by protocol family
239 An address incompatible with the requested protocol was used.
240 For example, you shouldn't necessarily expect to be able to use
241 NS addresses with ARPA Internet protocols.
242 48 EADDRINUSE Address already in use
244 Only one usage of each address is normally permitted.
245 49 EADDRNOTAVAIL Can't assign requested address
247 Normally results from an attempt to create a socket with an
248 address not on this machine.
249 50 ENETDOWN Network is down
251 A socket operation encountered a dead network.
252 51 ENETUNREACH Network is unreachable
254 A socket operation was attempted to an unreachable network.
255 52 ENETRESET Network dropped connection on reset
257 The host you were connected to crashed and rebooted.
258 53 ECONNABORTED Software caused connection abort
260 A connection abort was caused internal to your host machine.
261 54 ECONNRESET Connection reset by peer
263 A connection was forcibly closed by a peer. This normally
264 results from a loss of the connection on the remote socket due
265 to a timeout or a reboot.
266 55 ENOBUFS No buffer space available
268 An operation on a socket or pipe was not performed because the
269 system lacked sufficient buffer space or because a queue was
270 full.
271 56 EISCONN Socket is already connected
273 A connect request was made on an already connected socket; or, a
274 sendto or sendmsg request on a connected socket specified a des‐
275 tination when already connected.
276 57 ENOTCONN Socket is not connected
278 An request to send or receive data was disallowed because the
279 socket is not connected and (when sending on a datagram socket)
280 no address was supplied.
281 58 ESHUTDOWN Can't send after socket shutdown
283 A request to send data was disallowed because the socket had
284 already been shut down with a previous shutdown(2) call.
285 59 unused
287 60 ETIMEDOUT Connection timed out
289 A connect or send request failed because the connected party did
290 not properly respond after a period of time. (The timeout
291 period is dependent on the communication protocol.)
292 61 ECONNREFUSED Connection refused
294 No connection could be made because the target machine actively
295 refused it. This usually results from trying to connect to a
296 service that is inactive on the foreign host.
297 62 ELOOP Too many levels of symbolic links
299 A path name lookup involved more than 8 symbolic links.
300 63 ENAMETOOLONG File name too long
302 A component of a path name exceeded 255 (MAXNAMELEN) characters,
303 or an entire path name exceeded 1023 (MAXPATHLEN-1) characters.
304 64 EHOSTDOWN Host is down
306 A socket operation failed because the destination host was down.
307 65 EHOSTUNREACH Host is unreachable
309 A socket operation was attempted to an unreachable host.
310 66 ENOTEMPTY Directory not empty
312 A directory with entries other than “.” and “..” was supplied
313 to a remove directory or rename call.
314 69 EDQUOT Disc quota exceeded
316 A write to an ordinary file, the creation of a directory or sym‐
317 bolic link, or the creation of a directory entry failed because
318 the user's quota of disk blocks was exhausted, or the allocation
319 of an inode for a newly created file failed because the user's
320 quota of inodes was exhausted.
321
323 Process ID
324 Each active process in the system is uniquely identified by a pos‐
325 itive integer called a process ID. The range of this ID is from 0
326 to 30000.
327 Parent process ID
329 A new process is created by a currently active process; (see
330 fork(2)). The parent process ID of a process is the process ID of
331 its creator.
332 Process Group ID
334 Each active process is a member of a process group that is identi‐
335 fied by a positive integer called the process group ID. This is
336 the process ID of the group leader. This grouping permits the
337 signaling of related processes (see killpg(2)) and the job control
338 mechanisms of csh(1).
339 Tty Group ID
341 Each active process can be a member of a terminal group that is
342 identified by a positive integer called the tty group ID. This
343 grouping is used to arbitrate between multiple jobs contending for
344 the same terminal; (see csh(1) and tty(4)).
345 Real User ID and Real Group ID
347 Each user on the system is identified by a positive integer termed
348 the real user ID.
349 Each user is also a member of one or more groups. One of these
351 groups is distinguished from others and used in implementing
352 accounting facilities. The positive integer corresponding to this
353 distinguished group is termed the real group ID.
354 All processes have a real user ID and real group ID. These are
356 initialized from the equivalent attributes of the process that
357 created it.
358 Effective User Id, Effective Group Id, and Access Groups
360 Access to system resources is governed by three values: the effec‐
361 tive user ID, the effective group ID, and the group access list.
