Up: Optimized Code
11.1 Inline Functions
Inlining is an optimization that inserts a copy of the function
body directly at each call site, instead of jumping to a shared
routine. gdb displays inlined functions just like
non-inlined functions. They appear in backtraces. You can view their
arguments and local variables, step into them with
next, and escape from them with
You can check whether a function was inlined by using the
info frame command.
For gdb to support inlined functions, the compiler must record information about inlining in the debug information — gcc using the dwarf 2 format does this, and several other compilers do also. gdb only supports inlined functions when using dwarf 2. Versions of gcc before 4.1 do not emit two required attributes (DW_AT_call_file and DW_AT_call_line); gdb does not display inlined function calls with earlier versions of gcc. It instead displays the arguments and local variables of inlined functions as local variables in the caller.
The body of an inlined function is directly included at its call site; unlike a non-inlined function, there are no instructions devoted to the call. gdb still pretends that the call site and the start of the inlined function are different instructions. Stepping to the call site shows the call site, and then stepping again shows the first line of the inlined function, even though no additional instructions are executed.
This makes source-level debugging much clearer; you can see both the
context of the call and then the effect of the call. Only stepping by
a single instruction using
nexti does not do
this; single instruction steps always show the inlined body.
There are some ways that gdb does not pretend that inlined function calls are the same as normal calls:
- You cannot set breakpoints on inlined functions. gdb either reports that there is no symbol with that name, or else sets the breakpoint only on non-inlined copies of the function. This limitation will be removed in a future version of gdb; until then, set a breakpoint by line number on the first line of the inlined function instead.
- Setting breakpoints at the call site of an inlined function may not work, because the call site does not contain any code. gdb may incorrectly move the breakpoint to the next line of the enclosing function, after the call. This limitation will be removed in a future version of gdb; until then, set a breakpoint on an earlier line or inside the inlined function instead.
- gdb cannot locate the return value of inlined calls after
finishcommand. This is a limitation of compiler-generated debugging information; after
finish, you can step to the next line and print a variable where your program stored the return value.