1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
|
<title>STRUCT keyword </title>
<head>
<script language="JavaScript">
</script>
</head>
<body bgcolor="#ffffcc">
<hr>
<center><h1>STRUCT keyword </h1></center>
<hr>
<p>
<ul>
<li><a href="#basics">Structure basics.</a>
<li><a href="#membership">Structure membership.</a>
<li><a href="#pointers">Pointers to structures.</a>
<li><a href="#array">Array of structures.</a>
<li><a href="#geek">The bottom draw..</a>
<li><a href="../../CPLUSPLUS/SYNTAX/struct.html">C++ extensions</a>
</ul>
<a name=basics>
<hr>
<h2>Structure basics</h2>
<b>struct</b> is used to declare a new
<a href="../CONCEPT/data_types.html">data-type</a>.
Basically this means
grouping variables together. For example, a struct data type could be used
to declare the format of the following file.<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
Jo Loss Maths A
Harry Carpenter English A
Billy King Maths C
</pre>
</td></tr>
</table>
</center>
<p>
The records above could be described in a struct as follows:
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
struct
{
char cname[8];
char sname[16];
char exam[16];
char grade;
} record;
</pre>
</td></tr>
</table>
</center>
<p>
The statement above declares a variable called <b>record</b> with 4 members
called <b>cname, sname, exam, grade</b>. The structure as a whole can be
referred to as <b>record</b> and a member can be referenced as
<b>record.exam</b>
<p>
Structures can be declared in various forms...
<p>
<pre>
struct x {int a; int b; int c;}; /* declaration */
struct {int a; int b; int c;} z;
struct x z;
</pre>
All the examples above are structure declarations,
<ul>
<li>The first gives <b>x</b> as a 'structure tag' - this is optional.
<li>The first and second declare the members of the structure.
<li>Second and third give <b>z</b> this is the variable that assumes the
structure type.
</ul>
<p>
<a name=membership>
<hr>
<h2>Structure membership</h2>
We can access individual members of a structure with the . operator.<p>
For example to assign a value, we can enter:
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
struct x {int a; int b; int c;};
main()
{
struct x z;
z.a = 10;
z.b = 20;
z.c = 30;
}
</pre>
</td></tr>
</table>
</center>
<p>
And to retrieve a value from a structure member:
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
struct x
{
int a;
int b;
int c;
};
main()
{
struct x z;
z.a = 10;
z.a++;
printf(" first member is %d \n", z.a);
}
</pre>
</td></tr>
</table>
</center>
<p>
<a name=pointers>
<hr>
<h2>Pointers to structures</h2>
<a href="#arrow">Fast path to an explanation of the -> operator.</a>
<p>
All that we have discussed so far has been OK but runs into problems
when structures have to be moved between functions for the following
reasons.
<ul>
<li>If the structure is large it is more effiecent to pass a
<a href="../CONCEPT/pointers.html">pointer</a> to the structure
instead of the structure its self. This technic is also used to pass
<a href="../CONCEPT/pointers.html#arrays">pointers to arrays</a> between
<a href="functions.html#2.5">functions.</a>
<p>
<li>When passing a structure to a function, you actually pass a COPY of the
structure. Therefore it is not possible to change the values of members
within the structure as the copy is destroyed when the function ends.
</ul>
<p>
So how does it all work? Here is an example. (make your browser W-I-D-E so
you can see the two examples).
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
|
|
struct x {int a; int b; int c;} ; | struct x {int a; int b; int c;} ;
|
void function(struct x); | void function(struct x *);
|
main() | main()
{ | {
struct x z; | struct x z, *pz; <font color=red>/* 3 */</font>
| pz = &z; <font color=red>/* 4 */</font>
z.a = 10; <font color=red>/* 1 */</font> | z.a = 10;
z.a++; | z.a++;
|
function(z); <font color=red>/* 2 */</font> | function(pz); <font color=red>/* 5 */</font>
} | }
|
void function( struct x z) | void function(struct x * pz)
{ | { <font color=red>/* 6 */</font>
printf(" first member %d \n", z.a);| printf(" first member %d \n", (*pz).a);
} | }
|
</pre>
</td></tr>
</table>
</center>
<p>
Here is the annotation.
<ol>
<li>Give a structure member a value.
<li>Pass a COPY of the whole structure to the function.
<li>Define 'pz' a pointer to a structure of type 'x'.
