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1 721 jeremybenn
See README.alpha for Linux on DEC AXP info.
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This file applies mostly to Linux/Intel IA32.  Ports to Linux on an M68K, IA64,
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SPARC, MIPS, Alpha and PowerPC are also integrated.  They should behave
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similarly, except that the PowerPC port lacks incremental GC support, and
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it is unknown to what extent the Linux threads code is functional.
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See below for M68K specific notes.
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Incremental GC is generally supported.
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Dynamic libraries are supported on an ELF system.  A static executable
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should be linked with the gcc option "-Wl,-defsym,_DYNAMIC=0".
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The collector appears to work reliably with Linux threads, but beware
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of older versions of glibc and gdb.
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The garbage collector uses SIGPWR and SIGXCPU if it is used with
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Linux threads.  These should not be touched by the client program.
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To use threads, you need to abide by the following requirements:
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1) You need to use LinuxThreads or NPTL (which are included in libc6).
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   The collector relies on some implementation details of the LinuxThreads
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   package.  This code may not work on other
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   pthread implementations (in particular it will *not* work with
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   MIT pthreads).
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2) You must compile the collector with -DGC_LINUX_THREADS and -D_REENTRANT
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   specified in the Makefile.
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3a) Every file that makes thread calls should define GC_LINUX_THREADS and
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   _REENTRANT and then include gc.h.  Gc.h redefines some of the
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   pthread primitives as macros which also provide the collector with
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   information it requires.
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3b) A new alternative to (3a) is to build the collector and compile GC clients
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   with -DGC_USE_LD_WRAP, and to link the final program with
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   (for ld) --wrap read --wrap dlopen --wrap pthread_create \
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            --wrap pthread_join --wrap pthread_detach \
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            --wrap pthread_sigmask --wrap sleep
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   (for gcc) -Wl,--wrap -Wl,read -Wl,--wrap -Wl,dlopen -Wl,--wrap \
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             -Wl,pthread_create -Wl,--wrap -Wl,pthread_join -Wl,--wrap \
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             -Wl,pthread_detach -Wl,--wrap -Wl,pthread_sigmask \
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             -Wl,--wrap -Wl,sleep
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   In any case, _REENTRANT should be defined during compilation.
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4) Dlopen() disables collection during its execution.  (It can't run
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   concurrently with the collector, since the collector looks at its
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   data structures.  It can't acquire the allocator lock, since arbitrary
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   user startup code may run as part of dlopen().)  Under unusual
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   conditions, this may cause unexpected heap growth.
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5) The combination of GC_LINUX_THREADS, REDIRECT_MALLOC, and incremental
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   collection fails in seemingly random places.  This hasn't been tracked
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   down yet, but is perhaps not completely astonishing.  The thread package
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   uses malloc, and thus can presumably get SIGSEGVs while inside the
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   package.  There is no real guarantee that signals are handled properly
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   at that point.
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6) Thread local storage may not be viewed as part of the root set by the
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   collector.  This probably depends on the linuxthreads version.  For the
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   time being, any collectable memory referenced by thread local storage should
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   also be referenced from elsewhere, or be allocated as uncollectable.
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   (This is really a bug that should be fixed somehow.)
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M68K LINUX:
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(From Richard Zidlicky)
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The bad news is that it can crash every linux-m68k kernel on a 68040,
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so an additional test is needed somewhere on startup. I have meanwhile
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patches to correct the problem in 68040 buserror handler but it is not
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yet in any standard kernel.
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Here is a simple test program to detect whether the kernel has the
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problem. It could be run as a separate check in configure or tested
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upon startup. If it fails (return !0) than mprotect can't be used
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on that system.
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/*
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 * test for bug that may crash 68040 based Linux
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 */
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#include 
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#include 
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#include 
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#include 
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#include 
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char *membase;
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int pagesize=4096;
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int pageshift=12;
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int x_taken=0;
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int sighandler(int sig)
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{
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   mprotect(membase,pagesize,PROT_READ|PROT_WRITE);
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   x_taken=1;
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}
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main()
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{
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  long l;
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   signal(SIGSEGV,sighandler);
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   l=(long)mmap(NULL,pagesize,PROT_READ,MAP_PRIVATE | MAP_ANON,-1,0);
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  if (l==-1)
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     {
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       perror("mmap/malloc");
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       abort();
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     }
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  membase=(char*)l;
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    *(long*)(membase+sizeof(long))=123456789;
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  if (*(long*)(membase+sizeof(long)) != 123456789 )
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    {
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      fprintf(stderr,"writeback failed !\n");
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      exit(1);
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    }
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  if (!x_taken)
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    {
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      fprintf(stderr,"exception not taken !\n");
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      exit(1);
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    }
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  fprintf(stderr,"vmtest Ok\n");
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  exit(0);
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}
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