Stack Backtracing Inside Your Program

A couple of points are important to keep in mind when you use the backtrace functions. First, backtrace_symbols() internally calls malloc() and, thus, can fail if the memory heap is corrupted--which might be the case if you are dealing with a fault signal handler. If you need to resolve the return addresses in such a situation, calling backtrace_symbols_fd() is safer, because it directly writes to the given file descriptor without allocating memory. The same reasoning implies that it is safer to use either static or automatic (non dynamic) storage space for the array passed to backtrace().

Also, there are some limitations to the ability of automatically tracing back the execution of a program. The most relevant are some compiler optimizations that, in one way or another, alter the contents of the stack frame or even prevent a function from having one (think of function inlining). Obviously, the stack frame does not even exist for macros, which are not function calls at all. Finally, a stack backtrace is impossible to perform if the stack itself has been corrupted by a memory trash.

Regarding symbol resolution, the current glibc (version 2.3.1 at the time of this writing) allows users to obtain the function name and offset only on systems based on the ELF binary format. Furthermore, static symbols' names cannot be resolved internally, because they cannot be accessed by the dynamic linking facilities. In this case, the external command addr2line can be used instead.

In case you wonder how would you access stack information in a C program, the answer is simple: you can't. Stack handling, in fact, depends heavily on the platform your program runs on, and the C language does not provide any means to do it in a standard way. The implementation of backtrace() in the glibc library contains platform-specific code for each platform, which is based either on GCC internal variables (__builtin_frame_address and __builtin_return_address) or on assembly code.

In the case of the i386 platform (in glibc-x.x.x/sysdeps/i386/backtrace.c), a couple of lines of assembly code are used to access the contents of the ebp and esp CPU registers, which hold the address of the current stack frame and of the stack pointer for any given function:

register void *ebp __asm__ ("ebp");
register void *esp __asm__ ("esp");

Starting from the value of ebp, it is easy to follow the chain of pointers and move up to the initial stack frame. In this way you gather the sequence of return addresses and build the backtrace.

At this point, you still have to resolve the return addresses into function names, an operation dependent on the binary format you are using. In the case of ELF, it is performed by using a dynamic linker internal function (_dl_addr(), see glibc-x.x.x/sysdeps/generic/elf/backtracesyms.c).

Are you working on a complex program that contains a lot of different execution paths that make you cluelessly wander through hundreds of functions, desperately trying to understand which one called which other function? Wander no more and print a backtrace. It's free, fast and easy. While you are at it, do yourself a favour and also use that function inside a fault signal handler--it's guaranteed to help you with those nasty bugs that appear once in a thousand runs.

Gianluca Insolvibile has been a Linux enthusiast since kernel 0.99pl4. He currently deals with networking and digital video research and development.