what is the difference between a label and a function in assembly

Question

have been following an assembly tutorial on youtube here through AT&T syntax. I have just learned about declaring(if that's the correct term here) a function with the .type directive, such as:

.type MyFunction, @function

Now I can define my function like:

MyFunction:
    <code here>

And subsequently call it whenever:

call MyFunction

I know that previous to this in the tutorials, we were simply creating a label that was attached to some code:

MyLabel:
    <code here>

which could then be called like this:

call MyLabel

So my questions are:

What exactly is the difference between a function declared with .type, and and 'function' declared simply with a label? When should one be used over the other, or does it matter?

Answer 1

Here's what the binutils docs say about the .type directive (assuming you are using the GNU assembler):

This directive is used to set the type of a symbol.

...

For ELF targets, the .type directive is used like this:

.type name , type description

According to the docs on Symbol Type:

The type attribute of a symbol contains relocation (section) information, any flag settings indicating that a symbol is external, and (optionally), other information for linkers and debuggers. The exact format depends on the object-code output format in use.

.type MyFunction, @function marks the label MyFunction (which is a symbol) as a function for the linker or debugger.

When writing pure assembly files by hand, this is unnecessary. Here's an example of a file in which an itoa function is called. As you can see, there are no directives preceding the label.

In summary, labels can be marked as functions, but to executing code, it's just a jump target either way.