How to allocate memory
======================

[Published 2021-06-02]

From the man page of malloc:

    The malloc() function allocates size bytes and returns a pointer to the
    allocated  memory.

allocate (from Wiktionary):

    3. (computing) To reserve a portion of memory for use by a computer program.

Does that mean that malloc reserves the memory?  What does it mean to
reserve memory?  What does reserving memory do to the memory?
Nothing.  Malloc often doesn't change the memory in any way.

Maybe reserving just means to decide that the memory will be used for
something.  Who makes the decision?  The computer?  No, computers
don't have brains.  The programmer does.  The programmer allocates the
memory, not malloc.  That's because the programmer makes the decision
that the memory will be used.  The computer doesn't know anything
about how things will be used.

So what does malloc do then?  It's a tool that programmers can use to
manage memory allocations.  That's all.  What people call memory
allocators should really be called memory allocation management tools.

- - - - -

How do you allocate memory then?  Just think "This memory will be
used." That's how you allocate memory.  You don't even need to use
memory allocation management tools.  Here is a C program that shows
how I can allocate memory and then use it without using any memory
allocation management tools:

    #include <string.h>
    #include <stdio.h>
    int main() {
        char *my_string = (char *)1234;
        strcpy(my_string, "hello world");
        printf("%s\n", my_string);
    }

On the strcpy or printf line, is the memory that my_string points to
allocated?  It can be, if I say so.  The only problem is that the
program does not always work on some computers.  That is not because
the memory is not allocated; it is because the operating system
usually stops you from accessing some random memory like that.  That's
why memory allocation management tools are useful.  They do the work
necessary to get permission to use memory from the operating system
plus some additional work to keep track of what memory you have or
haven't requested.

Finally, there are multiple levels of allocation.  On the hardware
level, all of the memory in the computer is allocated to be used by
the operating system (or possibly other things).  At the same time,
many memory addresses are allocated to other hardware parts than the
memory; the tool used to keep track of these allocations is called a
memory map and can be found in the computer's documentation.  On the
next level, some memory is allocated by the operating system to a
running process; that's because the people who made the operating
system intended that memory to be used by the process.  Inside the
process, some memory can be allocated to be used by the programmer who
wrote that program.

All of this means that the computer or compiler can't know what memory
is allocated, so there are no memory safe languages (all languages
allow you to access memory that you didn't intend to access).