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#### 5.5.2 Source Location Availability

Source location information is only available when the `debug` optimization quality is at least `2`. If source location information is unavailable, the source commands will give an error message.

If source location information is available, but the source location is unknown because of an interrupt or unexpected hardware error (see Unknown Locations and Interrupts), then the command will print:

         Unknown location: using block start.

and then proceed to print the source location for the start of the basic block enclosing the code location. It's a bit complicated to explain exactly what a basic block is, but here are some properties of the block start location:

- The block start location may be the same as the true location.
- The block start location will never be later in the program's flow of control than the true location.
- No conditional control structures (such as `if`, `cond`, `or`) will intervene between the block start and the true location (but note that some conditionals present in the original source could be optimized away.) Function calls do not end basic blocks.
- The head of a loop will be the start of a block.
- The programming language concept of “block structure” and the Lisp `block` special form are totally unrelated to the compiler's basic block.

In other words, the true location lies between the printed location and the next conditional (but watch out because the compiler may have changed the program on you.)