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Continuing and stepping

Continuing means resuming program execution until your program completes normally. In contrast, stepping means executing just one more "step" of your program, where "step" may mean either one line of source code, or one machine instruction (depending on what particular command you use). Either when continuing or when stepping, your program may stop even sooner, due to a breakpoint or a signal. (If due to a signal, you may want to use handle, or use `signal 0' to resume execution. See section Signals.)

continue [ignore-count]

c [ignore-count]

fg [ignore-count]

Resume program execution, at the address where your program last stopped; any breakpoints set at that address are bypassed. The optional argument ignore-count allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of ignore (see section Break conditions).

The argument ignore-count is meaningful only when your program stopped due to a breakpoint. At other times, the argument to continue is ignored.

The synonyms c and fg are provided purely for convenience, and have exactly the same behavior as continue.

To resume execution at a different place, you can use return (see section Returning from a function) to go back to the calling function; or jump (see section Continuing at a different address) to go to an arbitrary location in your program.

A typical technique for using stepping is to set a breakpoint (see section Breakpoints, watchpoints, and exceptions) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.

step

Continue running your program until control reaches a different source line, then stop it and return control to GDB. This command is abbreviated s.

Warning: If you use the step command while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information.

step count

Continue running as in step, but do so count times. If a breakpoint is reached, or a signal not related to stepping occurs before count steps, stepping stops right away.

next [count]

Continue to the next source line in the current (innermost) stack frame. Similar to step, but any function calls appearing within the line of code are executed without stopping. Execution stops when control reaches a different line of code at the stack level which was executing when the next command was given. This command is abbreviated n.

An argument count is a repeat count, as for step.

next within a function that lacks debugging information acts like step, but any function calls appearing within the code of the function are executed without stopping.

finish

Continue running until just after function in the selected stack frame returns. Print the returned value (if any).

Contrast this with the return command (see section Returning from a function).

until

u

Continue running until a source line past the current line, in the current stack frame, is reached. This command is used to avoid single stepping through a loop more than once. It is like the next command, except that when until encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump.

This means that when you reach the end of a loop after single stepping though it, until makes your program continue execution until it exits the loop. In contrast, a next command at the end of a loop simply steps back to the beginning of the loop, which forces you to step through the next iteration.

until always stops your program if it attempts to exit the current stack frame.

until may produce somewhat counterintuitive results if the order of machine code does not match the order of the source lines. For example, in the following excerpt from a debugging session, the f (frame) command shows that execution is stopped at line 206; yet when we use until, we get to line 195:

(gdb) f
#0  main (argc=4, argv=0xf7fffae8) at m4.c:206
206                 expand_input();
(gdb) until
195             for ( ; argc > 0; NEXTARG) {

This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop--even though the test in a C for-loop is written before the body of the loop. The until command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement--not in terms of the actual machine code.

until with no argument works by means of single instruction stepping, and hence is slower than until with an argument.

until location

u location

Continue running your program until either the specified location is reached, or the current stack frame returns. location is any of the forms of argument acceptable to break (see section Setting breakpoints). This form of the command uses breakpoints, and hence is quicker than until without an argument.

stepi

si

Execute one machine instruction, then stop and return to the debugger.

It is often useful to do `display/i $pc' when stepping by machine instructions. This makes GDB automatically display the next instruction to be executed, each time your program stops. See section Automatic display.

An argument is a repeat count, as in step.

nexti

ni

Execute one machine instruction, but if it is a function call, proceed until the function returns.

An argument is a repeat count, as in next.

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