blob: 532f9d7aa7e19d72739c80b8e4cefe300ffd1b48 (
plain)
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
|
<title>Strings</title>
<body bgcolor="#ffffcc">
<hr>
<center><h1>Strings.</h1></center>
<hr>
<p>
<h2>Pointers to strings.</h2>
C does not have a "string" <a href=data_types.html>datatype</a>.
To create a string you have to use a <a href=arrays.html#char>char array</a>
or a <a href=data_types.html#char>char</a> pointer.
If you are not familur with <a href=arrays.html#char>char arrays</a>
I recomend that you read about them now.
<p>
To recap, a <a href=data_types.html#char>char</a> pointer is
<a href=../glossary.html#definitions>defined</A> like this:
<p>
<table border=2 width=100% bgcolor=ivory>
<tr><td>
<pre>
main()
{
char *Text;
}
</pre>
</td></tr>
</table>
<p>
All this program does is reserve storage that will hold an address.
At this point the address could be anything. To initalize <b>Text</b>
you can code:
<p>
<table border=2 width=100% bgcolor=ivory>
<tr><td>
<pre>
main()
{
char *Text = "Thunder";
}
</td></tr></table>
</pre>
<b>Text</b> now has the address of the first character in <b>Thunder</B>.
Graphically, things look like this.
<pre>
(Address) (Data)
---- ----
| F1 | 00 <------- Text
|----|----| (Data) (Adress)
| F2 | 00 | -------------
|----|----| -------> 54 (T) | D1 |
| F3 | 00 | | |--------|----|
|----|----| *Text | | 68 (h) | D2 |
| F4 | D1 | ------- |--------|----|
--------- | 75 (u) | D3 |
|--------|----|
| 6E (n) | D4 |
|--------|----|
| 64 (d) | D5 |
|--------|----|
| 65 (e) | D6 |
|--------|----|
| 72 (r) | D7 |
|--------|----|
| 00 | D8 |
-------------
</pre>
<p>
Please note the <b>00</b> at the end of <b>Thunder</b>. This is
the <a href=../SYNTAX/null.html>NULL</a> character and is used to mark the
end of a string.
<p>
If we wanted to O/P the data pointed to by a <b>char pointer</b>
we can code.
<p>
<table border=2 width=100% bgcolor=ivory>
<tr align=center><td>
<b>Source</b>
</td></tr><tr><td>
<pre>
main()
{
char *Text1 = "Thunder"; /* Define and initalize */
char *Text2; /* Define only */
Text2 = "Bird"; /* Point to some text */
<a href=../FUNCTIONS/printf.html>printf</a>("%s%s\n", Text1, Text2);
}
</pre>
</td></tr>
<tr align=center><td>
<b>Result</b>
</td></tr><tr><td>
<pre>
ThunderBird
</pre>
</td></tr></table>
<p>
This is all very well, but there is a MAJOR problem! <b>Thunder</b>
and <b>Bird</b> are
<a href=constants.html>constants</a>, they cannot be
changed in anyway.
We need a method of pointing to some storage that can be altered
and true to form, C provides a function called
<a href=../FUNCTIONS/malloc.html>malloc</a> to do just that.
<p>
<hr>
<h2>See Also:</h2>
<img src=../../GRAPHICS/whiteball.gif>
<a href=../SYNTAX/void.html>VOID keyword.</a>
<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=../SYNTAX/keywords.html>Keywords</a>
</td><td width=25%>
<a href="../FUNCTIONS/funcref.htm">Functions</a>
</td>
</tr>
</table>
</center>
<p>
<hr>
<address>Martin Leslie
<script language="JavaScript">
<!-- //
document.write(document.lastModified);
// -->
</script>
</address>
|
