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1 | @node System Management, System Configuration, Users and Groups, Top |
2 | @c %MENU% Controlling the system and getting information about it | |
3 | @chapter System Management | |
4 | ||
5 | This chapter describes facilities for controlling the system that | |
6 | underlies a process (including the operating system and hardware) and | |
7 | for getting information about it. Anyone can generally use the | |
8 | informational facilities, but usually only a properly privileged process | |
9 | can make changes. | |
28f540f4 | 10 | |
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11 | |
12 | @menu | |
13 | * Host Identification:: Determining the name of the machine. | |
99a20616 UD |
14 | * Platform Type:: Determining operating system and basic |
15 | machine type | |
faf2289f | 16 | * Filesystem Handling:: Controlling/querying mounts |
4b9a6d7c | 17 | * System Parameters:: Getting and setting various system parameters |
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18 | @end menu |
19 | ||
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20 | To get information on parameters of the system that are built into the |
21 | system, such as the maximum length of a filename, @ref{System | |
22 | Configuration}. | |
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23 | |
24 | @node Host Identification | |
25 | @section Host Identification | |
26 | ||
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27 | This section explains how to identify the particular system on which your |
28 | program is running. First, let's review the various ways computer systems | |
29 | are named, which is a little complicated because of the history of the | |
30 | development of the Internet. | |
31 | ||
32 | Every Unix system (also known as a host) has a host name, whether it's | |
33 | connected to a network or not. In its simplest form, as used before | |
34 | computer networks were an issue, it's just a word like @samp{chicken}. | |
35 | @cindex host name | |
36 | ||
37 | But any system attached to the Internet or any network like it conforms | |
38 | to a more rigorous naming convention as part of the Domain Name System | |
39 | (DNS). In DNS, every host name is composed of two parts: | |
40 | @cindex DNS | |
41 | @cindex Domain Name System | |
42 | ||
43 | @enumerate | |
44 | @item | |
68979757 | 45 | hostname |
4b9a6d7c UD |
46 | @cindex hostname |
47 | @item | |
48 | domain name | |
49 | @cindex domain name | |
50 | @end enumerate | |
51 | ||
52 | You will note that ``hostname'' looks a lot like ``host name'', but is | |
53 | not the same thing, and that people often incorrectly refer to entire | |
54 | host names as ``domain names.'' | |
55 | ||
68979757 | 56 | In DNS, the full host name is properly called the FQDN (Fully Qualified |
4b9a6d7c UD |
57 | Domain Name) and consists of the hostname, then a period, then the |
58 | domain name. The domain name itself usually has multiple components | |
59 | separated by periods. So for example, a system's hostname may be | |
68979757 | 60 | @samp{chicken} and its domain name might be @samp{ai.mit.edu}, so |
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61 | its FQDN (which is its host name) is @samp{chicken.ai.mit.edu}. |
62 | @cindex FQDN | |
63 | ||
64 | Adding to the confusion, though, is that DNS is not the only name space | |
68979757 | 65 | in which a computer needs to be known. Another name space is the |
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66 | NIS (aka YP) name space. For NIS purposes, there is another domain |
67 | name, which is called the NIS domain name or the YP domain name. It | |
68 | need not have anything to do with the DNS domain name. | |
69 | @cindex YP | |
70 | @cindex NIS | |
71 | @cindex NIS domain name | |
72 | @cindex YP domain name | |
73 | ||
74 | Confusing things even more is the fact that in DNS, it is possible for | |
75 | multiple FQDNs to refer to the same system. However, there is always | |
76 | exactly one of them that is the true host name, and it is called the | |
68979757 | 77 | canonical FQDN. |
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78 | |
79 | In some contexts, the host name is called a ``node name.'' | |
80 | ||
81 | For more information on DNS host naming, @xref{Host Names}. | |
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82 | |
83 | @pindex hostname | |
84 | @pindex hostid | |
85 | @pindex unistd.h | |
68979757 | 86 | Prototypes for these functions appear in @file{unistd.h}. |
4b9a6d7c UD |
87 | |
88 | The programs @code{hostname}, @code{hostid}, and @code{domainname} work | |
89 | by calling these functions. | |
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90 | |
91 | @comment unistd.h | |
92 | @comment BSD | |
93 | @deftypefun int gethostname (char *@var{name}, size_t @var{size}) | |
4b9a6d7c UD |
94 | This function returns the host name of the system on which it is called, |
95 | in the array @var{name}. The @var{size} argument specifies the size of | |
96 | this array, in bytes. Note that this is @emph{not} the DNS hostname. | |
97 | If the system participates in DNS, this is the FQDN (see above). | |
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98 | |
99 | The return value is @code{0} on success and @code{-1} on failure. In | |
100 | the GNU C library, @code{gethostname} fails if @var{size} is not large | |
101 | enough; then you can try again with a larger array. The following | |
102 | @code{errno} error condition is defined for this function: | |
103 | ||
104 | @table @code | |
105 | @item ENAMETOOLONG | |
106 | The @var{size} argument is less than the size of the host name plus one. | |
107 | @end table | |
108 | ||
109 | @pindex sys/param.h | |
110 | On some systems, there is a symbol for the maximum possible host name | |
111 | length: @code{MAXHOSTNAMELEN}. It is defined in @file{sys/param.h}. | |
112 | But you can't count on this to exist, so it is cleaner to handle | |
113 | failure and try again. | |
114 | ||
115 | @code{gethostname} stores the beginning of the host name in @var{name} | |
116 | even if the host name won't entirely fit. For some purposes, a | |
117 | truncated host name is good enough. If it is, you can ignore the | |
118 | error code. | |
119 | @end deftypefun | |
120 | ||
121 | @comment unistd.h | |
122 | @comment BSD | |
123 | @deftypefun int sethostname (const char *@var{name}, size_t @var{length}) | |
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124 | The @code{sethostname} function sets the host name of the system that |
125 | calls it to @var{name}, a string with length @var{length}. Only | |
126 | privileged processes are permitted to do this. | |
127 | ||
128 | Usually @code{sethostname} gets called just once, at system boot time. | |
129 | Often, the program that calls it sets it to the value it finds in the | |
130 | file @code{/etc/hostname}. | |
131 | @cindex /etc/hostname | |
132 | ||
133 | Be sure to set the host name to the full host name, not just the DNS | |
134 | hostname (see above). | |
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135 | |
136 | The return value is @code{0} on success and @code{-1} on failure. | |
137 | The following @code{errno} error condition is defined for this function: | |
138 | ||
139 | @table @code | |
140 | @item EPERM | |
141 | This process cannot set the host name because it is not privileged. | |
142 | @end table | |
143 | @end deftypefun | |
144 | ||
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145 | @comment unistd.h |
