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diff --git a/reference/C/CONCEPT/pointers.html b/reference/C/CONCEPT/pointers.html new file mode 100644 index 0000000..4830b09 --- /dev/null +++ b/reference/C/CONCEPT/pointers.html @@ -0,0 +1,559 @@ +<title>Pointers</title> +<body bgcolor="#ffffcc"> +<hr> +<center><h1>Pointers.</h1></center> +<hr> +<p> +Pointers are at the heart of C. When you crack this subject, you have got the worst +of C behind you. Before you tackle pointers though, you should get a grip +on <a href=arrays.html>arrays</a>.<p> +<ul> +<li><a href="#first">First principles.</a> +<li><a href="#def">Definition of a pointer.</a> +<li><a href="string.html">Pointers to strings.</a> +<li><a href="#arrays">Pointers to arrays.</a> +<li><a href="#compare">Char arrays verses char pointers.</a> +<li><a href="#void">Void pointers.</a> +<li><a href="#ptrs">Pointers to pointers.</a> +<li><a href="#functions">Pointers to functions.</a> +<li><a href=../MISC/linklists.html>Linked lists.</a> +</ul> +<a name=string> +<a name=basic> +<a name=first> +<hr> +<h2>First Principles.</h2> +To understand pointers, it may be worth understanding how normal +variables are stored. If you disagree, <A HREF="#def">Click here</A> +to move on. +<p> +What does the following program realy mean? +<p> +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + main() + { + int Length; + } +</pre> +</td> +</tr> +</table> +<p> +In my mind, it means, reserve enough storage to hold an integer and assign the variable +name 'Length' to it. The data held in this storage is undefined. +Graphically it looks like: +<pre> + + (Address) (Data) + ---- ---- + | F1 | <------- Length + |----|----| + | F2 | | + |----|----| + | F3 | | + |----|----| + | F4 | | + --------- + +</pre> +To put a known value into 'Length' we code, +<p> +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + main() + { + int Length; + Length = 20; + } +</pre> +</td> +</tr> +</table> +<p> +the deciamal value 20 (Hex 14) is placed into the storage location. +<pre> + + (Address) (Data) + ---- ---- + | F1 | 00 <------- Length + |----|----| + | F2 | 00 | + |----|----| + | F3 | 00 | + |----|----| + | F4 | 14 | + --------- + +</pre> +Finally, if the program is expanded to become +<p> +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + main() + { + int Length; + + Length = 20; + + <A HREF="../FUNCTIONS/printf.html">printf</a>("Length is %d\n", Length); + printf("Address of Length is %p\n", &Length); + } + +</pre> +</td> +</tr> +</table> +<p> +The output would look something like this ..... +<pre> + + Length is 20 + Address of Length is 0xF1 + +</pre> + +Please note the '&Length' on the second printf statement. +The <b>&</b> means <b>address of</b> Length. +If you are happy with this, you should push onto the pointers below. +<p> +<a name=def> +<hr> +<h2>Pointer definition.</h2> + +<b>A pointer contains an <a href=../glossary.html#address>address</a> that points + +to data.</b> +<p> +An example of code <a href=../glossary.html#definition>defining</a> a pointer could be... +<p> +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + main() + { + int *Width; + } + +</Pre> +</td> +</tr> +</table> +<p> +A graphical representation could be... +<pre> + + (Address) (Data) + ---- ---- + | F1 | <------- Width + |----|----| + | F2 | | + |----|----| + | F3 | | + |----|----| + | F4 | | + --------- + +</pre> +So far, this variable looks the same as above, + the value stored at 'Width' is unknown. +To place a value in 'Width' you could code. +<p> +<Table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + main() + { + int *Width; /* 1 */ + + Width = (int *)malloc(sizeof(int)); /* 2 */ + + *Width = 34; /* 3 */ + } + +</pre> +</td> +</tr> +</table> +<pre> + + (Address) (Data) + ---- ---- + | F1 | 00 <------- Width + |----|----| (Data) (Adress) + | F2 | 00 | --------- + |----|----| -------> 00 | D1 | + | F3 | 00 | | |----|----| + |----|----| *Width| | 00 | D2 | + | F4 | D1 | ------- |----|----| + --------- | 00 | D3 | + |----|----| + | 22 | D4 | + --------- + +</pre> + +Statements 2 and 3 are important here: +<p> +2) The <a href="../FUNCTIONS/malloc.html">malloc</a> function reserves some storage and puts the address of the +storage into Width. +<p> +3) *Width puts a value into the storage pointed to by Width. +<p> + +Unlike the +<a href="#first">Length = 20</a> example above, the storage pointed to by 'Width' +does NOT contain 34 (22 in Hex), it contains the address where the value +34 can be found. +The final program is... +<p> +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + main() + { + int *Width; + + Width = (int *)malloc(sizeof(int)); + *Width = 34; + + printf(" Data stored at *Width is %d\n", *Width); + printf(" Address of Width is %p\n", &Width); + printf("Address stored at Width is %p\n", Width); + } + +</pre> +</tr> +</td> +</table> +<p> +The program would O/P something like. +<p> +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + Data stored at *Width is 34 + Address of Width is 0xF1 + Address stored at Width is 0xD1 + +</pre> +</td> +</td> +</table> + +<p> +<a name=why>A pointer can point to any data type, ie +<a href=data_types.html#int>int</a>, +<a href=data_types.html#float>float</a>, +<a href=data_types.html#char>char</a>. +When <a href=../glossary.html#definition>defining</a> a pointer you +place an <b>*</b> (asterisk) character +between the data