Debugging with GDB (September 2007)

Chapter 5: Stopping and Continuing 45
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 infor-
mation for the function. Otherwise it acts like the next command. This avoids
problems when using cc -gl on MIPS machines. Previously, step entered s ub-
routines 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 com mand 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 occ ur in switch 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 Section 11.4 [Returning from a
function], page 99).
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 s tepping through a loop
more than once. It is like the next com mand, except that when until encoun-
ters 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 con-
trast, 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) comm and 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();