146 | @comment ??? | |
147 | @deftypefun int getdomainnname (char *@var{name}, size_t @var{length}) | |
148 | @cindex NIS domain name | |
149 | @cindex YP domain name | |
150 | ||
151 | @code{getdomainname} returns the NIS (aka YP) domain name of the system | |
152 | on which it is called. Note that this is not the more popular DNS | |
153 | domain name. Get that with @code{gethostname}. | |
154 | ||
155 | The specifics of this function are analogous to @code{gethostname}, above. | |
156 | ||
157 | @end deftypefun | |
158 | ||
159 | @comment unistd.h | |
160 | @comment ??? | |
87b56f36 | 161 | @deftypefun int setdomainname (const char *@var{name}, size_t @var{length}) |
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162 | @cindex NIS domain name |
163 | @cindex YP domain name | |
164 | ||
165 | @code{getdomainname} sets the NIS (aka YP) domain name of the system | |
166 | on which it is called. Note that this is not the more popular DNS | |
167 | domain name. Set that with @code{sethostname}. | |
168 | ||
169 | The specifics of this function are analogous to @code{sethostname}, above. | |
170 | ||
171 | @end deftypefun | |
172 | ||
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173 | @comment unistd.h |
174 | @comment BSD | |
175 | @deftypefun {long int} gethostid (void) | |
176 | This function returns the ``host ID'' of the machine the program is | |
4b9a6d7c | 177 | running on. By convention, this is usually the primary Internet IP address |
04b9968b | 178 | of that machine, converted to a @w{@code{long int}}. However, on some |
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179 | systems it is a meaningless but unique number which is hard-coded for |
180 | each machine. | |
4b9a6d7c UD |
181 | |
182 | This is not widely used. It arose in BSD 4.2, but was dropped in BSD 4.4. | |
183 | It is not required by POSIX. | |
184 | ||
185 | The proper way to query the IP address is to use @code{gethostbyname} | |
186 | on the results of @code{gethostname}. For more information on IP addresses, | |
187 | @xref{Host Addresses}. | |
28f540f4 RM |
188 | @end deftypefun |
189 | ||
190 | @comment unistd.h | |
191 | @comment BSD | |
192 | @deftypefun int sethostid (long int @var{id}) | |
193 | The @code{sethostid} function sets the ``host ID'' of the host machine | |
4b9a6d7c | 194 | to @var{id}. Only privileged processes are permitted to do this. Usually |
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195 | it happens just once, at system boot time. |
196 | ||
4b9a6d7c | 197 | The proper way to establish the primary IP address of a system |
68979757 | 198 | is to configure the IP address resolver to associate that IP address with |
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199 | the system's host name as returned by @code{gethostname}. For example, |
200 | put a record for the system in @file{/etc/hosts}. | |
201 | ||
202 | See @code{gethostid} above for more information on host ids. | |
203 | ||
28f540f4 | 204 | The return value is @code{0} on success and @code{-1} on failure. |
04b9968b | 205 | The following @code{errno} error conditions are defined for this function: |
28f540f4 RM |
206 | |
207 | @table @code | |
208 | @item EPERM | |
209 | This process cannot set the host name because it is not privileged. | |
210 | ||
211 | @item ENOSYS | |
212 | The operating system does not support setting the host ID. On some | |
213 | systems, the host ID is a meaningless but unique number hard-coded for | |
214 | each machine. | |
215 | @end table | |
216 | @end deftypefun | |
217 | ||
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218 | @node Platform Type |
219 | @section Platform Type Identification | |
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220 | |
221 | You can use the @code{uname} function to find out some information about | |
222 | the type of computer your program is running on. This function and the | |
223 | associated data type are declared in the header file | |
224 | @file{sys/utsname.h}. | |
225 | @pindex sys/utsname.h | |
226 | ||
68979757 | 227 | As a bonus, @code{uname} also gives some information identifying the |
4b9a6d7c | 228 | particular system your program is running on. This is the same information |
68979757 | 229 | which you can get with functions targetted to this purpose described in |
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230 | @ref{Host Identification}. |
231 | ||
232 | ||
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233 | @comment sys/utsname.h |
234 | @comment POSIX.1 | |
235 | @deftp {Data Type} {struct utsname} | |
236 | The @code{utsname} structure is used to hold information returned | |
237 | by the @code{uname} function. It has the following members: | |
238 | ||
239 | @table @code | |
240 | @item char sysname[] | |
241 | This is the name of the operating system in use. | |
242 | ||
28f540f4 RM |
243 | @item char release[] |
244 | This is the current release level of the operating system implementation. | |
245 | ||
246 | @item char version[] | |
247 | This is the current version level within the release of the operating | |
248 | system. | |
249 | ||
250 | @item char machine[] | |
251 | This is a description of the type of hardware that is in use. | |
252 | ||
253 | Some systems provide a mechanism to interrogate the kernel directly for | |
254 | this information. On systems without such a mechanism, the GNU C | |
255 | library fills in this field based on the configuration name that was | |
256 | specified when building and installing the library. | |
257 | ||
258 | GNU uses a three-part name to describe a system configuration; the three | |
259 | parts are @var{cpu}, @var{manufacturer} and @var{system-type}, and they | |
260 | are separated with dashes. Any possible combination of three names is | |
261 | potentially meaningful, but most such combinations are meaningless in | |
262 | practice and even the meaningful ones are not necessarily supported by | |
263 | any particular GNU program. | |
264 | ||
265 | Since the value in @code{machine} is supposed to describe just the | |
266 | hardware, it consists of the first two parts of the configuration name: | |
267 | @samp{@var{cpu}-@var{manufacturer}}. For example, it might be one of these: | |
268 | ||
269 | @quotation | |
37742e84 | 270 | @code{"sparc-sun"}, |
28f540f4 | 271 | @code{"i386-@var{anything}"}, |
37742e84 | 272 | @code{"m68k-hp"}, |
28f540f4 RM |
273 | @code{"m68k-sony"}, |
274 | @code{"m68k-sun"}, | |
275 | @code{"mips-dec"} | |
276 | @end quotation | |
4b9a6d7c UD |
277 | |
278 | @item char nodename[] | |
279 | This is the host name of this particular computer. In the GNU C | |
280 | library, the value is the same as that returned by @code{gethostname}; | |
281 | see @ref{Host Identification}. | |
282 | ||
283 | @ gethostname() is implemented with a call to uname(). | |
284 | ||
285 | @item char domainname[] | |
286 | This is the NIS or YP domain name. It is the same value returned by | |
68979757 | 287 | @code{getdomainname}; see @ref{Host Identification}. This element |
4b9a6d7c UD |
288 | is a relatively recent invention and use of it is not as portable as |
289 | use of the rest of the structure. | |
290 | ||
291 | @c getdomainname() is implemented with a call to uname(). | |
292 | ||
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293 | @end table |
294 | @end deftp | |
295 | ||
296 | @comment sys/utsname.h | |
297 | @comment POSIX.1 | |
298 | @deftypefun int uname (struct utsname *@var{info}) | |
