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] or c [ignore-count] or 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 lets you 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.

Continue running your program until control reaches a different source line, then stop it and return control to GDB. This command is abbreviated s.
Note: 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 doesn't step into a function that is compiled without debugging information. To step through functions without debugging information, use the stepi command, described below.

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

Also, the step command enters a subroutine only if there's line number information for the subroutine. Otherwise it acts like the next command. This avoids problems when using cc -gl on MIPS machines.

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.

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

The next command stops only at the first instruction of a source line. This prevents the multiple stops in switch statements, for loops, etc.

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 or 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's 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.

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

The until command may produce somewhat counterintuitive results if the order of machine code doesn't 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 hasn't really gone to an earlier statement—not in terms of the actual machine code.

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

until location or u location
Continue running your program until either the specified location is reached, or the current stack frame returns. The 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 [count] or si [count]
Execute one machine instruction, then stop and return to the debugger.

It's 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 "Automatic display."

The count argument is a repeat count, as in step.

nexti [count] or ni [count]
Execute one machine instruction, but if it's a function call, proceed until the function returns.

The count argument is a repeat count, as in next.