17.2 Continuing at a Different Address

Ordinarily, when you continue your program, you do so at the place where it stopped, with the continue command. You can instead continue at an address of your own choosing, with the following commands:

jump locspec

j locspec

Resume execution at the address of the code location that results from resolving locspec. See Location Specifications, for a description of the different forms of locspec. If locspec resolves to more than one address, those outside the current compilation unit are ignored. If considering just the addresses in the current compilation unit still doesn’t yield a unique address, the command aborts before jumping. Execution stops again immediately if there is a breakpoint there. It is common practice to use the tbreak command in conjunction with jump. See Setting Breakpoints.

The jump command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter. If locspec resolves to an address in a different function from the one currently executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables. For this reason, the jump command requests confirmation if the jump address is not in the function currently executing. However, even bizarre results are predictable if you are well acquainted with the machine-language code of your program.

On many systems, you can get much the same effect as the jump command by storing a new value into the register $pc. The difference is that this does not start your program running; it only changes the address of where it will run when you continue. For example,

set $pc = 0x485

makes the next continue command or stepping command execute at address 0x485, rather than at the address where your program stopped. See Continuing and Stepping.

However, writing directly to $pc will only change the value of the program-counter register, while using jump will ensure that any additional auxiliary state is also updated. For example, on SPARC, jump will update both $pc and $npc registers prior to resuming execution. When using the approach of writing directly to $pc it is your job to also update the $npc register.

The most common occasion to use the jump command is to back up—perhaps with more breakpoints set—over a portion of a program that has already executed, in order to examine its execution in more detail.