299 | The @code{uname} function fills in the structure pointed to by | |
300 | @var{info} with information about the operating system and host machine. | |
301 | A non-negative value indicates that the data was successfully stored. | |
302 | ||
303 | @code{-1} as the value indicates an error. The only error possible is | |
304 | @code{EFAULT}, which we normally don't mention as it is always a | |
305 | possibility. | |
306 | @end deftypefun | |
37742e84 UD |
307 | |
308 | ||
faf2289f | 309 | @node Filesystem Handling |
99a20616 | 310 | @section Controlling and Querying Mounts |
faf2289f UD |
311 | |
312 | All files are in filesystems, and before you can access any file, its | |
313 | filesystem must be mounted. Because of Unix's concept of | |
314 | @emph{Everything is a file}, mounting of filesystems is central to doing | |
315 | almost anything. This section explains how to find out what filesystems | |
316 | are currently mounted and what filesystems are available for mounting, | |
317 | and how to change what is mounted. | |
318 | ||
319 | The classic filesystem is the contents of a disk drive. The concept is | |
320 | considerably more abstract, though, and lots of things other than disk | |
dbacafe5 | 321 | drives can be mounted. |
faf2289f UD |
322 | |
323 | Some block devices don't correspond to traditional devices like disk | |
324 | drives. For example, a loop device is a block device whose driver uses | |
325 | a regular file in another filesystem as its medium. So if that regular | |
326 | file contains appropriate data for a filesystem, you can by mounting the | |
327 | loop device essentially mount a regular file. | |
328 | ||
329 | Some filesystems aren't based on a device of any kind. The ``proc'' | |
330 | filesystem, for example, contains files whose data is made up by the | |
331 | filesystem driver on the fly whenever you ask for it. And when you | |
332 | write to it, the data you write causes changes in the system. No data | |
333 | gets stored. | |
334 | ||
335 | @c It would be good to mention NFS mounts here. | |
336 | ||
337 | @menu | |
338 | * Mount Information:: What is or could be mounted? | |
99a20616 | 339 | * Mount-Unmount-Remount:: Controlling what is mounted and how |
faf2289f | 340 | @end menu |
37742e84 | 341 | |
99a20616 | 342 | @node Mount Information, Mount-Unmount-Remount, , Filesystem Handling |
f126ef67 | 343 | @subsection Mount Information |
faf2289f UD |
344 | |
345 | For some programs it is desirable and necessary to access information | |
346 | about whether a certain filesystem is mounted and, if it is, where, or | |
347 | simply to get lists of all the available filesystems. The GNU libc | |
348 | provides some functions to retrieve this information portably. | |
37742e84 UD |
349 | |
350 | Traditionally Unix systems have a file named @file{/etc/fstab} which | |
351 | describes all possibly mounted filesystems. The @code{mount} program | |
352 | uses this file to mount at startup time of the system all the necessary | |
353 | filesystems. The information about all the filesystems actually mounted | |
c756c71c UD |
354 | is normally kept in a file named @file{/etc/mtab}. Both files share |
355 | the same syntax and it is crucial that this syntax is followed all the | |
37742e84 UD |
356 | time. Therefore it is best to never directly write the files. The |
357 | functions described in this section can do this and they also provide | |
358 | the functionality to convert the external textual representation to the | |
359 | internal representation. | |
360 | ||
faf2289f UD |
361 | Note that the @file{fstab} and @file{mtab} files are maintained on a |
362 | system by @emph{convention}. It is possible for the files not to exist | |
363 | or not to be consistent with what is really mounted or available to | |
364 | mount, if the system's administration policy allows it. But programs | |
365 | that mount and unmount filesystems typically maintain and use these | |
366 | files as described herein. | |
367 | ||
37742e84 UD |
368 | @vindex _PATH_FSTAB |
369 | @vindex _PATH_MNTTAB | |
370 | @vindex FSTAB | |
371 | @vindex _PATH_MOUNTED | |
372 | The filenames given above should never be used directly. The portable | |
373 | way to handle these file is to use the macros @code{_PATH_FSTAB}, | |
c756c71c | 374 | defined in @file{fstab.h} and @code{_PATH_MNTTAB}, defined in |
37742e84 UD |
375 | @file{mntent.h}, respectively. There are also two alternate macro names |
376 | @code{FSTAB} and @code{_PATH_MOUNTED} defined but both names are | |
c756c71c UD |
377 | deprecated and kept only for backward compatibility. The two former |
378 | names should always be used. | |
37742e84 | 379 | |
faf2289f UD |
380 | @menu |
381 | * fstab:: The @file{fstab} file | |
382 | * mtab:: The @file{mtab} file | |
383 | * Other Mount Information:: Other (non-libc) sources of mount information | |
384 | @end menu | |
385 | ||
386 | @node fstab | |
f126ef67 | 387 | @subsubsection The @file{fstab} file |
faf2289f | 388 | |
37742e84 UD |
389 | The internal representation for entries of the file is @w{@code{struct |
390 | fstab}}, defined in @file{fstab.h}. | |
391 | ||
392 | @comment fstab.h | |
393 | @comment BSD | |
394 | @deftp {Data Type} {struct fstab} | |
395 | This structure is used with the @code{getfsent}, @code{getfsspec}, and | |
396 | @code{getfsfile} functions. | |
397 | ||
398 | @table @code | |
399 | @item char *fs_spec | |
fed8f7f7 | 400 | This element describes the device from which the filesystem is mounted. |
37742e84 UD |
401 | Normally this is the name of a special device, such as a hard disk |
402 | partition, but it could also be a more or less generic string. For | |
403 | @dfn{NFS} it would be a hostname and directory name combination. | |
404 | ||
405 | Even though the element is not declared @code{const} it shouldn't be | |
406 | modified. The missing @code{const} has historic reasons, since this | |
407 | function predates @w{ISO C}. The same is true for the other string | |
408 | elements of this structure. | |
409 | ||
410 | @item char *fs_file | |
fed8f7f7 UD |
411 | This describes the mount point on the local system. I.e., accessing any |
412 | file in this filesystem has implicitly or explicitly this string as a | |
37742e84 UD |
413 | prefix. |
414 | ||
415 | @item char *fs_vfstype | |
416 | This is the type of the filesystem. Depending on what the underlying | |
417 | kernel understands it can be any string. | |
418 | ||
419 | @item char *fs_mntops | |
420 | This is a string containing options passed to the kernel with the | |
421 | @code{mount} call. Again, this can be almost anything. There can be | |
422 | more than one option, separated from the others by a comma. Each option | |
423 | consists of a name and an optional value part, introduced by an @code{=} | |
424 | character. | |
425 | ||
04b9968b | 426 | If the value of this element must be processed it should ideally be done |
37742e84 UD |
427 | using the @code{getsubopt} function; see @ref{Suboptions}. |
428 | ||
429 | @item const char *fs_type | |
c756c71c | 430 | This name is poorly chosen. This element points to a string (possibly |
37742e84 UD |
431 | in the @code{fs_mntops} string) which describes the modes with which the |
432 | filesystem is mounted. @file{fstab} defines five macros to describe the | |
433 | possible values: | |
434 | ||
435 | @vtable @code | |
436 | @item FSTAB_RW | |
437 | The filesystems gets mounted with read and write enabled. | |
438 | @item FSTAB_RQ | |
