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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 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 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 Returning from a function) to go back to the calling function; or jump (see 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 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. Likewise, it will not step into a function which is compiled without debugging information. To step through functions without debugging information, use the stepi command, described in the following.

The step command now only stops at the first instruction of a source line. This prevents the multiple stops that used to occur in switch statements, for loops, etc. step continues to stop if a function that has debugging information is called within the line.

Also, the step command now only enters a subroutine if there is line number information for the subroutine. Otherwise it acts like the next command. This avoids problems when using cc -gl on MIPS machines. Previously, step entered subroutines if there was any debugging information about the routine.

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. This is similar to step, but function calls that appear within the line of code are executed without stopping. Execution stops when control reaches a different line of code at the original stack level that was executing when you gave the next command. This command is abbreviated n.

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

The next command now only stops at the first instruction of a source line. This prevents the multiple stops that used to occur in swtch statements, for loops, etc.

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 Returning from a function).

u

until
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 counter-intuitive results if the order of machine code does not match the order of the source lines. For instance, in the following example 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 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 use ‘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 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.