Dynamic Memory Allocation


 
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# 1  
Old 06-20-2011
Dynamic Memory Allocation

Hello Guys

I have a small confusion in the dynamic memory allocation concept.
If we declare a pointer say a char pointer, we need to allocate adequate memory space.

Code:
char* str = (char*)malloc(20*sizeof(char));
str = "This is a string";


But this will also work.

Code:
char* str = "This is a string";

So in which case do we have to allocate memory space?
# 2  
Old 06-20-2011
Both are wrong. In the first case you allocate 20 bytes of space for a string. However, instead of saving a string in that space, you change the pointer to point to a static, non-mutable string. The only difference to the second example is that you don't allocate space that you'll never be able to free again.

The correct way would be this:
Code:
char *str = (char *) malloc(20 * sizeof(char));
strncpy(str, "This is a string", 20 * sizeof(char));

# 3  
Old 06-20-2011
A pointer isn't a magical type that knows what you want it to do. It's just a numerical type with extra operators, = still means=. Ergo, when you use = on it, you're telling it to point to a completely different spot in memory. If you want to preserve memory you already have, you have to use string operations or array operations to copy it in.
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