439 | The filesystems gets mounted with read and write enabled. Write access | |
440 | is restricted by quotas. | |
441 | @item FSTAB_RO | |
c756c71c | 442 | The filesystem gets mounted read-only. |
37742e84 | 443 | @item FSTAB_SW |
c756c71c | 444 | This is not a real filesystem, it is a swap device. |
37742e84 UD |
445 | @item FSTAB_XX |
446 | This entry from the @file{fstab} file is totally ignored. | |
447 | @end vtable | |
448 | ||
449 | Testing for equality with these value must happen using @code{strcmp} | |
c756c71c | 450 | since these are all strings. Comparing the pointer will probably always |
37742e84 UD |
451 | fail. |
452 | ||
453 | @item int fs_freq | |
454 | This element describes the dump frequency in days. | |
455 | ||
456 | @item int fs_passno | |
457 | This element describes the pass number on parallel dumps. It is closely | |
c756c71c | 458 | related to the @code{dump} utility used on Unix systems. |
37742e84 UD |
459 | @end table |
460 | @end deftp | |
461 | ||
462 | ||
463 | To read the entire content of the of the @file{fstab} file the GNU libc | |
464 | contains a set of three functions which are designed in the usual way. | |
465 | ||
466 | @comment fstab.h | |
467 | @comment BSD | |
468 | @deftypefun int setfsent (void) | |
469 | This function makes sure that the internal read pointer for the | |
470 | @file{fstab} file is at the beginning of the file. This is done by | |
471 | either opening the file or resetting the read pointer. | |
472 | ||
473 | Since the file handle is internal to the libc this function is not | |
474 | thread-safe. | |
475 | ||
476 | This function returns a non-zero value if the operation was successful | |
477 | and the @code{getfs*} functions can be used to read the entries of the | |
478 | file. | |
479 | @end deftypefun | |
480 | ||
481 | @comment fstab.h | |
482 | @comment BSD | |
483 | @deftypefun void endfsent (void) | |
484 | This function makes sure that all resources acquired by a prior call to | |
c756c71c | 485 | @code{setfsent} (explicitly or implicitly by calling @code{getfsent}) are |
37742e84 UD |
486 | freed. |
487 | @end deftypefun | |
488 | ||
489 | @comment fstab.h | |
490 | @comment BSD | |
491 | @deftypefun {struct fstab *} getfsent (void) | |
492 | This function returns the next entry of the @file{fstab} file. If this | |
493 | is the first call to any of the functions handling @file{fstab} since | |
494 | program start or the last call of @code{endfsent}, the file will be | |
495 | opened. | |
496 | ||
04b9968b | 497 | The function returns a pointer to a variable of type @code{struct |
37742e84 UD |
498 | fstab}. This variable is shared by all threads and therefore this |
499 | function is not thread-safe. If an error occurred @code{getfsent} | |
c756c71c | 500 | returns a @code{NULL} pointer. |
37742e84 UD |
501 | @end deftypefun |
502 | ||
503 | @comment fstab.h | |
504 | @comment BSD | |
505 | @deftypefun {struct fstab *} getfsspec (const char *@var{name}) | |
506 | This function returns the next entry of the @file{fstab} file which has | |
507 | a string equal to @var{name} pointed to by the @code{fs_spec} element. | |
508 | Since there is normally exactly one entry for each special device it | |
509 | makes no sense to call this function more than once for the same | |
510 | argument. If this is the first call to any of the functions handling | |
511 | @file{fstab} since program start or the last call of @code{endfsent}, | |
512 | the file will be opened. | |
513 | ||
04b9968b | 514 | The function returns a pointer to a variable of type @code{struct |
37742e84 UD |
515 | fstab}. This variable is shared by all threads and therefore this |
516 | function is not thread-safe. If an error occurred @code{getfsent} | |
c756c71c | 517 | returns a @code{NULL} pointer. |
37742e84 UD |
518 | @end deftypefun |
519 | ||
520 | @comment fstab.h | |
521 | @comment BSD | |
522 | @deftypefun {struct fstab *} getfsfile (const char *@var{name}) | |
523 | This function returns the next entry of the @file{fstab} file which has | |
524 | a string equal to @var{name} pointed to by the @code{fs_file} element. | |
525 | Since there is normally exactly one entry for each mount point it | |
526 | makes no sense to call this function more than once for the same | |
527 | argument. If this is the first call to any of the functions handling | |
528 | @file{fstab} since program start or the last call of @code{endfsent}, | |
529 | the file will be opened. | |
530 | ||
04b9968b | 531 | The function returns a pointer to a variable of type @code{struct |
37742e84 UD |
532 | fstab}. This variable is shared by all threads and therefore this |
533 | function is not thread-safe. If an error occurred @code{getfsent} | |
c756c71c | 534 | returns a @code{NULL} pointer. |
37742e84 UD |
535 | @end deftypefun |
536 | ||
37742e84 | 537 | |
faf2289f | 538 | @node mtab |
f126ef67 | 539 | @subsubsection The @file{mtab} file |
faf2289f | 540 | The following functions and data structure access the @file{mtab} file. |
37742e84 | 541 | |
faf2289f | 542 | @comment mntent.h |
37742e84 UD |
543 | @comment BSD |
544 | @deftp {Data Type} {struct mntent} | |
545 | This structure is used with the @code{getmntent}, @code{getmntent_t}, | |
546 | @code{addmntent}, and @code{hasmntopt} functions. | |
547 | ||
548 | @table @code | |
549 | @item char *mnt_fsname | |
550 | This element contains a pointer to a string describing the name of the | |
551 | special device from which the filesystem is mounted. It corresponds to | |
552 | the @code{fs_spec} element in @code{struct fstab}. | |
553 | ||
554 | @item char *mnt_dir | |
555 | This element points to a string describing the mount point of the | |
556 | filesystem. It corresponds to the @code{fs_file} element in | |
557 | @code{struct fstab}. | |
558 | ||
559 | @item char *mnt_type | |
560 | @code{mnt_type} describes the filesystem type and is therefore | |
561 | equivalent to @code{fs_vfstype} in @code{struct fstab}. @file{mntent.h} | |
04b9968b UD |
562 | defines a few symbolic names for some of the values this string can have. |
563 | But since the kernel can support arbitrary filesystems it does not | |
37742e84 UD |
564 | make much sense to give them symbolic names. If one knows the symbol |
565 | name one also knows the filesystem name. Nevertheless here follows the | |
04b9968b | 566 | list of the symbols provided in @file{mntent.h}. |
37742e84 UD |
567 | |
568 | @vtable @code | |
569 | @item MNTTYPE_IGNORE | |
570 | This symbol expands to @code{"ignore"}. The value is sometime used in | |
571 | @file{fstab} files to make sure entries are not used without removing them. | |
572 | @item MNTTYPE_NFS | |
573 | Expands to @code{"nfs"}. Using this macro sometimes could make sense | |
574 | since it names the default NFS implementation, in case both version 2 | |
575 | and 3 are supported. | |
576 | @item MNTTYPE_SWAP | |
577 | This symbol expands to @code{"swap"}. It names the special @file{fstab} | |
578 | entry which names one of the possibly multiple swap partitions. | |
579 | @end vtable | |
580 | ||
581 | @item char *mnt_opts | |
582 | The element contains a string describing the options used while mounting | |
583 | the filesystem. As for the equivalent element @code{fs_mntops} of | |
584 | @code{struct fstab} it is best to use the function @code{getsubopt} | |
585 | (@pxref{Suboptions}) to access the parts of this string. | |
586 | ||
587 | The @file{mntent.h} file defines a number of macros with string values | |
588 | which correspond to some of the options understood by the kernel. There | |