type and the variable name, here are a few examples. +<p> +<table border=2 width=100% bgcolor=ivory> +<tr><td> +<pre> + + main() + { + int count; /* an integer variable */ + int *pcount; /* a pointer to an integer variable */ + float miles; /* a floating point variable. */ + float *m; /* a pointer */ + char ans; /* character variable */ + char *charpointer; /* pointer to a character variable */ + } + +</pre> +</td></tr></table> +<p> + + +<p> + +<a name=arrays> +<hr> +<h2>Pointers to arrays</h2> +When looking at +<a href=arrays.html>arrays</a> we had a program that accessed data within a +two dimensional character array. This is what the code looked like. + +<pre> + main() + { + char colours[3][6]={"red","green","blue"}; + } +</pre> +The code above has defined an array of 3 elements, each pointing to 6 +character strings. +You can also code it like this. Which is actually more descriptive +because it indicates what is actually going on in storage. +<pre> + main() + { + char *colours[]={"red","green","blue"}; + } +</pre> + +Graphically it looks like this: +<p> +<table border=2 width=100% bgcolor=ivory> +<tr><td> +<pre> + +<font color="gray"> + + + colours *colours *(colours+2) **colours + | | | | + | | | | + V V V | </font> + --- ----------- <font color="gray"> | </font> + | |---->| | | | <font color="gray"> | </font> + --- ----------- <font color="gray"> | </font> + | | | <font color="gray"> V </font> + | | | ----------------------- + --------------->| r | e | d | | | | + | | ----------------------- + | | + | | ----------------------- + ---|-------->| g | r | e | e | n | | + | ----------------------- + | + | ----------------------- + -------->| b | l | u | e | | | + ----------------------- +<font color="gray"> A A + | | + | | + **(colours+2) *(*(colours+2)+3) +</font> +</pre> +</td></tr></table> +<p> +<pre> + printf("%s \n", colours[1]); + printf("%s \n", *(colours+1)) +</pre> +will both return <b>green</b>. +<p> + +<a name=compare> +<hr> +<h2>Char Arrays verses Char pointers</h2> + +What is the difference +between these two lumps of code? +<p> +<table border=2 width=100% bgcolor=ivory> +<tr><td> +<pre> + + main() + { + char colour[]="red"; + printf("%s \n",colour); + } + +</pre> +</td><td> +<pre> + + main() + { + char *colour="red"; + printf("%s \n",colour); + } + +</pre> +</td></tr></table> +<p> +The answer is, NOTHING! They both print the word +<b>red</b> because in both cases 'printf' is being passed a pointer to a +string. +<p> +An <a href=arrays.html>array</a> of 4 bytes is +reserved and the text 'red' placed +into the storage array. The contents of the array can be chaged later +BUT on the left, the size of the array is fixed. +<p> +Here is a picture of the example on the right. The 'r' of 'red' is stored +at address 10, the 'e' is at address 11 etc. + +<p> +<img src=../../GRAPHICS/ptr.gif> +<p> +At this point it maybe worth looking at +the <a href=../FUNCTIONS/malloc.html>malloc</a> function. + +<p> +<a name=void> +<hr> +<h2>Void Pointers</h2> + +There are times when you write a function but do not know the datatype +of the returned value. When this is the case, you can use a void pointer. +<p> + +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + int func(void *Ptr); + + main() + { + char *Str = "abc"; + + func(Str); + } + + int func(void *Ptr) + { + printf("%s\n", Ptr); + } +</pre> +</td> +</tr> +</table> +<p> + +<b>Please note, you cant do pointer arithmatic on void pointers.</b> +<p> + +<a name=ptrs> +<hr> +<h2>Pointers to pointers</h2> + +So far we have looked at pointers to data, but there is no reason why +we should not define a pointer to a pointer. The syntax looks like this. +<p> + +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + + main() + { + char **DoublePtr; + } +</pre> +</td> +</tr> +</table> +<p> + +A common use for these is when you want to return a +pointer in a function parameter. +<p> + +<table border=2 width=100% bgcolor=ivory> +<tr> +<td> +<pre> + #include <stdlib.h> /* malloc */ + + void Func(char **DoublePtr); + + main() + { + char *Ptr; + + Func(&Ptr); + } + + void Func(char **DoublePtr) + { + *DoublePtr = <a href=../FUNCTIONS/malloc.html>malloc</a>(50); + } +</pre> +</td> +</tr> +</table> +<p> + +<p> +<a name=functions> +<hr> + +<h2>Pointers to functions</h2> + +Pointers to functions can be used to create 'callback' functions. +An example of these pointers can be seen in the +<a href=../FUNCTIONS/qsort.html>qsort</a> function. +<p> +<h3>Examples.</h3> + +<img src="../../GRAPHICS/computer.gif"> +<a href="../EXAMPLES/funcpt1.c">Simple example passing a function pointer.</a><p> + +<img src="../../GRAPHICS/computer.gif"> +<a href="../EXAMPLES/funcpt2.c"> Example passing 'int' variables.</a><p> + +<img src="../../GRAPHICS/computer.gif"> +<a href="../EXAMPLES/funcpt3.c"> Example passing 'char' and 'char *' variables.</a><p> + +<hr> +<h2>See Also:</h2> + +<img src="../../GRAPHICS/whiteball.gif"> +<a href="../SYNTAX/void.html">VOID keyword.</a> +<p> +<img src="../../GRAPHICS/whiteball.gif"> +<a href="../SYNTAX/functions.html">function</a> arguments. +<p> +<img src="../../GRAPHICS/whiteball.gif"> +<a href="../MISC/linklists.html">linked lists</a>. +<p> +<img src="../../GRAPHICS/whiteball.gif"> +<a href="string.html">Strings</a>. +<p> +<img src="../../GRAPHICS/whiteball.gif"> +<a href="arrays.html">Arrays.</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> |