Debugging with GDB (February 2008)

Chapter 5: Stopping and Continuing 45
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 sub-
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 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.
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 stepping through a loop
more than once. It is like the next command, 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) 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—