04b9968b | 589 | might be many more options which are possible so it doesn't make much sense |
37742e84 UD |
590 | to rely on these macros but to be consistent here is the list: |
591 | ||
592 | @vtable @code | |
593 | @item MNTOPT_DEFAULTS | |
594 | Expands to @code{"defaults"}. This option should be used alone since it | |
49c091e5 | 595 | indicates all values for the customizable values are chosen to be the |
37742e84 UD |
596 | default. |
597 | @item MNTOPT_RO | |
fed8f7f7 | 598 | Expands to @code{"ro"}. See the @code{FSTAB_RO} value, it means the |
37742e84 UD |
599 | filesystem is mounted read-only. |
600 | @item MNTOPT_RW | |
fed8f7f7 | 601 | Expand to @code{"rw"}. See the @code{FSTAB_RW} value, it means the |
37742e84 UD |
602 | filesystem is mounted with read and write permissions. |
603 | @item MNTOPT_SUID | |
604 | Expands to @code{"suid"}. This means that the SUID bit (@pxref{How | |
605 | Change Persona}) is respected when a program from the filesystem is | |
606 | started. | |
607 | @item MNTOPT_NOSUID | |
608 | Expands to @code{"nosuid"}. This is the opposite of @code{MNTOPT_SUID}, | |
c756c71c | 609 | the SUID bit for all files from the filesystem is ignored. |
37742e84 UD |
610 | @item MNTOPT_NOAUTO |
611 | Expands to @code{"noauto"}. At startup time the @code{mount} program | |
612 | will ignore this entry if it is started with the @code{-a} option to | |
613 | mount all filesystems mentioned in the @file{fstab} file. | |
614 | @end vtable | |
615 | ||
616 | As for the @code{FSTAB_*} entries introduced above it is important to | |
617 | use @code{strcmp} to check for equality. | |
618 | ||
619 | @item mnt_freq | |
620 | This elements corresponds to @code{fs_freq} and also specifies the | |
621 | frequency in days in which dumps are made. | |
622 | ||
623 | @item mnt_passno | |
624 | This element is equivalent to @code{fs_passno} with the same meaning | |
625 | which is uninteresting for all programs beside @code{dump}. | |
626 | @end table | |
627 | @end deftp | |
628 | ||
629 | For accessing the @file{mtab} file there is again a set of three | |
630 | functions to access all entries in a row. Unlike the functions to | |
631 | handle @file{fstab} these functions do not access a fixed file and there | |
c756c71c UD |
632 | is even a thread safe variant of the get function. Beside this the GNU |
633 | libc contains functions to alter the file and test for specific options. | |
37742e84 UD |
634 | |
635 | @comment mntent.h | |
636 | @comment BSD | |
637 | @deftypefun {FILE *} setmntent (const char *@var{file}, const char *@var{mode}) | |
638 | The @code{setmntent} function prepares the file named @var{FILE} which | |
639 | must be in the format of a @file{fstab} and @file{mtab} file for the | |
640 | upcoming processing through the other functions of the family. The | |
641 | @var{mode} parameter can be chosen in the way the @var{opentype} | |
642 | parameter for @code{fopen} (@pxref{Opening Streams}) can be chosen. If | |
643 | the file is opened for writing the file is also allowed to be empty. | |
644 | ||
645 | If the file was successfully opened @code{setmntent} returns a file | |
646 | descriptor for future use. Otherwise the return value is @code{NULL} | |
647 | and @code{errno} is set accordingly. | |
648 | @end deftypefun | |
649 | ||
650 | @comment mntent.h | |
651 | @comment BSD | |
652 | @deftypefun int endmntent (FILE *@var{stream}) | |
653 | This function takes for the @var{stream} parameter a file handle which | |
654 | previously was returned from the @code{setmntent} call. | |
655 | @code{endmntent} closes the stream and frees all resources. | |
656 | ||
c756c71c UD |
657 | The return value is @math{1} unless an error occurred in which case it |
658 | is @math{0}. | |
37742e84 UD |
659 | @end deftypefun |
660 | ||
661 | @comment mntent.h | |
662 | @comment BSD | |
663 | @deftypefun {struct mntent *} getmntent (FILE *@var{stream}) | |
664 | The @code{getmntent} function takes as the parameter a file handle | |
665 | previously returned by successful call to @code{setmntent}. It returns | |
666 | a pointer to a static variable of type @code{struct mntent} which is | |
667 | filled with the information from the next entry from the file currently | |
668 | read. | |
669 | ||
c7f7281e UD |
670 | The file format used prescribes the use of spaces or tab characters to |
671 | separate the fields. This makes it harder to use name containing one of | |
672 | these characters (e.g., mount points using spaces). Therefore these | |
673 | characters are encoded in the files and the @code{getmntent} function | |
674 | takes care of the decoding while reading the entries back in. | |
675 | @code{'\040'} is used to encode a space character, @code{'\012'} to | |
676 | encode a tab character and @code{'\\'} to encode a backslash. | |
677 | ||
37742e84 UD |
678 | If there was an error or the end of the file is reached the return value |
679 | is @code{NULL}. | |
680 | ||
681 | This function is not thread-safe since all calls to this function return | |
682 | a pointer to the same static variable. @code{getmntent_r} should be | |
c756c71c | 683 | used in situations where multiple threads access the file. |
37742e84 UD |
684 | @end deftypefun |
685 | ||
686 | @comment mntent.h | |
687 | @comment BSD | |
688 | @deftypefun {struct mntent *} getmntent_r (FILE *@var{stream}, struct mentent *@var{result}, char *@var{buffer}, int @var{bufsize}) | |
689 | The @code{getmntent_r} function is the reentrant variant of | |
690 | @code{getmntent}. It also returns the next entry from the file and | |
691 | returns a pointer. The actual variable the values are stored in is not | |
692 | static, though. Instead the function stores the values in the variable | |
693 | pointed to by the @var{result} parameter. Additional information (e.g., | |
694 | the strings pointed to by the elements of the result) are kept in the | |
695 | buffer of size @var{bufsize} pointed to by @var{buffer}. | |
696 | ||
c7f7281e UD |
697 | Escaped characters (space, tab, backslash) are converted back in the |
698 | same way as it happens for @code{getmentent}. | |
699 | ||
c756c71c | 700 | The function returns a @code{NULL} pointer in error cases. Errors could be: |
37742e84 UD |
701 | @itemize @bullet |
702 | @item | |
703 | error while reading the file, | |
704 | @item | |
705 | end of file reached, | |
706 | @item | |
707 | @var{bufsize} is too small for reading a complete new entry. | |
708 | @end itemize | |
709 | @end deftypefun | |
710 | ||
711 | @comment mntent.h | |
712 | @comment BSD | |
713 | @deftypefun int addmntent (FILE *@var{stream}, const struct mntent *@var{mnt}) | |
04b9968b | 714 | The @code{addmntent} function allows adding a new entry to the file |
37742e84 UD |
715 | previously opened with @code{setmntent}. The new entries are always |
716 | appended. I.e., even if the position of the file descriptor is not at | |
c756c71c | 717 | the end of the file this function does not overwrite an existing entry |
37742e84 UD |
718 | following the current position. |
719 | ||
720 | The implication of this is that to remove an entry from a file one has | |
721 | to create a new file while leaving out the entry to be removed and after | |
722 | closing the file remove the old one and rename the new file to the | |
723 | chosen name. | |
724 | ||
c7f7281e UD |
725 | This function takes care of spaces and tab characters in the names to be |
726 | written to the file. It converts them and the backslash character into | |
