`testl` eax against eax?

The meaning of test is to AND the arguments together, and check the result for zero. So this code tests if EAX is zero or not. je will jump if zero.

BTW, this generates a smaller instruction than cmp eax, 0 which is the reason that compilers will generally do it this way.

The test instruction does a logical AND-operation between the operands but does not write the result back into a register. Only the flags are updated.

In your example the test eax, eax will set the zero flag if eax is zero, the sign-flag if the highest bit set and some other flags as well.

The Jump if Equal (je) instruction jumps if the zero flag is set.

You can translate the code to a more readable code like this:

cmp eax, 0
je  somewhere

That has the same functionality but requires some bytes more code-space. That's the reason why the compiler emitted a test instead of a compare.

It tests whether eax is 0, or above, or below. In this case, the jump is taken if eax is 0.