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Diffstat (limited to 'REORG.TODO/manual/debug.texi')
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diff --git a/REORG.TODO/manual/debug.texi b/REORG.TODO/manual/debug.texi new file mode 100644 index 0000000000..ac5121b061 --- /dev/null +++ b/REORG.TODO/manual/debug.texi @@ -0,0 +1,136 @@ +@node Debugging Support +@c @node Debugging Support, POSIX 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 + +@comment execinfo.h +@comment GNU +@deftypefun int backtrace (void **@var{buffer}, int @var{size}) +@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 + +@comment execinfo.h +@comment GNU +@deftypefun {char **} backtrace_symbols (void *const *@var{buffer}, int @var{size}) +@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 + +@comment execinfo.h +@comment GNU +@deftypefun void backtrace_symbols_fd (void *const *@var{buffer}, int @var{size}, int @var{fd}) +@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 |