727 | the format describe in the @code{getmntent} description above. | |
728 | ||
c756c71c UD |
729 | This function returns @math{0} in case the operation was successful. |
730 | Otherwise the return value is @math{1} and @code{errno} is set | |
37742e84 UD |
731 | appropriately. |
732 | @end deftypefun | |
733 | ||
734 | @comment mntent.h | |
735 | @comment BSD | |
736 | @deftypefun {char *} hasmntopt (const struct mntent *@var{mnt}, const char *@var{opt}) | |
737 | This function can be used to check whether the string pointed to by the | |
738 | @code{mnt_opts} element of the variable pointed to by @var{mnt} contains | |
739 | the option @var{opt}. If this is true a pointer to the beginning of the | |
740 | option in the @code{mnt_opts} element is returned. If no such option | |
c756c71c | 741 | exists the function returns @code{NULL}. |
37742e84 UD |
742 | |
743 | This function is useful to test whether a specific option is present but | |
744 | when all options have to be processed one is better off with using the | |
745 | @code{getsubopt} function to iterate over all options in the string. | |
746 | @end deftypefun | |
faf2289f UD |
747 | |
748 | @node Other Mount Information | |
f126ef67 | 749 | @subsubsection Other (Non-libc) Sources of Mount Information |
faf2289f UD |
750 | |
751 | On a system with a Linux kernel and the @code{proc} filesystem, you can | |
752 | get information on currently mounted filesystems from the file | |
753 | @file{mounts} in the @code{proc} filesystem. Its format is similar to | |
754 | that of the @file{mtab} file, but represents what is truly mounted | |
755 | without relying on facilities outside the kernel to keep @file{mtab} up | |
756 | to date. | |
757 | ||
758 | ||
99a20616 | 759 | @node Mount-Unmount-Remount, , Mount Information, Filesystem Handling |
f126ef67 | 760 | @subsection Mount, Unmount, Remount |
faf2289f UD |
761 | |
762 | This section describes the functions for mounting, unmounting, and | |
763 | remounting filesystems. | |
764 | ||
765 | Only the superuser can mount, unmount, or remount a filesystem. | |
766 | ||
767 | These functions do not access the @file{fstab} and @file{mtab} files. You | |
768 | should maintain and use these separately. @xref{Mount Information}. | |
769 | ||
770 | The symbols in this section are declared in @file{sys/mount.h}. | |
771 | ||
772 | @comment sys/mount.h | |
dbacafe5 | 773 | @comment SVID, BSD |
faf2289f UD |
774 | @deftypefun {int} mount (const char *@var{special_file}, const char *@var{dir}, const char *@var{fstype}, unsigned long int @var{options}, const void *@var{data}) |
775 | ||
dbacafe5 | 776 | @code{mount} mounts or remounts a filesystem. The two operations are |
faf2289f | 777 | quite different and are merged rather unnnaturally into this one function. |
dbacafe5 | 778 | The @code{MS_REMOUNT} option, explained below, determines whether |
faf2289f UD |
779 | @code{mount} mounts or remounts. |
780 | ||
781 | For a mount, the filesystem on the block device represented by the | |
782 | device special file named @var{special_file} gets mounted over the mount | |
783 | point @var{dir}. This means that the directory @var{dir} (along with any | |
784 | files in it) is no longer visible; in its place (and still with the name | |
785 | @var{dir}) is the root directory of the filesystem on the device. | |
786 | ||
787 | As an exception, if the filesystem type (see below) is one which is not | |
788 | based on a device (e.g. ``proc''), @code{mount} instantiates a | |
789 | filesystem and mounts it over @var{dir} and ignores @var{special_file}. | |
790 | ||
791 | For a remount, @var{dir} specifies the mount point where the filesystem | |
792 | to be remounted is (and remains) mounted and @var{special_file} is | |
793 | ignored. Remounting a filesystem means changing the options that control | |
794 | operations on the filesystem while it is mounted. It does not mean | |
795 | unmounting and mounting again. | |
796 | ||
797 | For a mount, you must identify the type of the filesystem as | |
798 | @var{fstype}. This type tells the kernel how to access the filesystem | |
799 | and can be thought of as the name of a filesystem driver. The | |
800 | acceptable values are system dependent. On a system with a Linux kernel | |
801 | and the @code{proc} filesystem, the list of possible values is in the | |
802 | file @file{filesystems} in the @code{proc} filesystem (e.g. type | |
803 | @kbd{cat /proc/filesystems} to see the list). With a Linux kernel, the | |
804 | types of filesystems that @code{mount} can mount, and their type names, | |
805 | depends on what filesystem drivers are configured into the kernel or | |
806 | loaded as loadable kernel modules. An example of a common value for | |
807 | @var{fstype} is @code{ext2}. | |
808 | ||
809 | For a remount, @code{mount} ignores @var{fstype}. | |
810 | ||
811 | @c This is traditionally called "rwflag" for historical reasons. | |
812 | @c No point in confusing people today, though. | |
813 | @var{options} specifies a variety of options that apply until the | |
3566d33c | 814 | filesystem is unmounted or remounted. The precise meaning of an option |
faf2289f UD |
815 | depends on the filesystem and with some filesystems, an option may have |
816 | no effect at all. Furthermore, for some filesystems, some of these | |
817 | options (but never @code{MS_RDONLY}) can be overridden for individual | |
818 | file accesses via @code{ioctl}. | |
819 | ||
820 | @var{options} is a bit string with bit fields defined using the | |
821 | following mask and masked value macros: | |
822 | ||
823 | @table @code | |
824 | @item MS_MGC_MASK | |
825 | This multibit field contains a magic number. If it does not have the value | |
826 | @code{MS_MGC_VAL}, @code{mount} assumes all the following bits are zero and | |
827 | the @var{data} argument is a null string, regardless of their actual values. | |
828 | ||
829 | @item MS_REMOUNT | |
830 | This bit on means to remount the filesystem. Off means to mount it. | |
831 | @c There is a mask MS_RMT_MASK in mount.h that says only two of the options | |
4b9a6d7c | 832 | @c can be reset by remount. But the Linux kernel has its own version of |
faf2289f UD |
833 | @c MS_RMT_MASK that says they all can be reset. As far as I can tell, |
834 | @c libc just passes the arguments straight through to the kernel. | |
835 | ||
836 | @item MS_RDONLY | |
837 | This bit on specifies that no writing to the filesystem shall be allowed | |
dbacafe5 | 838 | while it is mounted. This cannot be overridden by @code{ioctl}. This |
faf2289f UD |
839 | option is available on nearly all filesystems. |
840 | ||
841 | @item S_IMMUTABLE | |
842 | This bit on specifies that no writing to the files in the filesystem | |
843 | shall be allowed while it is mounted. This can be overridden for a | |
844 | particular file access by a properly privileged call to @code{ioctl}. | |
845 | This option is a relatively new invention and is not available on many | |
846 | filesystems. | |
847 | ||
848 | @item S_APPEND | |
849 | This bit on specifies that the only file writing that shall be allowed | |
850 | while the filesystem is mounted is appending. Some filesystems allow | |
851 | this to be overridden for a particular process by a properly privileged | |
852 | call to @code{ioctl}. This is a relatively new invention and is not | |
853 | available on many filesystems. | |
854 | ||
855 | @item MS_NOSUID | |
856 | This bit on specifies that Setuid and Setgid permissions on files in the | |
