From Gambit wiki
(Give more details on the object representation)
|Line 84:||Line 84:|
=== Object size ===
=== Object size ===
. , and .
fixnumshave in the -2^.. 2^is -2^.. 2^.
stored as , and .
Revision as of 18:31, 4 October 2008
(This document is a stub.)
Gambit features an internal source debugger, see the User Documentation.
There are currently multiple interactive debuggers in development, please see the mailing list archives.
Tracking down mistyped identifiers (i.e. variable and procedure names)
Typically you solve this through compiling your code, i.e. (compile-file X), and then load the compiled object file, i.e. (load "file") or (load "file.o1"). The typical warning message is
*** WARNING -- Variable "namespace-name#identifier-name" used in module "filename.o1" is undefined
In order to track down the line number of the undefined identifier, compile with the with-gvm option. The addressed row numbers are found on the rows starting with "line " above the rows with occurrences of namespace-name#identifier-name in them, in the gvm file.
Tracking down segmentation faults
Typical problem sources
Running (#!void) in code compiled with (declare (not safe)) does produce sigsegv:s. Addressing nonexistant variables in code compiled with (declare (not safe)) may cause this as well, i.e. (string-ref nonexistant-variable 0) or (set! nonexistant-variable 'a-value).
Also, pay attention that all Gambit-internal functions without typechecks are always invoked with correct parameters. These are prefixed with ##. I.e., the car function with typechecks is (car), and the one without typechecks is (##car). Thus, (car #!void) will not be fatal in code compiled with (declare (safe)), though, (##car #!void) may be fatal in code compiled with (declare (safe)). And, as we already concluded, both these expressions are fatal to execute in code compiled with (declare (not safe)).
Compiling Gambit for debugging purposes
Download the latest Gambit sourcecode.
Unpack it, usually you do that through executing tar xvfz gambc-v4_X_X.tgz in your shell.
- Run the configure script. It is normally compiled with --enable-single-host flipped on. If you want Gambit to dump internal messages to the file "console" in the current directory, also pass the --enable-debug parameter. To see all options, run ./configure --help.
- Edit the file makefile using your favourite editor. There are at least two rows that contain the sequence -O1 (i.e. dash O one), in the current version that's the two rows starting with FLAGS_OBJ and FLAGS_DYN. Replace -O1 with -g -O0 . The -g option will make your C compiler include debugging symbols on compilation, and the -O0 will force it not to do any optimizations on the assembly/binary code it generates, thus the mapping of rows of C/C++ code into memory addresses during program execution will be the most precise your C/C++ compiler is capable of generating.
- Compile Gambit through typing make
- If you want this Gambit to replace your current installation, run make install
Running compile-file with debugging options flipped on
When running (compile-file ), remember to pass the cc-opts: "-g -O0" .
Also, there are the options:
- track-scheme to make the C/C++ compiler use the source Scheme code instead of the source C/C++ code for debugging info. This is an option that you may want to vary during debugging work.
- keep-c to make compile-file not remove the intermediary C/C++ code file.
Run Gambit in GDB
GDB is found on http://sourceware.org/gdb/, and is typically included as an optional package with Linux and BSD distributions.
From your shell, run
GDB will start. If you want to pass gsc argument, type
set args=[the arguments]
set args=-e "(display \"test\n\")" -
To start Gambit witihn GDB, type run.
If your Gambit application crashes, you will get a prompt in GDB indicating so. To get a backtrace of the stack of your program, type bt . The backtrace should also be interesting for anyone who would assist you in finding the reason to the crash.
To quit, type quit.
Other things you may want to do
There are ample of debug utilities, for different operating systems, that may be interesting for you to use.
For instance, strace is an utility that displays all Kernel invocations your application does. Remember to run it with the -Ff options.
valgrind is an utility that traces memory leaks. (Gambit will not leak memory in itself, but a faulty Foreign Function Interface library could do that.)
Tracking down excessive memory use
Symbols are not garbage collected. Thus, if you do (string->symbol X) on gigabytes of string data, then gigabytes of heap space will be permanently allocated for this. If you want to use ample of symbols in your application, use uninterned symbols, generated using make-uninterned-symbol .
Memory leaks in FFI:s
While Gambit garbage collects Scheme objects on its own, objects created by FFI:s may not be garbage collected automatically, and, there may be bugs in FFI:s. If you use an FFI, look at how it should be used carefully, and if you suspect there's a memory leak in it, analyze its sourcecode, and use debug utilities such as valgrind.
Garbage collection threshold
Pay attention to the runtime options h (maximum heapsize in kilobytes) and l (livepercent). See the reference manual for more information. Setting livepercent to five means that garbage collection will take place at the time that there are nineteen times more memory allocated for objects that should be garbage collected, than there is memory allocated for objects that should not. The reason the livepercent option is there, is to give a way to control how sparing/generous the garbage collector should be about memory consumption, vs. how heavy/light it should be in CPU load.
The internal representation of objects depends on the word size of the processor. Object references are encoded in a single word, either 32 or 64 bits wide, whose lower 2 bits is a type tag giving some partial type information on the object. The upper bits are either a literal value (such as fixnums, characters, booleans, and so on) or an encoding of a pointer (in the case of memory allocated objects such as flonums, bignums, pairs, vectors, and other structured objects).
In particular small integers (fixnums) have a type tag of zero. The upper bits contain the integer value in 2's complement representation. For example the fixnum 15 is encoded with a word whose value is 60 (i.e. 15 * 4). On a 32 bit processor the range is -536870912 .. 536870911 (i.e. -2^(31-2) .. 2^(31-2)-1) and on a 64 bit processor the range is -2305843009213693952 .. 2305843009213693951 (i.e. -2^(63-2) .. 2^(63-2)-1). Exact integers outside of the fixnum range are represented using bignums, which are memory allocated.
Memory is allocated in multiples of the word size (4 bytes on a 32 bit processor and 8 bytes on a 64 bit processor) and aligned on the word size. The alignment constraint implies that (at least) the two lower bits of the address are always zero and can thus be overwritten with the type tag to form an object reference. The first word is a header whose lower 8 bits contain the type of the object, and some bits used by the garbage collector. The upper bits encode the length of the object in bytes. Consequently there is a limit on the size of objects and vectors (homogeneous or not) in particular. On a 64 bit processor this is not a problem because lengths up to 64 petabytes are possible (that's a pretty big object!). On a 32 bit processor the length is limited to 16 megabytes, which implies that vectors of more than 4 million elements cannot be represented. The runtime system will raise a heap overflow exception when attempting to allocate an object bigger than this limit.
Strings are memory allocated object stored in memory as uniform vectors of 8, 16 or 32 bit integers depending on the --enable-char-size=N configure option used (i.e. --enable-char-size=1 for 8 bit characters, --enable-char-size=2 for 16 bit characters, and --enable-char-size=4 for 32 bit characters). The default is --enable-char-size=4, which means all the Unicode characters are storable in strings by default. With the default 4 byte per character representation, the longest string that can be allocated on a 32 bit processor is just over 4 million characters long.