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

Question

I understand how malloc() works. My question is, I'll see things like this:

#define A_MEGABYTE (1024 * 1024)

char *some_memory;
size_t size_to_allocate = A_MEGABYTE;
some_memory = (char *)malloc(size_to_allocate);
sprintf(some_memory, "Hello World");
printf("%s\n", some_memory);
free(some_memory);

I omitted error checking for the sake of brevity. My question is, can't you just do the above by initializing a pointer to some static storage in memory? perhaps:

char *some_memory = "Hello World";

At what point do you actually need to allocate the memory yourself instead of declaring/initializing the values you need to retain?

Answer 1

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"