857 | filesystem shall be ignored while it is mounted. | |
858 | ||
859 | @item MS_NOEXEC | |
860 | This bit on specifies that no files in the filesystem shall be executed | |
861 | while the filesystem is mounted. | |
862 | ||
863 | @item MS_NODEV | |
dbacafe5 | 864 | This bit on specifies that no device special files in the filesystem |
faf2289f UD |
865 | shall be accessible while the filesystem is mounted. |
866 | ||
867 | @item MS_SYNCHRONOUS | |
dbacafe5 UD |
868 | This bit on specifies that all writes to the filesystem while it is |
869 | mounted shall be synchronous; i.e. data shall be synced before each | |
faf2289f UD |
870 | write completes rather than held in the buffer cache. |
871 | ||
872 | @item MS_MANDLOCK | |
873 | This bit on specifies that mandatory locks on files shall be permitted while | |
874 | the filesystem is mounted. | |
875 | ||
876 | @item MS_NOATIME | |
877 | This bit on specifies that access times of files shall not be updated when | |
878 | the files are accessed while the filesystem is mounted. | |
879 | ||
880 | @item MS_NODIRATIME | |
881 | This bit on specifies that access times of directories shall not be updated | |
882 | when the directories are accessed while the filesystem in mounted. | |
883 | ||
884 | @c there is also S_QUOTA Linux fs.h (mount.h still uses its former name | |
885 | @c S_WRITE), but I can't see what it does. Turns on quotas, I guess. | |
886 | ||
887 | @end table | |
888 | ||
889 | Any bits not covered by the above masks should be set off; otherwise, | |
890 | results are undefined. | |
891 | ||
892 | The meaning of @var{data} depends on the filesystem type and is controlled | |
893 | entirely by the filesystem driver in the kernel. | |
894 | ||
895 | Example: | |
896 | ||
897 | @smallexample | |
898 | @group | |
899 | #include <sys/mount.h> | |
900 | ||
901 | mount("/dev/hdb", "/cdrom", MS_MGC_VAL | MS_RDONLY | MS_NOSUID, ""); | |
902 | ||
903 | mount("/dev/hda2", "/mnt", MS_MGC_VAL | MS_REMOUNT, ""); | |
904 | ||
905 | @end group | |
906 | @end smallexample | |
907 | ||
908 | Appropriate arguments for @code{mount} are conventionally recorded in | |
909 | the @file{fstab} table. @xref{Mount Information}. | |
910 | ||
911 | The return value is zero if the mount or remount is successful. Otherwise, | |
dbacafe5 | 912 | it is @code{-1} and @code{errno} is set appropriately. The values of |
faf2289f UD |
913 | @code{errno} are filesystem dependent, but here is a general list: |
914 | ||
915 | @table @code | |
916 | @item EPERM | |
917 | The process is not superuser. | |
918 | @item ENODEV | |
919 | The file system type @var{fstype} is not known to the kernel. | |
920 | @item ENOTBLK | |
921 | The file @var{dev} is not a block device special file. | |
922 | @item EBUSY | |
923 | ||
68979757 | 924 | @itemize @bullet |
faf2289f | 925 | |
dbacafe5 | 926 | @item |
faf2289f UD |
927 | The device is already mounted. |
928 | ||
929 | @item | |
930 | The mount point is busy. (E.g. it is some process' working directory or | |
931 | has a filesystem mounted on it already). | |
932 | ||
933 | @item | |
934 | The request is to remount read-only, but there are files open for write. | |
935 | @end itemize | |
936 | ||
937 | @item EINVAL | |
68979757 | 938 | @itemize @bullet |
faf2289f UD |
939 | |
940 | @item | |
941 | A remount was attempted, but there is no filesystem mounted over the | |
942 | specified mount point. | |
943 | ||
dbacafe5 | 944 | @item |
faf2289f UD |
945 | The supposed filesystem has an invalid superblock. |
946 | ||
947 | @end itemize | |
948 | ||
949 | @item EACCESS | |
68979757 | 950 | @itemize @bullet |
faf2289f UD |
951 | |
952 | @item | |
dbacafe5 | 953 | The filesystem is inherently read-only (possibly due to a switch on the |
faf2289f UD |
954 | device) and the process attempted to mount it read/write (by setting the |
955 | @code{MS_RDONLY} bit off). | |
956 | ||
957 | @item | |
958 | @var{special_file} or @var{dir} is not accessible due to file permissions. | |
959 | ||
960 | @item | |
961 | @var{special_file} is not accessible because it is in a filesystem that is | |
962 | mounted with the @code{MS_NODEV} option. | |
963 | ||
964 | @end itemize | |
965 | ||
966 | @item EM_FILE | |
967 | The table of dummy devices is full. @code{mount} needs to create a | |
968 | dummy device (aka ``unnamed'' device) if the filesystem being mounted is | |
969 | not one that uses a device. | |
970 | ||
971 | @end table | |
972 | ||
973 | @end deftypefun | |
974 | ||
975 | ||
976 | @comment sys/mount.h | |
dbacafe5 | 977 | @comment GNU |
faf2289f UD |
978 | @deftypefun {int} umount2 (const char *@var{file}, int @var{flags}) |
979 | ||
980 | @code{umount2} unmounts a filesystem. | |
981 | ||
982 | You can identify the filesystem to unmount either by the device special | |
983 | file that contains the filesystem or by the mount point. The effect is | |
984 | the same. Specify either as the string @var{file}. | |
985 | ||
dbacafe5 | 986 | @var{flags} contains the one-bit field identified by the following |
faf2289f UD |
987 | mask macro: |
988 | ||
989 | @table @code | |
990 | ||
991 | @item MNT_FORCE | |
992 | This bit on means to force the unmounting even if the filesystem is | |
993 | busy, by making it unbusy first. If the bit is off and the filesystem is | |
994 | busy, @code{umount2} fails with @code{errno} = @code{EBUSY}. Depending | |
995 | on the filesystem, this may override all, some, or no busy conditions. | |
996 | ||
997 | @end table | |
998 | ||
999 | All other bits in @var{flags} should be set to zero; otherwise, the result | |
1000 | is undefined. | |
1001 | ||
1002 | Example: | |
1003 | ||
1004 | @smallexample | |
1005 | @group | |
1006 | #include <sys/mount.h> | |
1007 | ||
1008 | umount2("/mnt", MNT_FORCE); | |
1009 | ||
1010 | umount2("/dev/hdd1", 0); | |
1011 | ||
1012 | @end group | |
1013 | @end smallexample | |
1014 | ||
1015 | After the filesystem is unmounted, the directory that was the mount point | |
1016 | is visible, as are any files in it. | |
1017 | ||
1018 | As part of unmounting, @code{umount2} syncs the filesystem. | |
1019 | ||
1020 | If the unmounting is successful, the return value is zero. Otherwise, it | |
1021 | is @code{-1} and @code{errno} is set accordingly: | |
1022 | ||
1023 | @table @code | |
1024 | @item EPERM | |
1025 | The process is not superuser. | |
1026 | @item EBUSY | |
1027 | The filesystem cannot be unmounted because it is busy. E.g. it contains | |
1028 | a directory that is some process's working directory or a file that some | |
1029 | process has open. With some filesystems in some cases, you can avoid | |
1030 | this failure with the @code{MNT_FORCE} option. | |
1031 | ||
1032 | @item EINVAL | |
dbacafe5 | 1033 | @var{file} validly refers to a file, but that file is neither a mount |
faf2289f UD |
1034 | point nor a device special file of a currently mounted filesystem. |
1035 | ||
1036 | @end table | |
dbacafe5 UD |
1037 | |
1038 | This function is not available on all systems. | |
faf2289f UD |
1039 | @end deftypefun |
1040 | ||
1041 | @comment sys/mount.h | |
dbacafe5 | 1042 | @comment SVID, GNU |
faf2289f UD |
1043 | @deftypefun {int} umount (const char *@var{file}) |
1044 | ||
dbacafe5 UD |
1045 | @code{umount} does the same thing as @code{umount2} with @var{flags} set |
1046 | to zeroes. It is more widely available than @code{umount2} but since it | |
1047 | lacks the possibility to forcefully unmount a filesystem is deprecated | |
1048 | when @code{umount2} is also available. | |
