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111 lines
4.8 KiB
Plaintext
111 lines
4.8 KiB
Plaintext
@node Debugging Support
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@c @node Debugging Support, , Cryptographic Functions, Top
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@c %MENU% Functions to help debugging applications.
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@chapter Debugging support
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Applications are usually debugged using dedicated debugger programs.
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But sometimes this is not possible and, in any case, it is useful to
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provide the developer with as much information as possible at the time
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the problems are experienced. For this reason a few functions are
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provided which a program can use to help the developer more easily
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locate the problem.
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@menu
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* Backtraces:: Obtaining and printing a back trace of the
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current stack.
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@end menu
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@node Backtraces, , , Debugging Support
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@section Backtraces
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@cindex backtrace
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@cindex backtrace_symbols
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@cindex backtrace_fd
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A @dfn{backtrace} is a list of the function calls that are currently
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active in a thread. The usual way to inspect a backtrace of a program
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is to use an external debugger such as gdb. However, sometimes it is
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useful to obtain a backtrace programatically from within a program,
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e.g., for the purposes of logging or diagnostics.
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The header file @file{execinfo.h} declares three functions that obtain
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and manipulate backtraces of the current thread.
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@pindex execinfo.h
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@comment execinfo.h
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@comment GNU
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@deftypefun int backtrace (void **@var{buffer}, int @var{size})
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The @code{backtrace} function obtains a backtrace for the current
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thread, as a list of pointers, and places the information into
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@var{buffer}. The argument @var{size} should be the number of
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@w{@code{void *}} elements that will fit into @var{buffer}. The return
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value is the actual number of entries of @var{buffer} that are obtained,
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and is at most @var{size}.
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The pointers placed in @var{buffer} are actually return addresses
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obtained by inspecting the stack, one return address per stack frame.
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Note that certain compiler optimisations may interfere with obtaining a
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valid backtrace. Function inlining causes the inlined function to not
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have a stack frame; tail call optimisation replaces one stack frame with
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another; frame pointer elimination will stop @code{backtrace} from
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interpreting the stack contents correctly.
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@end deftypefun
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@comment execinfo.h
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@comment GNU
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@deftypefun {char **} backtrace_symbols (void *const *@var{buffer}, int @var{size})
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The @code{backtrace_symbols} function translates the information
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obtained from the @code{backtrace} function into an array of strings.
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The argument @var{buffer} should be a pointer to an array of addresses
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obtained via the @code{backtrace} function, and @var{size} is the number
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of entries in that array (the return value of @code{backtrace}).
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The return value is a pointer to an array of strings, which has
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@var{size} entries just like the array @var{buffer}. Each string
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contains a printable representation of the corresponding element of
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@var{buffer}. It includes the function name (if this can be
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determined), an offset into the function, and the actual return address
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(in hexadecimal).
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Currently, the function name and offset only be obtained on systems that
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use the ELF binary format for programs and libraries. On other systems,
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only the hexadecimal return address will be present. Also, you may need
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to pass additional flags to the linker to make the function names
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available to the program. (For example, on systems using GNU ld, you
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must pass (@code{-rdynamic}.)
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The return value of @code{backtrace_symbols} is a pointer obtained via
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the @code{malloc} function, and it is the responsibility of the caller
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to @code{free} that pointer. Note that only the return value need be
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freed, not the individual strings.
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The return value is @code{NULL} if sufficient memory for the strings
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cannot be obtained.
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@end deftypefun
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@comment execinfo.h
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@comment GNU
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@deftypefun void backtrace_symbols_fd (void *const *@var{buffer}, int @var{size}, int @var{fd})
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The @code{backtrace_symbols_fd} function performs the same translation
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as the function @code{backtrace_symbols} function. Instead of returning
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the strings to the caller, it writes the strings to the file descriptor
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@var{fd}, one per line. It does not use the @code{malloc} function, and
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can therefore be used in situations where that function might fail.
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@end deftypefun
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The following program illustrates the use of these functions. Note that
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the array to contain the return addresses returned by @code{backtrace}
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is allocated on the stack. Therefore code like this can be used in
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situations where the memory handling via @code{malloc} does not work
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anymore (in which case the @code{backtrace_symbols} has to be replaced
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by a @code{backtrace_symbols_fd} call as well). The number of return
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addresses is normally not very large. Even complicated programs rather
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seldom have a nesting level of more than, say, 50 and with 200 possible
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entries probably all programs should be covered.
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@smallexample
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@include execinfo.c.texi
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@end smallexample
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