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3.5.2 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), 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:

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.)