<div>
&gt;&gt; I'm not quite clear about &quot;(char*)&amp;va_alist&quot; here.<br />
<br />
The &amp;&nbsp;takes the address (in memory) of the va_alist variable.<br />
The (char*) then casts that address to a pointer to char.<br />
<br />
So :<br />
<br />
&gt;&gt;&nbsp;Is &quot;list = (char*)&amp;va_alist&quot; equals to &quot;list = va_alist&quot;<br />
<br />
No.<br />
<br />
<br />
&gt;&gt;&nbsp;Why is &quot;typedef char* va_list&quot; but not &quot;typedef void* va_list&quot; or even &quot;typedef int* va_list&quot;<br />
<br />
These three are different - ie. they are not interchangeable.<br />
<br />
char* defines a pointer to char<br />
void* defines a generic pointer (pointing to data of which the type is undefined)<br />
int* defines a pointer to int<br />
<br />
Which you need depends on the situation - how you'll use it, etc.
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<div>
the typedef of va_list is implementation defined, and conforming code should not depend on it being char* or void* or &nbsp;int*
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<div>
As the type of va_list is &quot;char *&quot;,would it be the same that using &quot;#define va_start(list) list = &amp;va_list&quot; instead of &quot;#define va_start(list) list = (char *)&amp;va_list&quot; ?<br />
<br />
Int is 4-byte on 32-bit platform or 2-byte on 16-bit platform while char is 1-byte.If pInt is pointer to int and pChar is pointer to char,is it right that &quot;pInt+1&quot; adding 2 or 4 bytes and &quot;pChar+1&quot; adding 1 byte?<br />
So we choose &quot;char *&quot; because it has the same unit with sizeof().<br />
pChar + n = Address of pChar + n * (1-byte) = pChar + n * sizeof()<br />
pInt + n = Address of pInt + n * (2-bytes) = pInt + 2n * sizeof()<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;or= Address of pInt + n * (4-bytes) = pInt + 4n * sizeof() &nbsp;<br />
Is that right,efn?<br />
And Thanks for all of you!<br />

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<div>
&gt; As the type of va_list is &quot;char *&quot;,would it be the same that using &quot;#define va_start(list) list = &amp;va_list&quot; instead of &quot;#define va_start(list) list = (char *)&amp;va_list&quot; ?<br />
<br />
No. &nbsp;If you examine the code you posted carefully, you will see that the object of which the address is being taken is va_alist, which is not the same as the type va_list. &nbsp;va_alist is presumably declared with some type somewhere, but if that type is not char (and it's probably not), the cast is necessary to convert &amp;va_alist from pointer to &nbsp;to pointer to char.<br />
<br />
&gt;&nbsp;Int is 4-byte on 32-bit platform or 2-byte on 16-bit platform while char is 1-byte.If pInt is pointer to int and pChar is pointer to char,is it right that &quot;pInt+1&quot; adding 2 or 4 bytes and &quot;pChar+1&quot; adding 1 byte?<br />
<br />
Yes, basically. &nbsp;pInt + 1 addresses the next int after *pInt, wherever that is.<br />
<br />
&gt;&nbsp;Is that right,efn?<br />
<br />
Yes, I think you've got it.
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<div>
&gt;&gt; No.<br />
<br />
Indeed. But why would you want to leave it out ?
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<div class="content wysiwyg-content">
Hi,experts here.<br />
I have found a typical definition of variant length argument list here:<br />
<br />
typedef char* va_list;<br />
#define va_start(list) list = (char*)&amp;va_alist<br />
#define va_end(list)<br />
#define va_arg(list, mode)<br />
((mode*) (list += sizeof(mode)))[-1]<br />
<br />
(1)First,I'm not quite clear about &quot;(char*)&amp;va_alist&quot; here.Is &quot;list = (char*)&amp;va_alist&quot; equals to &quot;list = va_alist&quot;,or Could it be written as &quot;#define va_start(list) list = va_alist&quot;?<br />
(2)Second,Why is &quot;typedef char* va_list&quot; but not &quot;typedef void* va_list&quot; or even &quot;typedef int* va_list&quot;,<br />
What's its specialities?<br />
Wish your help and<br />
Thanks for any hint here.
</div>
</div>


Hi,experts here.
I have found a typical definition of variant length argument list here:

typedef char* va_list;
#define va_start(list) list = (char*)&va_alist
#define va_end(list)
#define va_arg(list, mode)
((mode*) (list += sizeof(mode)))[-1]

(1)First,I'm not quite clear about "(char*)&va_alist" here.Is "list = (char*)&va_alist" equals to "list = va_alist",or Could it be written as "#define va_start(list) list = va_alist"?
(2)Second,Why is "typedef char* va_list" but not "typedef void* va_list" or even "typedef int* va_list",
What's its specialities?
Wish your help and
Thanks for any hint here.

Question on definition of Variant Length Argument List

C is a general-purpose, imperative computer programming language, supporting structured programming, lexical variable scope and recursion, while a static type system prevents many unintended operations. By design, C provides constructs that map efficiently to typical machine instructions, so it has found lasting use in applications that had formerly been coded in assembly language, including operating systems as well as various application software for computers ranging from supercomputers to embedded systems. It is distinct from C++ (which has its roots in C) and C#, and many later languages have borrowed directly or indirectly from C.