C ++ memory problems (many companies' interview questions are worth reading. Don't forget to tell me if you understand them)

C ++ memory problems (many companies' interview questions are worth reading. Don't forget to tell me if you understand them) Void getmemory (char * P)
{
P = (char *) malloc (100 );
}

Void test (void)
{
Char * STR = NULL;
Getmemory (STR );
Strcpy (STR, "helloworld ");
Printf (STR );
}
What are the results of running the test function?
A:ProgramCrash. Because getmemory cannot pass dynamic memory, STR in the test function is always null. Strcpy (STR, "helloworld"); will crash the program.

Char * getmemory (void)
{
Char P [] = "helloworld ";
Return P;
}
Void test (void)
{
Char * STR = NULL;
STR = getmemory ();
Printf (STR );
}
What are the results of running the test function?
A: It may be garbled. Because getmemory returns a pointer to "stack memory", the address of this pointer is not null, but its original content has been cleared and the new content is unknown.

Void getmemory2 (char ** P, int num)
{
* P = (char *) malloc (Num );
}
Void test (void)
{
Char * STR = NULL;
Getmemory (& STR, 100 );
Strcpy (STR, "hello ");
Printf (STR );
}
What are the results of running the test function?
Answer: (1) Hello output (2) Memory leakage

The above are all about memory. I would like to ask the first reason why getmemory cannot pass dynamic memory, but the third getmemory2 (char ** P, int num) can; the second one said that getmemory returned a pointer pointing to the "stack memory". The address of the pointer is not null. In short, I don't understand it. I 'd better understand what people can explain. ========================================================== ================================== They are all simple questions.
Void getmemory (char * P)
{
P = (char *) malloc (100 );
}
The p Parameter copies the value of the original pointer instead of the original pointer. Therefore, even if P points to the new address again, the original Pointer Points to the address unchanged.
Char * getmemory (void)
{
Char P [] = "helloworld ";
Return P;
}
P is a local variable. After leaving the scope, the stack space is recycled and the result is unpredictable.
Void getmemory (char ** P, int num)
{
* P = (char *) malloc (Num );
}
P is the pointer to the passed pointer. * P is the value assigned to the passed pointer. Therefore, the value of the passed pointer can be changed as follows:
Void getmemory (char * & P)
{
P = (char *) malloc (100 );
}
Same effect ========================================================== ================================== During VC debugging, press Alt + 8, TC or BC to use TD for debugging. Open the Assembly window and check whether the Assembly corresponding to each sentence C is understood.
To understand the C pointer in essence, you must learn assembly and the correspondence between C and assembly.
From the perspective of compilation, understanding and learning the pointers of C language makes it very simple to look at something complicated! From: http://topic.csdn.net/u/20100318/21/ae111d42-27e4-413a-9fd9-7132367d9f91.html