faf2289f | 1049 | @end deftypefun |
4b9a6d7c UD |
1050 | |
1051 | ||
1052 | ||
1053 | @node System Parameters | |
1054 | @section System Parameters | |
1055 | ||
1056 | This section describes the @code{sysctl} function, which gets and sets | |
1057 | a variety of system parameters. | |
1058 | ||
1059 | The symbols used in this section are declared in the file @file{sysctl.h}. | |
1060 | ||
1061 | @comment sysctl.h | |
1062 | @comment BSD | |
1063 | @deftypefun int sysctl (int *@var{names}, int @var{nlen}, void *@var{oldval}, | |
1064 | size_t *@var{oldlenp}, void *@var{newval}, size_t @var{newlen}) | |
1065 | ||
1066 | @code{sysctl} gets or sets a specified system parameter. There are so | |
1067 | many of these parameters that it is not practical to list them all here, | |
1068 | but here are some examples: | |
1069 | ||
1070 | @itemize @bullet | |
1071 | @item network domain name | |
1072 | @item paging parameters | |
1073 | @item network Address Resolution Protocol timeout time | |
1074 | @item maximum number of files that may be open | |
1075 | @item root filesystem device | |
1076 | @item when kernel was built | |
1077 | @end itemize | |
1078 | ||
1079 | The set of available parameters depends on the kernel configuration and | |
1080 | can change while the system is running, particularly when you load and | |
1081 | unload loadable kernel modules. | |
1082 | ||
1083 | The system parameters with which @code{syslog} is concerned are arranged | |
1084 | in a hierarchical structure like a hierarchical filesystem. To identify | |
1085 | a particular parameter, you specify a path through the structure in a | |
1086 | way analogous to specifying the pathname of a file. Each component of | |
1087 | the path is specified by an integer and each of these integers has a | |
1088 | macro defined for it by @file{sysctl.h}. @var{names} is the path, in | |
1089 | the form of an array of integers. Each component of the path is one | |
1090 | element of the array, in order. @var{nlen} is the number of components | |
1091 | in the path. | |
1092 | ||
1093 | For example, the first component of the path for all the paging | |
1094 | parameters is the value @code{CTL_VM}. For the free page thresholds, the | |
1095 | second component of the path is @code{VM_FREEPG}. So to get the free | |
1096 | page threshold values, make @var{names} an array containing the two | |
1097 | elements @code{CTL_VM} and @code{VM_FREEPG} and make @var{nlen} = 2. | |
1098 | ||
1099 | ||
1100 | The format of the value of a parameter depends on the parameter. | |
1101 | Sometimes it is an integer; sometimes it is an ASCII string; sometimes | |
1102 | it is an elaborate structure. In the case of the free page thresholds | |
1103 | used in the example above, the parameter value is a structure containing | |
1104 | several integers. | |
1105 | ||
1106 | In any case, you identify a place to return the parameter's value with | |
1107 | @var{oldval} and specify the amount of storage available at that | |
1108 | location as *@var{oldlenp}. *@var{oldlenp} does double duty because it | |
1109 | is also the output location that contains the actual length of the | |
1110 | returned value. | |
1111 | ||
1112 | If you don't want the parameter value returned, specify a null pointer | |
1113 | for @var{oldval}. | |
1114 | ||
68979757 | 1115 | To set the parameter, specify the address and length of the new value |
4b9a6d7c UD |
1116 | as @var{newval} and @var{newlen}. If you don't want to set the parameter, |
1117 | specify a null pointer as @var{newval}. | |
1118 | ||
1119 | If you get and set a parameter in the same @code{sysctl} call, the value | |
1120 | returned is the value of the parameter before it was set. | |
1121 | ||
1122 | Each system parameter has a set of permissions similar to the | |
1123 | permissions for a file (including the permissions on directories in its | |
1124 | path) that determine whether you may get or set it. For the purposes of | |
1125 | these permissions, every parameter is considered to be owned by the | |
1126 | superuser and Group 0 so processes with that effective uid or gid may | |
1127 | have more access to system parameters. Unlike with files, the superuser | |
1128 | does not invariably have full permission to all system parameters, because | |
1129 | some of them are designed not to be changed ever. | |
1130 | ||
1131 | ||
1132 | @code{sysctl} returns a zero return value if it succeeds. Otherwise, it | |
1133 | returns @code{-1} and sets @code{errno} appropriately. Besides the | |
1134 | failures that apply to all system calls, the following are the | |
1135 | @code{errno} codes for all possible failures: | |
1136 | ||
1137 | @table @code | |
1138 | @item EPERM | |
68979757 | 1139 | The process is not permitted to access one of the components of the |
4b9a6d7c UD |
1140 | path of the system parameter or is not permitted to access the system parameter |
1141 | itself in the way (read or write) that it requested. | |
68979757 | 1142 | @c There is some indication in the Linux 2.2 code that the code is trying to |
4b9a6d7c UD |
1143 | @c return EACCESS here, but the EACCESS value never actually makes it to the |
1144 | @c user. | |
1145 | @item ENOTDIR | |
1146 | There is no system parameter corresponding to @var{name}. | |
1147 | @item EFAULT | |
1148 | @var{oldval} is not null, which means the process wanted to read the parameter, | |
1149 | but *@var{oldlenp} is zero, so there is no place to return it. | |
1150 | @item EINVAL | |
1151 | @itemize @bullet | |
1152 | @item | |
1153 | The process attempted to set a system parameter to a value that is not valid | |
1154 | for that parameter. | |
1155 | @item | |
1156 | The space provided for the return of the system parameter is not the right | |
1157 | size for that parameter. | |
1158 | @end itemize | |
1159 | @item ENOMEM | |
1160 | This value may be returned instead of the more correct @code{EINVAL} in some | |
1161 | cases where the space provided for the return of the system parameter is too | |
1162 | small. | |
1163 | ||
1164 | @end table | |
1165 | ||
1166 | @end deftypefun | |
1167 | ||
1168 | If you have a Linux kernel with the @code{proc} filesystem, you can get | |
68979757 | 1169 | and set most of the same parameters by reading and writing to files in |
4b9a6d7c UD |
1170 | the @code{sys} directory of the @code{proc} filesystem. In the @code{sys} |
1171 | directory, the directory structure represents the hierarchical structure | |
1172 | of the parameters. E.g. you can display the free page thresholds with | |
1173 | @smallexample | |
1174 | cat /proc/sys/vm/freepages | |
1175 | @end smallexample | |
1176 | @c In Linux, the sysctl() and /proc instances of the parameter are created | |
1177 | @c together. The proc filesystem accesses the same data structure as | |
1178 | @c sysctl(), which has special fields in it for /proc. But it is still | |
1179 | @c possible to create a sysctl-only parameter. | |
1180 | ||
1181 | Some more traditional and more widely available, though less general, | |
1182 | GNU C library functions for getting and setting some of the same system | |
1183 | parameters are: | |
1184 | ||
1185 | @itemize @bullet | |
1186 | @item | |
1187 | @code{getdomainname}, @code{setdomainname} | |
1188 | @item | |
1189 | @code{gethostname}, @code{sethostname} (@xref{Host Identification}.) | |
1190 | @item | |
99a20616 | 1191 | @code{uname} (@xref{Platform Type}.) |
4b9a6d7c UD |
1192 | @item |
1193 | @code{bdflush} | |
1194 | @end itemize |