| Debugging with GDB: Continuing and Stepping |
|---|
Next: Skipping Over Functions and Files, Previous: Breakpoints, Up: Stopping [Contents][Index]
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
it stops due to a signal, you may want to use handle, or use
‘signal 0’ to resume execution (see Signals),
or you may step into the signal’s handler (see stepping and signal handlers).)
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 (for foreground, as the
debugged program is deemed to be the foreground program) 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 Catchpoints) 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
stepcommand 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 thestepicommand, described below.
The step command only stops at the first instruction of a source
line. This prevents the multiple stops that could otherwise occur in
switch statements, for loops, etc. step continues
to stop if a function that has debugging information is called within
the line. In other words, step steps inside any functions
called within the line.
Also, the step command only enters a function if there is line
number information for the function. 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 only stops at the first instruction of a
source line. This prevents multiple stops that could otherwise occur in
switch statements, for loops, etc.
set step-mode
set step-mode on
The set step-mode on command causes the step command to
stop at the first instruction of a function which contains no debug line
information rather than stepping over it.
This is useful in cases where you may be interested in inspecting the machine instructions of a function which has no symbolic info and do not want GDB to automatically skip over this function.
set step-mode off
Causes the step command to step over any functions which contains no
debug information. This is the default.
show step-mode
Show whether GDB will stop in or step over functions without source line debug information.
finish
Continue running until just after function in the selected stack frame
returns. Print the returned value (if any). This command can be
abbreviated as fin.
Contrast this with the return command (see 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. The location is any of
the forms described in Specify Location.
This form of the command uses temporary breakpoints, and
hence is quicker than until without an argument. The specified
location is actually reached only if it is in the current frame. This
implies that until can be used to skip over recursive function
invocations. For instance in the code below, if the current location is
line 96, issuing until 99 will execute the program up to
line 99 in the same invocation of factorial, i.e., after the inner
invocations have returned.
94 int factorial (int value)
95 {
96 if (value > 1) {
97 value *= factorial (value - 1);
98 }
99 return (value);
100 }
advance location
Continue running the program up to the given location. An argument is
required, which should be of one of the forms described in
Specify Location.
Execution will also stop upon exit from the current stack
frame. This command is similar to until, but advance will
not skip over recursive function calls, and the target location doesn’t
have to be in the same frame as the current one.
stepi
stepi arg
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 Automatic Display.
An argument is a repeat count, as in step.
nexti
nexti arg
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.
By default, and if available, GDB makes use of
target-assisted range stepping. In other words, whenever you
use a stepping command (e.g., step, next), GDB
tells the target to step the corresponding range of instruction
addresses instead of issuing multiple single-steps. This speeds up
line stepping, particularly for remote targets. Ideally, there should
be no reason you would want to turn range stepping off. However, it’s
possible that a bug in the debug info, a bug in the remote stub (for
remote targets), or even a bug in GDB could make line
stepping behave incorrectly when target-assisted range stepping is
enabled. You can use the following command to turn off range stepping
if necessary:
set range-stepping
show range-stepping
Control whether range stepping is enabled.
If on, and the target supports it, GDB tells the
target to step a range of addresses itself, instead of issuing
multiple single-steps. If off, GDB always issues
single-steps, even if range stepping is supported by the target. The
default is on.
Next: Skipping Over Functions and Files, Previous: Breakpoints, Up: Stopping [Contents][Index]