362 The effective user ID and effective group ID are initially the
364 process's real user ID and real group ID respectively. Either may
365 be modified through execution of a set-user-ID or set-group-ID
366 file (possibly by one its ancestors) (see execve(2)).
367 The group access list is an additional set of group ID's used only
369 in determining resource accessibility. Access checks are per‐
370 formed as described below in ``File Access Permissions''.
371 Super-user
373 A process is recognized as a super-user process and is granted
374 special privileges if its effective user ID is 0.
375 Special Processes
377 The processes with a process ID's of 0, 1, and 2 are special.
378 Process 0 is the scheduler. Process 1 is the initialization
379 process init, and is the ancestor of every other process in the
380 system. It is used to control the process structure. Process 2
381 is the paging daemon.
382 Descriptor
384 An integer assigned by the system when a file is referenced by
385 open(2) or dup(2), or when a socket is created by pipe(2),
386 socket(2) or socketpair(2), which uniquely identifies an access
387 path to that file or socket from a given process or any of its
388 children.
389 File Name
391 Names consisting of up to 255 (MAXNAMELEN) characters may be used
392 to name an ordinary file, special file, or directory.
393 These characters may be selected from the set of all ASCII charac‐
395 ter excluding 0 (null) and the ASCII code for / (slash). (The
396 parity bit, bit 8, must be 0.)
397 Note that it is generally unwise to use *, ?, [ or ] as part of
399 file names because of the special meaning attached to these char‐
400 acters by the shell.
401 Path Name
403 A path name is a null-terminated character string starting with an
404 optional slash (/), followed by zero or more directory names sepa‐
405 rated by slashes, optionally followed by a file name. The total
406 length of a path name must be less than 1024 (MAXPATHLEN) charac‐
407 ters.
408 If a path name begins with a slash, the path search begins at the
410 root directory. Otherwise, the search begins from the current
411 working directory. A slash by itself names the root directory. A
412 null pathname refers to the current directory.
413 Directory
415 A directory is a special type of file that contains entries that
416 are references to other files. Directory entries are called
417 links. By convention, a directory contains at least two links, .
418 and .., referred to as dot and dot-dot respectively. Dot refers
419 to the directory itself and dot-dot refers to its parent direc‐
420 tory.
421 Root Directory and Current Working Directory
423 Each process has associated with it a concept of a root directory
424 and a current working directory for the purpose of resolving path
425 name searches. A process's root directory need not be the root
426 directory of the root file system.
427 File Access Permissions
429 Every file in the file system has a set of access permissions.
430 These permissions are used in determining whether a process may
431 perform a requested operation on the file (such as opening a file
432 for writing). Access permissions are established at the time a
433 file is created. They may be changed at some later time through
434 the chmod(2) call.
435 File access is broken down according to whether a file may be:
437 read, written, or executed. Directory files use the execute per‐
438 mission to control if the directory may be searched.
439 File access permissions are interpreted by the system as they
441 apply to three different classes of users: the owner of the file,
442 those users in the file's group, anyone else. Every file has an
443 independent set of access permissions for each of these classes.
444 When an access check is made, the system decides if permission
445 should be granted by checking the access information applicable to
446 the caller.
447 Read, write, and execute/search permissions on a file are granted
449 to a process if:
450 The process's effective user ID is that of the super-user.
452 The process's effective user ID matches the user ID of the owner
454 of the file and the owner permissions allow the access.
455 The process's effective user ID does not match the user ID of the
457 owner of the file, and either the process's effective group ID
458 matches the group ID of the file, or the group ID of the file is
459 in the process's group access list, and the group permissions
460 allow the access.
461 Neither the effective user ID nor effective group ID and group
463 access list of the process match the corresponding user ID and
464 group ID of the file, but the permissions for ``other users''
465 allow access.
466 Otherwise, permission is denied.
468 Sockets and Address Families
470 A socket is an endpoint for communication between processes. Each
472 socket has queues for sending and receiving data.
473 Sockets are typed according to their communications properties.
475 These properties include whether messages sent and received at a
476 socket require the name of the partner, whether communication is
477 reliable, the format used in naming message recipients, etc.
478 Each instance of the system supports some collection of socket
480 types; consult socket(2) for more information about the types
481 available and their properties.
482 Each instance of the system supports some number of sets of commu‐
484 nications protocols. Each protocol set supports addresses of a
485 certain format. An Address Family is the set of addresses for a
486 specific group of protocols. Each socket has an address chosen
487 from the address family in which the socket was created.
488
490 intro(3), perror(3)
4914th Berkeley Distribution May 23, 1986 INTRO(2)