When should I use malloc in C and when don’t I?

by 
char *some_memory = "Hello World";

is creating a pointer to a string constant. That means the string “Hello World” will be somewhere in the read-only part of the memory and you just have a pointer to it. You can use the string as read-only. You cannot make changes to it. Example:

some_memory[0] = 'h';

Is asking for trouble.

On the other hand 

some_memory = (char *)malloc(size_to_allocate);

is allocating a char array ( a variable) and some_memory points to that allocated memory. Now this array is both read and write. You can now do:

some_memory[0] = 'h';

and the array contents change to “hello World”

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