<li>Put the address of 'z' into 'pz'. 'pz' now POINTS to 'z'.
PLEASE NOTE. 'z' is defined to reserve memory equal to the size of the
structure. 'pz' only holds an address so will be 4 bytes long.
<li>Pass the pointer into the function.
<li>Print the value of the member 'a'.
</ol>
<a name=arrow>
The <b>(*pz).a</b> syntax is used a great deal in C and it was decided to create
a short hand for it. So:
<pre>
(*pz).a == pz->a
</pre>
Here is the final picture.
<pre>
/*************************************************************************/
struct x {int a; int b; int c;} ; /* Declare the structure. */
void function(struct x * ); /* Declare the function. */
/*************************************************************************/
main()
{
/* Declare two variables.
* z == type struct x
* pz == a pointer to type struct x
*/
struct x z, *pz;
pz = &z; /* put the address of 'z' into 'pz' */
z.a = 10; /* initialize z.a */
z.a++; /* Increment z.a */
/* print the contents of 'z.a'
* using the pointer 'pz' */
printf(" first member before the function call %d \n", pz->a);
/* Call 'function' passing the
* pointer 'pz' */
function(pz);
/* Print the NEW value of 'z.a'
* using three different notations */
printf(" first member after the function call %d \n", pz->a);
printf(" first member after the function call %d \n", (*pz).a);
printf(" first member after the function call %d \n", z.a);
}
/*************************************************************************/
void function(struct x * pz)
{
/* Print the value of 'z.a' by
* referencing the pointer 'pz'
* which holds the address of 'z' */
printf(" first member inside the function %d \n", pz->a);
/* Increment the value of 'z.a'
* this is the source location
* in memory. */
pz->a++;
}
/*************************************************************************/
</pre>
<a name=geek>
<p>
<hr>
<h2>The Bottom Draw</h2>
Finally, here is a little feature that allows you to save a little
space.
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
main()
{
struct Flags
{
unsigned int Online <font color=red>:1</font>;
unsigned int Mounted <font color=red>:1</font>;
}
struct Flags TapeInfo;
TapeInfo.Online = 1;
TapeInfo.Mounted = 0;
}
</pre>
</td></tr>
</table>
</center>
<p>
The <font color=red>:1</font> tells the compiler that only 1 byte is
required for <b>Online</b> and <b>Mounted</b>. There are a few points to
note about this though.
<ul>
<li>You may expect the compiler to reserve 2 bytes for the structure, it
actually reserves one word (usually 4 bytes) as this is the smallest unit
that can be reserved, the remaining 2 bytes are unavailable.
This is still better than the 2 words that wold normally get reserved.
<li>You can put any number into the variable, if the number is too large
to fit, the high order bits are lost without warning.
<li>Only <b>signed int</b>, <b>unsigned int</b>, <b>int</b> support this syntax.
</ul>
<hr>
<h2>Examples</h2>
<p>
<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct1.c">
This is the most basic <b>struct</b> example I could think of.</a>
<br>
<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct2.c">
Using structure elements, and passing them into a function.</a>
<br>
<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct3.c">
Passing a whole structure to a function.</a> This performs a copy of the
structure so the same rules apply as for <b>int</b> etc.
Pointers to structures can be passed but I have not got to them yet....
<br>
<a name=array>
<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct4.c">Define and use an array of structures.</a>
<br>
<img src=../../GRAPHICS/help.gif>
<a href="../PROBLEMS/problems.html#struct">
Here is a <b>struct</b> problem for you.</a>
<hr>
<h2>See Also:</h2>
<img src=../../GRAPHICS/whiteball.gif>
<a href="typedef.html">typedef</a> keyword.
<br>
<img src=../../GRAPHICS/whiteball.gif>
<a href="../MISC/linklists.html">Linked lists</a>.
<br>
<p>
<hr>
<p>
<center>
<table border=2 width="80%" bgcolor="ivory">
<tr align=center>
<td width="25%">
<a href="../cref.html"> Top</a>
</td><td width="25%">
<a href="../master_index.html"> Master Index</a>
</td><td width="25%">
<a href="keywords.html"> Keywords</a>
</td><td width="25%">
<a href="../FUNCTIONS/funcref.htm"> Functions</a>
</td>
</tr>
</table>
</center>
<p>
<hr>
<address>Martin Leslie
</address><p>
</body>
</html>
|
