@node Debugging Support

@c @node Debugging Support, Threads, Cryptographic Functions, Top

@c %MENU% Functions to help debugging applications

@chapter Debugging support

Applications are usually debugged using dedicated debugger programs.

But sometimes this is not possible and, in any case, it is useful to

provide the developer with as much information as possible at the time

the problems are experienced.  For this reason a few functions are

provided which a program can use to help the developer more easily

locate the problem.

@menu

* Backtraces::                Obtaining and printing a back trace of the

                               current stack.

@end menu

@node Backtraces, , , Debugging Support

@section Backtraces

@cindex backtrace

@cindex backtrace_symbols

@cindex backtrace_fd

A @dfn{backtrace} is a list of the function calls that are currently

active in a thread.  The usual way to inspect a backtrace of a program

is to use an external debugger such as gdb.  However, sometimes it is

useful to obtain a backtrace programmatically from within a program,

e.g., for the purposes of logging or diagnostics.

The header file @file{execinfo.h} declares three functions that obtain

and manipulate backtraces of the current thread.

@pindex execinfo.h

@deftypefun int backtrace (void **@var{buffer}, int @var{size})

@standards{GNU, execinfo.h}

@safety{@prelim{}@mtsafe{}@asunsafe{@asuinit{} @ascuheap{} @ascudlopen{} @ascuplugin{} @asulock{}}@acunsafe{@acuinit{} @acsmem{} @aculock{} @acsfd{}}}

@c The generic implementation just does pointer chasing within the local

@c stack, without any guarantees that this will handle signal frames

@c correctly, so it's AS-Unsafe to begin with.  However, most (all?)

@c arches defer to libgcc_s's _Unwind_* implementation, dlopening

@c libgcc_s.so to that end except in a static version of libc.

@c libgcc_s's implementation may in turn defer to libunwind.  We can't

@c assume those implementations are AS- or AC-safe, but even if we

@c could, our own initialization path isn't, and libgcc's implementation

@c calls malloc and performs internal locking, so...

The @code{backtrace} function obtains a backtrace for the current

thread, as a list of pointers, and places the information into

@var{buffer}.  The argument @var{size} should be the number of

@w{@code{void *}} elements that will fit into @var{buffer}.  The return

value is the actual number of entries of @var{buffer} that are obtained,

and is at most @var{size}.

The pointers placed in @var{buffer} are actually return addresses

obtained by inspecting the stack, one return address per stack frame.

Note that certain compiler optimizations may interfere with obtaining a

valid backtrace.  Function inlining causes the inlined function to not

have a stack frame; tail call optimization replaces one stack frame with

another; frame pointer elimination will stop @code{backtrace} from

interpreting the stack contents correctly.

@end deftypefun

@deftypefun {char **} backtrace_symbols (void *const *@var{buffer}, int @var{size})

@standards{GNU, execinfo.h}

@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{} @aculock{}}}

@c Collects info returned by _dl_addr in an auto array, allocates memory

@c for the whole return buffer with malloc then sprintfs into it storing

@c pointers to the strings into the array entries in the buffer.

@c _dl_addr takes the recursive dl_load_lock then calls

@c _dl_find_dso_for_object and determine_info.

@c _dl_find_dso_for_object calls _dl-addr_inside_object.

@c All of them are safe as long as the lock is held.

@c @asucorrupt?  It doesn't look like the dynamic loader's data

@c structures could be in an inconsistent state that would cause

@c malfunction here.

The @code{backtrace_symbols} function translates the information

obtained from the @code{backtrace} function into an array of strings.

The argument @var{buffer} should be a pointer to an array of addresses

obtained via the @code{backtrace} function, and @var{size} is the number

of entries in that array (the return value of @code{backtrace}).

The return value is a pointer to an array of strings, which has

@var{size} entries just like the array @var{buffer}.  Each string

contains a printable representation of the corresponding element of

@var{buffer}.  It includes the function name (if this can be

determined), an offset into the function, and the actual return address

(in hexadecimal).

Currently, the function name and offset can only be obtained on systems that

use the ELF binary format for programs and libraries.  On other systems,

only the hexadecimal return address will be present.  Also, you may need

to pass additional flags to the linker to make the function names

available to the program.  (For example, on systems using GNU ld, you

must pass @code{-rdynamic}.)

The return value of @code{backtrace_symbols} is a pointer obtained via

the @code{malloc} function, and it is the responsibility of the caller

to @code{free} that pointer.  Note that only the return value need be

freed, not the individual strings.

The return value is @code{NULL} if sufficient memory for the strings

cannot be obtained.

@end deftypefun

@deftypefun void backtrace_symbols_fd (void *const *@var{buffer}, int @var{size}, int @var{fd})

@standards{GNU, execinfo.h}

@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}}

@c Single loop of _dl_addr over addresses, collecting info into an iovec

@c written out with a writev call per iteration.  Addresses and offsets

@c are converted to hex in auto buffers, so the only potential issue

@c here is leaking the dl lock in case of cancellation.

The @code{backtrace_symbols_fd} function performs the same translation

as the function @code{backtrace_symbols} function.  Instead of returning

the strings to the caller, it writes the strings to the file descriptor

@var{fd}, one per line.  It does not use the @code{malloc} function, and

can therefore be used in situations where that function might fail.

@end deftypefun

The following program illustrates the use of these functions.  Note that

the array to contain the return addresses returned by @code{backtrace}

is allocated on the stack.  Therefore code like this can be used in

situations where the memory handling via @code{malloc} does not work

anymore (in which case the @code{backtrace_symbols} has to be replaced

by a @code{backtrace_symbols_fd} call as well).  The number of return

addresses is normally not very large.  Even complicated programs rather

seldom have a nesting level of more than, say, 50 and with 200 possible

entries probably all programs should be covered.

@smallexample

@include execinfo.c.texi

@end smallexample